Part 4. Diagnostic Tools in Heart Disease

Normal electrical activation of the heart begins pacemaker activity in the sinoatrial node, and the wave of activation spreads through the right and left atria (Fig. 493-1). In the right atrium the wave of depolarization passes inferiorly, and the left atrium is activated via Bachmann bundle, which also triggers an inferiorly directed activation front. These activation fronts generate a potential that is detected on the body surface as the P wave. Any force that has magnitude and direction is termed a vector and can be represented by an arrow with direction and magnitude proportional to the force. The impulse is delayed at the atrioventricular (AV) node, producing the PR interval. This allows ventricular filling to be completed before ventricular contraction begins. Beyond the AV node, the impulse moves rapidly down the bundle of His into the right and left bundle branches. As the impulses pass down the septum, they activate septal muscle predominantly from the left side, so that the initial ventricular vector passes from left to right, anteriorly and superiorly (Fig. 493-2), and begins the Q wave in lead V6 or the first part of the R wave in lead V1.1 After reaching the apex, the impulse activates the ventricular free walls from endocardium to epicardium and from apex toward the base, thus inscribing the R and S waves; the last part of the heart to be activated is the posterior ventricular muscle just under the AV ring. In adults and older children, there is more left than right ventricular muscle, so the major cardiac vectors point to the left and posteriorly and produce a tall R wave in V6 and a deep S wave in V1. In a normal newborn infant with a thick right ventricle, the major cardiac vectors pass to the right and anteriorly and produce a dominant R wave in V1 and a large S wave in V6. After depolarization has occurred, there is slower repolarization that produces the T wave.

The vectors at each point in the cardiac cycle can be drawn as if arising from a common central point; thus, they can be represented by a series of arrows fanning out (Fig. 493-2B). The ends of the arrows can be joined and the shafts omitted to give a vector loop (Fig. 493-2C). These vector loops can be recorded directly by connecting electrodes to the body and recording the voltages between 3 sets of electrodes at right angles to each other. Leads from right to left define an X axis, those from top to bottom define a Y axis, and those from front to back define a Z axis. (A lead is a line joining 2 electrodes or 2 sets of electrodes.) Simultaneous recording of X and Y axes gives a loop representing the projection of the actual vector loop on the frontal plane of the body, whereas simultaneous recording of voltages in the X and Z axes gives the horizontal plane projection of the vector loop (eFig. 493.1). The Y and Z axes define the sagittal plane vector loop, but this adds no additional information beyond that contained in the frontal and horizontal planes. Various lead systems give similar patterns, but different values of various measurements, so that each needs its own standards. The beam is interrupted every 0.0025 seconds and usually has a blunt leading edge, thus it is possible to tell which way the vector is moving and at what speed.

The “scalar” electrocardiogram is a recording of voltage against time, usually made at a paper speed of 25 mm/s, with 1 mV giving 10 mm of deflection. At any instant the mean vector present can be resolved into 2 components, 1 parallel to a lead and 1 at right angles to it. The latter affects both electrodes equally, so that there is no potential difference between them, whereas the former causes a voltage to appear between the 2 electrodes. This voltage is proportional to the projection made by the vector on the lead (Fig. 493-3A). The limb leads (I, II, III, aVR, aVL, aVF) represent forces in the frontal plane, and the precordial leads essentially represent forces in the horizontal plane (Fig. 493-3B); of these, the right chest leads (V4R,V3R,V1,V2) indicate anteroposterior forces, and the left chest leads (V5,V6) indicate left-to-right forces. Furthermore, certain parts of the left ventricle can be roughly reflected in certain groupings of leads: leads V1 and V2 show activity in the anteroseptal muscle; V2 to V5 in the anterior wall; V5 and V6 in the apex; I and aVL in the anterolateral wall; and II, III, and aVF in the inferior (diaphragmatic) wall. Remember that there is actually a 3-dimensional vector in the heart and that the electrocardiogram and vectorcardiogram merely indicate the projection of that vector on a particular lead or plane, respectively. The electrocardiogram is easier to take, and time intervals are more easily measured with it, whereas the vectorcardiogram gives more precise information about the vector at each moment in the cardiac cycle. Although vectorcardiograms are now rarely performed, the experienced electrocardiographer sees the electrocardiogram as a representation of the vectorcardiogram and visualizes the vector forces in interpreting the tracings.

The routine pediatric electrocardiogram involves the recording of 15 leads, the standard 12 plus leads to the right of V1 (V3R, V4R), and at least 1 lead (V7) to the left of V6; if no Q wave is seen in V7, then leads to the left of V7 should be recorded as well. Ideally, the electrocardiogram should be interpreted knowing the history; physical examination; drug therapy; and heart size, shape, and position as seen on the chest roentgenogram, as part of the standardized cardiac evaluation. However, the electrocardiographer often does not have access to this information. At a minimum, the reason for the study should be given.

It is best to read the electrocardiogram systematically. First, measure atrial and ventricular rates and define the rhythm. Then record the PR interval, QRS duration, and QT interval. Measure the frontal-plane mean axes of the P waves, QRS complex, and T waves. Finally, look for abnormalities of pattern in the waves and their interconnecting segments and note the voltages of P, Q, R, S, and T waves.

The electrocardiogram can provide information about arrhythmias, hypertrophy of cardiac chambers, electrolyte changes, myocardial or pericardial infections, and myocardial ischemia and infarction. Specific electrocardiogram (ECG) patterns occur in certain congenital heart diseases. The electrocardiogram gives little information about myocardial function or ventricular cavity size.

Measuring the QRS Axis

To evaluate the mean frontal plane QRS axis, the following features must be noted:

• Lead I records electrical activity from the left to the right arm; by agreement, a deflection passing toward the left arm is positive, and a deflection passing away from it is negative (Fig. 493-4A).
• Leads II and III are at 60° to each other and to lead I, and they can be superimposed on a common center (Fig. 493-4B). Note that the lower half of the frontal plane is assigned positive numbers and the upper half negative numbers.
• Lead aVF records electrical activity from the legs up to the heart and thus lies in the axis +90° to 90° (Fig. 493-4C). An impulse passing toward the leg thus gives a positive deflection.
• Leads aVR and aVL record electrical activity from the right and left shoulders, respectively, to the heart (Fig. 493-4D), and their axes are at 120° to each other and to lead aVF. Because they are unipolar leads, an impulse passing toward either shoulder will give a positive deflection despite the negative angles associated with the superior ends of the axes in the figure.
• All 6 leads may be superimposed on a central point (Fig. 493-4E) to give axes that are at 30° to each other.
• To use this hexaxial reference frame, first decide whether lead I is predominantly positive or negative. In general, this can be determined from the heights of the R and S waves. However, because it is actually area that is assessed, a tall but narrow R wave might contribute less positivity than the negativity contributed by a short but wide S wave; if the Q wave is large, it should also be included in the assessment. If the result is positive, the net vector is passing to the left; if it is negative, the net vector is passing to the right. If the net voltage is zero (that is, equiphasic R and S complexes), the vector must be passing up at 90° or down at +90° (Fig. 493-4A). Next, examine aVF for its net voltage: if positive, the vector is passing down; if negative, it is passing up; if zero, it is passing directly right or left (Fig. 493-4C). These 2 leads can be quickly integrated to assign the vector to 1 of 4 quadrants (Fig. 493-4F). Lead I+, aVF+ indicates the left lower quadrant; lead I+, aVF– indicates the left upper quadrant (left axis); lead I–, aVF+ indicates the right lower quadrant (right axis); lead I–, aVF– indicates the right upper quadrant, sometimes termed the northwest quadrant. The axis can be measured more closely by referring to the remaining leads; on modern electrocardiograms, the frontal plane axis is computed and reported in the ECG print-out.
• Occasionally, the R and S waves are almost equiphasic in several limb leads, perhaps in 4 or in all 6. This implies that the QRS vector is mainly perpendicular to the frontal plane and in the frontal plane forms a rough circle so that there is no true mean axis. This is sometimes termed an indeterminate frontal plane axis. It has little specific clinical importance except insofar as it prevents the accurate determination of a mean QRS axis.
• The mean frontal axis of the P and T waves can be calculated in the same way.

Measuring the QT Interval

By convention, the measured QT interval is reported as well as the “corrected” interval, termed the QTc, which is the raw QT divided by the square root of the R-R interval. Measurement of the QT interval is, unfortunately, not straightforward. This is partly because determining the end of the slowly-changing T wave can be difficult and partly because the QT interval changes with heart rate on a beat-to-beat basis. The latter feature becomes important in children who often display marked normal sinus arrhythmia. Finally, the normal standards for the QT interval are based on a normal QRS complex and so are not valid in patients with preexcitation, ventricular pacing, or bundle branch block. An approach to measuring the QT interval has been described by Garson,2 which adjusts for marked phasic sinus arrhythmia. Two principles should be emphasized. First, machine measurements of the QT interval provided by modern electrocardiographs are not sufficiently reliable, and so all ECGs should still be remeasured manually. Second, the ECG reader should avoid commenting on the QT interval on faxed ECG copies, as the degradation in image quality will make the QT measurement unreliable.

In normal sinus rhythm, the right atrium is activated before the left atrium so that the first part of the P wave is right atrial, the last part is left atrial, but forces from both atria make up the middle of the P wave. With right atrial hypertrophy, the duration of the P wave is not typically lengthened, but the P waves become taller and peaked, especially in lead II, and exceed the normal upper limit of 0.25 mV (2.5 mm at full standardization). In lead V1, too, there may be a large biphasic P wave. With left atrial hypertrophy, the initial part of the P wave is unaltered, but the later part is larger and often lasts longer, so that the P waves are wider than normal and bifid, with a large second component.

The normal P frontal plane axis is about +30° to +60°; any marked change from this axis suggests an abnormal focus of atrial activation. Thus, in true mirror-image dextrocardia, the P axis is about +120°, giving a negative P wave in lead I, whereas retrograde activation of the atria from a junctional focus gives a P axis of about 30° to 60°. Consequently, P waves are negative in leads II and III. One subtle feature of sinus rhythm is the biphasic nature of the P wave in lead V1, which is upright in the early part of the P wave but inverted in the latter part. Some forms of atrial ectopic rhythm arise from sites close to the sinus node and give rise to a normal frontal plane axis, but they are clearly ectopic. One example is a focus from the anterior right atrium in which the frontal axis is normal, but the P wave is entirely inverted in V1, reflecting anterior to posterior activation. Another is a focus in the right upper pulmonary vein, in which the frontal place axis is also normal but in which the P wave in V1 is entirely upright.

The PR interval, from the beginning of the P wave to the onset of the QRS complex, is about 0.13 second in newborns; it increases to about 0.16 second at 16 years of age and can be up to 0.21 second in adults. The interval is slightly shorter during sinus tachycardia at any age, due to increased sympathetic tone.

Because ventricular activation begins in the septum and generates a septal vector that normally passes to the right, anteriorly, and superiorly, a Q wave is normally present in V6 and absent in the right chest leads. A Q wave in right but not in left chest leads may indicate septal activation from right to left because of left bundle branch block or ventricular inversion (L-transposition of the great arteries). It can occur also if the heart is rotated so that the septum lies in the sagittal plane; the septal vector may then be found by taking leads to the right and left of the routine leads. In some patients with very severe right ventricular hypertrophy, there may be a qR pattern in the right chest leads. Finally, a tiny initial r wave in right chest leads may be missed, especially if the electrocardiogram is recorded by an older instrument with limited ability to record high frequency content.

At birth, with the relatively thick right ventricle of the term infant, the mean QRS axis points anteriorly and to the right, giving right axis deviation (95% limits, 90–190°) and large R waves in right precordial leads. The QRS axis gradually shifts to the left in the frontal plane, and by about 3 months of age averages +65°, with a range in normal infants of 0° to +105°. It normally remains in the left lower quadrant throughout life. In the horizontal plane, with development the axis gradually rotates posteriorly until it is pointing leftward at 0° at about 3 months of age and posteriorly at about 45° from age 3 years onward.1 This explains why with age the R wave decreases in V1 and increases in V6, whereas V1 shows a large S wave after infancy. Although premature infants also show right ventricular dominance at birth, the QRS vector begins to swing posteriorly and to the left at about 1 month of age; this normal variant may cause overdiagnosis of left ventricular hypertrophy in preterm infants.

The mean T vector undergoes rapid and marked changes after birth. For the first 12 hours it points to the left and posteriorly and has a frontal plane axis of about +60°; this gives a negative T wave in V1. The T vector then rotates anteriorly and to the right, giving a positive T wave in V1 by 24 hours after birth. It remains anterior for 2 to 7 days and then moves posteriorly and to the left, and the T wave again inverts in V1. The reasons for these rapid changes are not clear. At about 12 years of age, the T wave may become upright once again in V1 in some children. T waves normally may be inverted in leads V5 and V6 during but not beyond the first day after birth; they may be inverted normally in V4 until 5 years of age and in V3 until 10 years of age. The frontal plane T axis stays at about +60° throughout childhood. Because the T and QRS vectors do not follow the same course in early childhood, the angle between them changes. At birth the frontal plane mean QRS-T angle is about +130°, and it slowly falls until it reaches 30° to 60° by about 3 years of age; thereafter, it remains under 60°, and any increase suggests a myocardial abnormality.

Ventricular hypertrophy is difficult to diagnose reliably by electrocardiography. Differences in heart position and chest shape affect how much of the cardiac potential is recorded from the body surface, and minor degrees of asynchrony of ventricular depolarization can alter the algebraic sum of the electrical forces at any moment. Because of the wide range of normality, it is impossible to diagnose slight hypertrophy, especially as the voltages before the onset of hypertrophy are seldom known. Furthermore, no single sign on its own can be considered reliable; in particular, neither an abnormal QRS axis nor increased voltage in a single lead should result in the reading of hypertrophy if there are no other supportive features.

Left Ventricular Hypertrophy

In infancy, with the increased mass of left ventricular muscle in left ventricular hypertrophy, the mean QRS axis moves to the left and posteriorly. Therefore, in the frontal plane the QRS axis moves to between 0° and –90°; an axis less than 30° is uncommon in infancy and suggests the possibility of left ventricular hypertrophy. This leftward shift of the QRS axis increases the R wave and decreases the S wave in V5 and V6, and the posterior shift of the QRS axis decreases R waves and increases S waves in V1. In older children and adults, the QRS axis is normally to the left, posterior, and inferior, so that with left ventricular hypertrophy there is no further axis shift but only an increase in voltage. A left superior frontal axis (0° to –90°) cannot be caused by increased inferiorly placed left ventricular muscle and is not the mark of left ventricular hypertrophy, but of a conduction defect.

In practice, the reading of left ventricular hypertrophy is based largely on voltage changes and T wave abnormalities. It should be noted here that the normal standards used in children derive from a study by Davignon et al in which the subjects were predominantly of European descent.3 There are clear data showing that in the African American population mean QRS voltages are higher without a higher incidence of actual left ventricular hypertrophy.4 Therefore, the diagnosis of ventricular hypertrophy should be made with caution when the only indicator is increased voltage.

Increased posterior forces are suggested by R waves below the fifth percentile or S waves above the 95th percentile in V3R and V1 (Fig. 493-5A).3 The increased left forces are suggested by R waves above the 95th percentile in V5 and V6 (Fig. 493-5B); smaller than normal S waves in V5 and V6 are not helpful because they can normally be very small. In addition, because the septum is usually involved in the hypertrophy, there is often an enlarged Q wave, which, when greater than 0.4 mV (4 mm at full standardization), suggests left ventricular hypertrophy. T-wave changes in the absence of voltage changes do not indicate left ventricular hypertrophy, but if the two changes are associated, they suggest either myocardial ischemia, as in severe aortic stenosis, or else what is termed left ventricular strain.

Right Ventricular Hypertrophy

At birth the term infant has a right ventricular wall as thick as, or slightly thicker than, that of the left ventricle. Thus, the newborn has a pattern of physiological right ventricular hypertrophy compared with the older child or adult. If there is pathological right ventricular hypertrophy, the mean QRS vector may move farther right and anteriorly, so that frontal plane QRS axes more than 190° under 1 week of age or 105° over 1 month of age suggest right ventricular hypertrophy, provided that there is no conduction defect. The increased anterior and rightward forces produce taller R waves and smaller S waves in right chest leads and smaller R waves and larger S waves in left chest leads.

In addition, a pure R wave or a qR pattern in the right chest leads strongly suggests pathologic right ventricular hypertrophy, as does an upright T wave in V4R and V1 between 7 days and about 8 years of age. Note that neither left nor right ventricular hypertrophy can be diagnosed confidently if there is an abnormally wide QRS complex, as occurs in bundle branch block or Wolff-Parkinson-White syndrome.

Right Bundle Branch Block

If the right bundle is interrupted, septal activation from left to right is unchanged, and then the left ventricle is depolarized to give a force to the left and posteriorly; this force is more marked than usual because the counteracting right ventricular forces have not yet begun. As left ventricular depolarization is ending, the impulse eventually penetrates the right ventricular muscle and spreads slowly through it to give late-onset slow right and anterior forces. Therefore, in right chest leads there is a normal initial R wave followed by a deep narrow S wave and then a tall and wide R' that brings the QRS duration to over 0.12 second in adults. (By convention, a deflection under 5 mm is written as a lowercase letter, for example, r, and a deflection over 5 mm is written as a capital letter, for example, R. Also, a second positive deflection is described as r' or R'.) In left chest leads the pattern is of a qRS type, with a deep and wide terminal S wave. The terminal right and anterior forces, being unopposed by left ventricular forces, are large and serve to produce a marked right-axis shift in the frontal plane.

Because in the newborn infant the normal QRS duration is about 0.05 to 0.06 second, it is possible for complete right bundle branch block to widen the QRS but still be under the limit of 0.12 second that pertains to adults. If the child has had a right ventriculotomy, there may be a typical right bundle branch block pattern as a result of the cutting of many terminal conduction fibers in the free wall and not from damage to the main right bundle. On the other hand, patients may develop a right bundle branch block due to surgical closure of a ventricular septal defect in which the proximal portion of the right bundle is interrupted at the margin of the defect. Finally, many normal children have an rSr pattern that results from a delayed but otherwise normal terminal deflection caused by late depolarization of the posterior part of the right ventricle under the AV groove. This pattern has been termed incomplete right bundle branch block, but in children it usually has nothing to do with conduction changes in the right bundle and its major branches. Therefore, it is better to refer to this as a minor right ventricular conduction delay, and this pattern does not prevent one from reading the ECG as normal. A similar but more marked pattern of delayed terminal rightward conduction may also be seen in children with atrial septal defects and other causes of right ventricular volume overload.

In some newborn infants, there may be a very tall and pure R wave in right chest leads that raises the suspicion of right ventricular hypertrophy. Close inspection shows a slur or notch on the upstroke of the R wave; with time the slur becomes more marked, a deep notch develops to produce an rR' pattern, and finally a typical rSr' pattern develops. Therefore, a conduction defect in babies can simulate right ventricular hypertrophy, but absence of other evidence of right ventricular pathologic changes and the evolution of the electrocardiogram lead to the correct diagnosis.

ST-segment and T-wave changes other than those related to maturation have similar causes in children and adults. Frequently, they are secondary to changes in the QRS complex, such as widening of the QRS complex in conduction abnormalities or an increased QRS amplitude in hypertrophy. These secondary changes of T waves are usually associated with a normal QRS-T angle in the frontal plane. Primary T-wave changes are those occurring in the absence of QRS changes and may be classified as changes related to electrolyte disturbances; physiologic changes (see below); or pathologic changes caused by drugs (especially digitalis glycosides), myocarditis or pericarditis, cardiomyopathy, degenerative neurologic diseases, and myocardial ischemia. The pathologic changes are discussed in their respective sections, but the other 2 groups are discussed briefly here.

Electrolyte Abnormalities

Hypocalcemia lengthens and hypercalcemia shortens the QT interval. Because the QT interval normally varies with heart rate, a rate-independent corrected QT interval (QTc) is calculated as QT/ÐR-R. The normal value is 0.40, with a range of 0.36 to 0.44. A low magnesium level may intensify the effects of low calcium; clinically, the long QT interval of hypocalcemia in infancy may remain after the calcium level has been restored to normal and improve only after magnesium is given. Other factors can also alter the QT interval: Digitalis glycosides and pericarditis may shorten it slightly, and myocarditis and certain genetic syndromes may lengthen it.

A high serum potassium level gives high, peaked T waves that are clearly abnormal at concentrations above 7 meq/L. Higher potassium concentrations not only raise T waves further, but reduce QRS amplitude, widen QRS duration, and lengthen PR interval. Above 9 meq/L there are usually atrial arrest and very wide QRS complexes that may lead to ventricular fibrillation. Preterm infants may be more resistant to these changes. Low serum potassium levels lower T waves, the effect being detectable below 3.5 meq/L. As potassium falls more, a U wave appears, and the ST segment becomes depressed.

Physiologic Changes

Physiologic changes are important because, if not appreciated, they may lead to the incorrect diagnosis of heart disease. Drinking iced liquids may cool the inferior wall of the left ventricle and cause transient deep T-wave inversion in the left chest leads. After heavy meals, there can be T-wave inversion in left chest leads that some clinicians associate with hyperglycemia. A history of recent ingestion of food or iced drinks will suggest these causes of T-wave changes, and a repeat ECG while fasting should then show normal T waves.

Anxiety and hyperventilation may also alter T waves. Therefore, one should not only try to assess the patient’s anxiety but also take an electrocardiogram with hyperventilation and then during exercise so that the effect of hyperventilation by itself can be assessed.

After paroxysmal tachycardia, the T waves may be abnormal for several hours or days, perhaps because of transient myocardial ischemia or potassium loss; the T waves usually return to normal, and there is generally no permanent injury to the myocardium.

Two other important normal variants should be considered. Some children and young adults have large upright T waves and elevated S-T segments in precordial or even limb leads, thus pericarditis might be suspected. This pattern is termed early repolarization, although it is not clear that it represents any abnormality of the order of ventricular repolarization, and it should not be considered abnormal. What distinguishes this variant from pericarditis is that here the T waves are very large and do not evolve as they do in pericarditis. The second variant is that of isolated inversion of the T waves in leads over the left ventricular apex, although the T waves are upright in earlier and later leads. This finding occurs most often in young men; it can vary from time to time and has no accepted explanation. Like other physiological T-wave changes, the T waves usually return to normal after oral potassium salts are ingested. Finally, athletes in peak training condition can have not only increased voltages diagnostic of left or right ventricular hypertrophy but also T-wave inversion in left ventricular leads. These can be normal findings in this setting, but hypertrophic cardiomyopathy must be ruled out, ideally by echocardiography.

Relatively recent advances in pediatric cardiovascular surgery, catheter-based interventional therapies, intensive care, and medical management have dramatically changed the landscape of the field of congenital heart disease (CHD). The complexity of the anatomy and physiology of patients surviving with CHD increases exponentially. The majority will survive to adulthood, and the need for reintervention is common. As such, the field is placing new demands on imaging to diagnose and plan medical management, as well as to identify need for and timing of reintervention. There are a number of imaging modalities available to the clinician and radiologist when it comes to these evaluations.

Echocardiography (ECG) has been and remains a mainstay of imaging in congenital heart disease. Despite its importance in rapid diagnosis and follow-up, it has limitations. The presence of postoperative scar, chest wall deformities, overlying lung tissue, and large body size as the patient ages often results in suboptimal transthoracic echocardiographic windows. Transesophageal echocardiography provides improved acoustic windows, but is limited by its small field of view and more invasive nature.

Cardiac catheterization, employing x-ray fluoroscopy and contrast angiography, has an expanding role in minimally invasive interventions, but its role as a diagnostic procedure is rapidly diminishing. This is in part due to its limitation as a 2-dimensional projection imaging technique with poor soft tissue contrast and the substantial ionizing radiation exposure involved and in part because both diagnostic and functional analyses are often better performed with noninvasive imaging techniques.

This chapter focuses on the evolving and expanding roles of other imaging modalities in diagnosing and monitoring patients with congenital heart disease, including cardiac magnetic resonance imaging (MRI), cardiac computed tomography (CT), and radionucleotide scintigraphy.

Cardiac magnetic resonance imaging (MRI) has emerged over the past few decades as an alternative, complementary, and frequently superior imaging modality for investigating the anatomy and function in the patient with congenital heart disease. It has many advantages over other imaging modalities. It does not require the use of iodinated contrast agents and does not involve exposure to ionizing radiation. This is particularly important in a population of patients who have been, and continue to be, exposed to large doses of contrast agent and radiation during hemodynamic and interventional catheterization. Additionally, many of these patients are children who are more susceptible to the adverse effects of radiation. Major advances in MRI hardware and software, including advanced coil design, faster gradients, new pulse sequences, and faster image reconstruction techniques, allow rapid, high-resolution imaging of complex anatomy and accurate, quantitative assessment of physiology and function.

Cardiac MRI Techniques

There are a number of MRI techniques useful in examining the anatomy and physiology of the congenital heart disease patient.

Cine MRI

ECG-gated gradient-echo sequences provide multiple images throughout the cardiac cycle in prescribed anatomic locations. Display of these images in a cine mode allows visualization of the dynamic motion of the heart and vessels.1-3 Cine MRI techniques, at a minimum, allow assessment of anatomy. More importantly, they allow qualitative and quantitative assessment of physiology and function. Specifically, cine MRI allows quantification of chamber volumes, myocardial mass, and ventricular function. furthermore, cine MRI allows qualitative assessment of focal and global wall motion abnormalities; qualitative and quantitative assessment of valve pathology, including the mechanism and severity of valve regurgitation and the location and severity of valve stenoses; identification and quantification of intra- and extracardiac shunts; and visualization of other areas of flow turbulence.

The use of such cine MRI techniques for the quantification of chamber volumes, myocardial mass, and ventricular function, both fast gradient-echo4-7 and balanced steady-state precession (SSTP),1,2 have been extensively evaluated and validated.8-10 Briefly, evaluation of function begins with obtaining a series of contiguous slices (cines) along the short axis of the ventricles, extending from base to apex. These images are played back in a cine loop, and the end-systolic and end-diastolic phases are chosen. The endocardial borders are traced at both time points, and the epicardial borders are traced at one of the 2 time points (Fig. 494-1). Ventricular volumes are then calculated as the sum of the traced volumes (area times slice thickness). Myocardial mass is calculated as the myocardial muscle volume times 1.05 g/mm3 (density of myocardium). From these data, ventricular end-diastolic volume, end-systolic volume, stroke volume, ejection fraction, myocardial mass, and mass/volume ratio can be calculated for both the right and left ventricles.

Spin-Echo (Black Blood) Imaging

ECG-gated spin-echo sequences, or black blood imaging represent another important tool for imaging in the congenital heart disease patient. Despite providing only static information, black blood imaging has many benefits. allows assessment of anatomy with thin slices, high spatial resolution, and excellent blood-myocardium and blood-vessel wall contrast (Fig. 494-2). Black blood techniques are superb for evaluating the spatial relationship between cardiovascular and other intrathoracic structures such as the chest wall and the tracheo-bronchial tree. These features hold particular relevance when delineating complicated native and postsurgical cardiac anatomy. Such techniques are also less susceptible to artifact from metallic implanted devices such as stents, coils, occluder devices, clips, and sternal wires.

Flow Quantification

Cardiac-gated gradient-echo sequences with flow-encoding gradients are used to quantify the velocity and flow of blood (Fig. 494-3).11 These sequences are referred to as velocity-encoded cine MRI or phase-contrast MRI. Two-dimensional velocity-encoded cine MRI sequences are the most commonly used in clinical practice. They can be used to quantify cardiac output, pulmonary to systemic flow ratio (shunt), valvular regurgitation, differential lung perfusion, coronary flow reserve, and to observe the location and severity of flow obstruction.

Gadolinium-Enhanced 3-Dimensional Angiography

Three-dimensional angiography (3D-MRA) sequences are typically not cardiac gated and thus do not allow assessment of intracardiac structures. Regardless, they provide excellent depiction of arterial and venous vascular structures (Fig. 494-4). In the congenital heart disease patient, 3D-MRA fills a significant diagnostic role. It can be used to diagnose systemic arterial anomalies such as aortopulmonary collaterals, shunts, vascular rings, and coarctation. It is useful in diagnosing pulmonary arterial abnormalities such as focal and diffuse stenoses and abnormal distal arborization patterns. Using 3D-MRA methods is also useful for investigating systemic and pulmonary venous abnormalities, both congenital and postoperative. Also, 3D-MRA is useful for evaluating the relation between vascular and other thoracic structures. With faster techniques and navigator pulses, time-resolved 3D-MRA as well as noncontrast 3D-MRA are becoming alternatives.12

Coronary Artery Imaging, Perfusion Imaging, and Myocardial Viability

Coronary artery abnormalities and ischemia are important issues to be investigated in postoperative congenital heart disease patients. Not only is this patient population aging sufficiently to develop atherosclerotic coronary artery disease, they also commonly have congenitally abnormal or postoperatively acquired coronary artery lesions. It is not uncommon to find an anomalous origin or course of the left or right coronary artery, postsurgical coronary obstruction (ie, after arterial switch for transposition of the great arteries), coronary artery thrombus, and abnormal fistulous connections (ie, pulmonary atresia with intact ventricular septum and right ventricular-dependent coronary circulation). Identifying such abnormalities is often critical to planning reintervention and medical management. There is growing evidence to support the concept that myocardial delayed enhancement in a number of subsets of postoperative congenital heart disease patients predicts poor outcome.13-16 In summary, although still not as robust as routine coronary artery angiography with x-ray fluoroscopy or cardiac-gated CT angiography at investigating distal coronary artery lesions, MRI can image proximal coronary arteries well,17-20 evaluate myocardial perfusion and viability,21-24 and allow stress testing,25,26 all noninvasively without exposure to contrast agents and ionizing radiation.

General Clinical Applications of Cardiac MRI

Indications

The indications for magnetic resonance imaging (MRI) in the congenital heart disease (CHD) patient are evolving and expanding as MRI technology advances and the medical issues of this population become more complex. In general, MRI is indicated when transthoracic echocardiography does not provide adequate diagnostic information (eg, postoperative patients; adult with congenital heart disease), as an alternative to invasive and costly diagnostic catheterization, and when the unique capabilities of MRI can be exploited. The most common indications for MRI in the CHD patient are detailed. Cardiac MRI is quite effective in the segmental description of complex anatomy and it is unrivaled in the quantitative evaluation biventricular function, particularly the right ventricle, which is so often at risk in patients with CHD. It is useful in evaluating arterial and venous anomalies and detecting and quantifying shunts, valvular and vascular stenoses, and regurgitant lesions. Cardiac MRI is also excellent for characterizing coronary arterial anatomy and myocardial perfusion and viability.

Computed tomography (CT) has historically been quite useful in evaluating vascular anatomy. The advent of high resolution CT and cardiac gating has emerged as a useful tool for assessing intracardiac anatomy, coronary anatomy, and myocardial function. Multi-slice CT has advanced significantly over the past decade, particularly with the emergence of 64-slice multidetector scanners in 2004. This newest technology allows faster acquisition times, improved spatial resolution, and thinner slices. Improved gating has decreased motion artifacts. Although such technology is proving invaluable in cardiovascular imaging in adults, its use in the pediatric population in the assessment of congenital heart disease has just begun to be explored.27-29

The strengths of CT imaging include excellent spatial resolution (submillimeter isotropic resolution) and rapid acquisition times, often precluding the need for anesthesia. It is unparalleled in its ability to provide spatial relationships of intrathoracic structures, including the vasculature and the airways. Some argue that CT provides equal or even better image quality of the epicardial coronary vessels than does coronary angiography. It can also be used in congenital heart disease patients who have pacemakers or defibrillators.

Cardiac CT does have its weaknesses. There is less temporal resolution than cardiac MRI or echocardiography and thus is limited in its ability to characterize ventricular function. There are no CT techniques to quantify flow. CT contrast, albeit improved and less toxic, still has well-documented renal toxicity. Most importantly, there is significant radiation exposure associated with CT. It is critical for the clinician to consider this when ordering a CT, particularly in the young patient and the patient who has been, and continues to be, exposed to large doses of contrast agent and radiation during hemodynamic and interventional catheterization. It must also be recognized that gated studies and higher-resolution studies come at a cost of higher radiation doses. (See the as low as reasonably achievable [ALARA] guidelines.30)

General Clinical Applications of Cardiac CT

Indications

The clinical indications for cardiac CT in congenital heart disease patients are evolving and growing as technology advances. CT is particularly indicated in patients with pacemakers and defibrillators in whom cardiac MRI is precluded or in patients with metallic implants that create unmanageable artifacts on cardiac MRI (ie, steel coils). It also can take the place of cardiac MRI if the question being studied is strictly vascular anatomy and not intracardiac anatomy or cardiac function. CT can answer these questions with a rapid study, often without sedation. CT is quite useful in evaluating the coronary arteries, particularly if there is concern about distal coronary disease. Cardiac CT has the added benefit of accurately delineating the relationship of vascular structures and the airways.

Radionuclide imaging is used primarily for assessing cardiac physiology. Although rarely used today for the assessment of right and left ventricular function in pediatric patients, radionuclide studies continue to provide valuable information for the evaluation of intra- and extracardiac shunts and assessment of myocardial perfusion.

Assessment of Left‐to-Right Shunts

Echocardiography with Doppler is the most widely used method for determining the presence and magnitude of left-to-right shunts in congenital heart disease, although its accuracy in quantification of multiple level shunts or shunts in the presence of valve insufficiency or multiple shunts is not optimal. The radionuclide technique for left-to-right shunt quantification was established and validated more than 30 years ago and is not affected by such considerations.31

The radionuclide method is most often used to assess the size of left-to-right shunts in 4 major congenital lesions: atrial septal defect, ventricular septal defect, patent ductus arteriosis, and partial anomalous pulmonary venous return. In uncomplicated atrial septal defect, the shunt is principally from the left atrium to the right atrium, leading to an increased volume load in the right ventricle, which becomes dilated (eFig. 494.1). The left ventricle does not participate in the shunt. Anomalous connections of pulmonary veins, generally to the right atrium, also result in left-to-right shunts where the right ventricle handles the increased volume. In the typical perimembranous ventricular septal defect, the shunt is principally from the left ventricle, across the ventricular septum, into the right ventricular outflow tract, and out the pulmonary trunk (eFig. 494.2). The left ventricle handles the increased volume load and dilates. Large left to right shunts associated with ventricular septal defects can lead to pulmonary hypertension and subsequent right ventricular hypertrophy. In patent ductus arteriosus, the shunt occurs between the high-pressure aortic trunk to the lower-pressure pulmonary trunk (eFig. 494.3). The left ventricle handles the increased volume and dilates. As with large ventricular septal defects, large patent ductus arteriosus can lead to pulmonary hypertension and right ventricular hypertrophy. In each lesion, knowledge of the size of the shunt, expressed as pulmonary-to-systemic flow ratio (Qp/Qs), is essential for decisions regarding corrective surgery.

The scintigraphic technique involves the rapid injection of a bolus of radionuclide (usually technetium-99m–labeled diethylenetriamine pentaacetate, or 99mTc-DTPA) into the circulation while monitoring the transit through the heart and lungs with the gamma camera. For small infants (ie, premature newborn infants), a butterfly needle can be used in a temporal scalp vein to deliver a compact bolus of activity to the central circulation. In older children and adults, either a butterfly needle or a small cannula can be inserted, preferably into an external jugular vein, although an antecubital vein may also be used. The delivery of a compact, nonfragmented bolus of activity is essential to allow accurate determination of the size of the shunt. With good technique, the success rate should be greater than 90%. It may be necessary to sedate infants and some children, because crying simulates a Valsalva maneuver that can impede bolus entry into the thorax and lead to fragmentation of the bolus. As mentioned, 99mTc-DTPA is most commonly used for shunt studies. Doses are 200 microcuries (µCi) per kilogram of body weight, with a minimum dose of 2 mCi. The advantage of 99mTc-DTPA over other technetium based agents is the fairly rapid renal excretion and thus prompt clearance of background activity. This becomes important if it is necessary to perform a second injection to improve the quality of the bolus. Generally, no more than 2 sequential injections are performed due to dosimetry limitations.

The study is done in the anterior projection using a converging collimator (which provides magnification) in infants and ideally a high-sensitivity parallel collimator in older children and adults. A dynamic acquisition with a sampling rate of 2 to 4 frames per second is adequate for evaluation of shunts. If ejection fraction measurements are to be made by the first pass method, a rate of at least 25 frames per second should be used. The sequential flow study is reviewed in order to provide useful information regarding chamber orientation and vascular connections. In the presence of normal anatomic relationships, right heart structures will appear, followed by the main pulmonary artery, lungs, and subsequently, the left ventricle (levophase), and descending aorta. Persistent pulmonary activity, resulting in the absence of a distinct levophase is consistent with a moderate to large left-to-right shunt (eFig. 494.4). This appearance results from recirculation of activity from heart back to lungs and vice versa, across the shunt. This appearance has been called the “smudge sign,” and generally indicates a pulmonary to systemic flow of at least 1.6/1.

For quantification, time-versus-radioactivity curves are generated from regions of interest over the superior vena cava to assess the quality of the bolus and the periphery of the right lung, for shunt detection and magnitude (Fig. 494-5). A separate curve may be generated from a region over the left lung if differential shunting is expected (as may occur with a patent ductus arteriosus). The normal pulmonary arterial curve has an ascending limb, reflecting the arrival of tracer in the pulmonary circulation, and a symmetric descending limb, reflecting the tracer exiting the lungs and entering the left side of the heart. A late peak wsill appear, reflecting systemic recirculation. In a left-to-right shunt, a shoulder will be present on the downslope, indicating recirculation of activity back to the lungs across the shunt. For shunt quantification, the shape of the pulmonary portion of the curve is approximated by an algebraic expression called a gamma variate function (Fig. 494-6).31 In practice, the computer is given the coordinates of the upslope and initial downslope of the pulmonary curve, and a curve is generated that approximates the shape of the curve. The area under this pulmonary curve is proportional to pulmonary flow, Qp. This fitted curve is then subtracted from the initial time-versus-radioactivity curve, and another gamma variate fit is done on the remaining curve. The area under this second fitted curve is proportional to the shunt flow, Qsh. The difference between the two fitted curves is a measure of systemic flow, Qs. The resultant calculation of pulmonary to systemic flow, Qp/Qs, is performed as:

Qp/Qs = Qp/(Qp – Qsh)

Ratios less than 1.2:1 are consistent with the absence of left-to-right shunts. The Qp/Qs calculation, by the gamma variate method, has shown excellent correlation with shunt size determined at cardiac catheterization over a clinically significant range of 1.2:1 to 3.0:1.31 This relationship remains valid even in the presence of pulmonary hypertension, tricuspid regurgitation, and heart failure.31,32 In these conditions, extensive dilution and slow flow lead to a slow downslope to the pulmonary curve. However, the upslope should be proportionately slowed, and the curve fit method should generally apply, although caution should be exercised. Since the method depends on the full passage of the administered radionuclide through the lungs, left-to-right shunts will be overestimated in the presence of right-to-left shunts. Shunts greater than 3.0:1 are difficult to fit by the gamma variate method due to distortions in curve shape as a result of the large and torrential shunt flow. This is not a practical limitation, however, as any shunt greater than 3.0:1 is very large. In general, a shunt of 2.0 or greater is sufficient to warrant surgical correction.

With the anatomic detail provided by echocardiography, the hemodynamic correlates from Doppler examination, and the precise quantitation available from a radionuclide shunt study, it is sometimes possible to proceed directly to surgery without preoperative cardiac catheterization. This is particularly true with uncomplicated patent ductus and secundum atrial septal defect. In anomalous pulmonary venous return, the radionuclide determination of shunt size may be more accurate than that determined at catheterization by oximetric methods due to the inability to obtain a good mixed venous blood sample at catheterization.33 The radionuclide method has also been used to measure changes in shunt magnitude in response to oxygen therapy, and to assess the reactivity of the pulmonary vascular bed in patients with pulmonary arterial hypertension and large shunts.34 This is a very important consideration in determining operability in patients with moderate to large ventricular septal defects.

One of the leading indications for radionuclide shunt studies is the postoperative assessment of residual shunt size in patients with murmurs and echo Doppler evidence of persistent shunting after surgical correction of septal defects. Doppler quantification of shunt size is often not very reliable after patch closure of defects due to the turbulence generated in the vicinity of the patch. In this situation, the radionuclide technique has been helpful for assessing the need for repeat catheterization and possibly reoperation.

It is possible to calculate the extent of left-to-right shunts using the equilibrium blood pool method. Stroke volume or amplitude images can be used to measure the difference in stroke volume between the ventricles, as is commonly performed for the evaluation of regurgitant lesions (eFig. 494.5).35 With a ventricular septal defect or a patent ductus arteriosus, the left ventricle handles the excess volume of the shunt flow. The left ventricular stroke volume is proportional to the pulmonary blood flow, and the right ventricular stroke volume is proportional to the systemic blood flow. The pulmonary-to-systemic flow ration can be calculated as:

Qp/Qs = LV stroke volume/RV stroke volume

where, LV stroke volume equals LV end-diastolic volume minus end-systolic volume, and RV stroke volume equals RV end diastolic volume minus end-systolic volume.

For an atrial septal defect or anomalous pulmonary venous return, the right ventricle carries the excess shunt flow. The Qp/Qs can be calculated as:

Qp/Qs = RV stroke volume/LV stroke volume

A good correlation (r = 0.79) has been noted between the shunt Qp/Qs ratio calculated from stroke volume ratios and oximetry.36 This approach may be particularly useful in situations where attempts at a good bolus injection were unsuccessful.

Assessment of Right‐to-Left Shunts

Right-to-left shunts can be detected by inspection of the first-pass radionuclide angiogram, which reveals a premature appearance of radioactivity in the left-sided chambers or aorta. Time-versus-radioactivity curves generated from regions of interest over the carotid artery can be analyzed by curve-fitting methods to quantify shunt size.37 Intravenous injections of an inert radioactive gas, such as 133Xe or krypton-81m, can also be used for detecting right-to-left shunts.38 Significant systemic activity of these agents, which should be totally extracted by the lungs and exhaled in the alveolar gas, indicates shunting.

The easiest and most commonly used method to assess right-to-left shunts is the intravenous injection of 99mTc-labeled macroaggregated albumin particles, similar to those used for the assessment of pulmonary perfusion.39 In the absence of right-to-left shunting, all of the particles are trapped in the lungs. When right-to-left shunting occurs at any level, particles will enter the systemic circulation in proportion to the shunt flow, lodging in the capillary and precapillary beds of systemic organs. A series of whole body images are taken to determine the percentage of right-to-left shunt as:

Pulmonary-to-systemic flow ratio can be calculated as:

Qp/Qs = lung counts/whole body counts

Radiolabeled macroaggregrated albumin particles can be used to assess Qp/Qs in admixture lesions, where complete mixing of pulmonary and systemic blood occurs in 1 cardiac chamber. Images in eFigure 494.6 were obtained from a 16-year-old cyanotic patient who presented with a history of pulmonic atresia with intact ventricular septum and hypoplastic tricuspid valve. He received a Waterston aortic-to-right pulmonary artery shunt as a child. In addition, a left subclavian-to-central pulmonary shunt (Blalock-Taussig shunt) was done, with flow primarily to the left lung. The procedure was done to assess the patency of the shunts.

Although technically there is a left-to-right shunt of blood from the aorta to pulmonary artery, the major hemodynamic abnormality in this cyanotic lesion is complete admixture of systemic venous and pulmonary venous blood at the level of the left atrium. Both pulmonary and systemic blood arise from the aorta. Note the presence of brain and kidney uptake, consistent with right-to-left shunt. In this example, differential pulmonary perfusion was measured by determining relative counts in both lungs. Left lung activity represented 45% of total lung activity, whereas right lung activity was 55% of total. These findings suggested that both the Waterston and Blalock-Taussig shunts were functioning. Additional important data were obtainable from this macroaggregated albumin study. The pulmonary-to-systemic flow ratio, Qp/Qs, was calculated as Qp = lung counts, Qs = whole body counts – lung counts. This formula is applicable since the patient has an admixture lesion. There is complete mixing of the particles by the time the injected dose reaches the aorta. The particles are subsequently distributed in proportion to regional blood flow to the lungs and the body. The pulmonary/systemic flow ratio, Qp/Qs, is a very important determinant of systemic arterial oxygen saturation in patients with admixture lesions.40 In the example shown above, the Qp/Qs was 1.6. This modest ratio suggested that pulmonary flow was not excessive, and increased systemic saturation would likely result with a slightly larger degree of pulmonary blood flow, that is, a larger Qp/Qs.

In spite of the general reluctance to administer particles to patients with known right-to-left shunts, the method has proven to be safe, accurate, and very easy to perform.39 The particle number should be kept below 50,000 in pediatric patients.

The microparticle study is very useful for visualizing maldistribution of pulmonary blood flow after surgery for congenital heart disease. Pruckmayer et al41 studied 46 patients (mean age 8.2 years) with complex cardiac anomalies who had undergone either a Glenn shunt or Fontan procedure. Glenn shunt and Fontan procedures may be associated with abnormal pulmonary flow patterns and the development of pulmonary atrioventricular fistula.41 Imaging was done following sequential injections of ^ch494ref99mTc-microspheres into upper and lower limb veins. In 31 of 46 patients, blood flow from the superior vena cava (arm injection) drained preferentially to the right lung, whereas blood flow from the inferior vena cava (foot injection) drained equally to both lungs (eFig. 494.7). Lung perfusion scintigraphy after upper and lower extremity injections detected more abnormal pulmonary flow patterns than did contrast echo. In addition, the technique was able to quantify R-L shunt volumes individually from the superior vena cava and inferior vena cava, a unique attribute compared to any other imaging method. Quantitative assessment of relative pulmonary perfusion before and after percutaneous intervention for peripheral pulmonary artery stenosis42 has become one of the most commonly ordered scintigraphic procedures for cardiovascular assessment in pediatric patients in our practice (eFig. 494.8).

Assessment of Ventricular Function

Although radionuclide methods are well suited for the assessment of ventricular size and function in congenital heart lesions, these methods have been largely replaced by echocardiography in current clinical practice. Both first-pass and gated equilibrium methods for the determination of ejection fraction have been validated in the pediatric age group.43,44 Quantitative assessment of absolute ventricular volumes45 and determination of regurgitant fraction have been reported in children as well.46,47 For infants, the imaging is optimized with the use of a converging collimator to improve spatial resolution and increase the sensitivity. It is feasible to measure ejection fraction even in tiny premature infants with the pinhole collimator.48 Ventricular size and function evaluation are useful at rest and with dynamic stress in a variety of congenital lesions, both before and after surgical correction.49,50 Residual structural and functional abnormalities are very common, and careful, long-term follow-up is important.

Assessment of Myocardial Perfusion

Myocardial perfusion scintigraphy has been used for clinical assessment in children for a number of years.51,52 In the pediatric patient, perfusion imaging has been most widely used for the noninvasive identification of anomalous left coronary artery.53-56To evaluate patients for possible anomalous left coronary artery, a perfusion tracer is injected intravenously, at rest, and images are acquired in multiple planar projections, or single photon emission computed tomography (SPECT) imaging is done. The usual anatomy in this rather rare disease is for the left main coronary artery to arise from the main pulmonary artery. This situation can lead to regional ischemia and infarction of the left ventricle due to low perfusion pressure from the pulmonary artery, which can create a coronary steal. Perfusion scintigraphy typically reveals a segmental perfusion abnormality at rest (eFig. 494.9). This pattern is useful for identifying anomalous left coronary, as opposed to myocarditis or cardiomyopathy, as the etiology for poor ventricular function in infants. The condition is often associated with Q-waves on the electrocardiogram. Echocardiography is sometimes able to identify the aberrant origin of the left coronary, but catheterization is required for confirmation. Multidetector CT has shown promise for the noninvasive detection of anomalous origins of coronary arteries.57,58 Direct aortic implantation of the anomalous coronary artery is the preferred approach to surgical correction.59,60

Another clinical condition for which perfusion scintigraphy may be useful is Kawasaki disease.61 This syndrome is associated initially with persistent fevers, rash, adenopathy, and mucous membrane abnormalities. Before the introduction of intravenous gamma globulin therapy, about 20% to 25% of these patients developed aneurysms of the coronary arteries. Treatment with gamma globulin within 10 days of the onset of the illness reduces the frequency of coronary aneurysms to about 4%. About 30% to 50% of such aneurysms will spontaneously regress within the first 2 years of illness. The remaining aneurysms may later thrombose and cause myocardial ischemia and infarction.62,63 Bypass surgery has been advocated for some patients with objective evidence of ischemia.64

Assessment of myocardial perfusion is important in the evaluation and follow-up of patients with transposition of the great vessels who have undergone the arterial switch procedure.65,66 D-transposition of the great arteries is a lethal congenital malformation and represents the most common cardiac cause of cyanosis in the neonate. Historically, left untreated up to 90% of infants with the condition died within the first year of life.67 The prognosis for these infants, however, has been remarkably improved with the introduction of palliative and corrective surgical techniques. Among the earliest and most successful surgical procedures is a physiologic correction of the circulation by redirecting the pulmonary and systemic venous returns to the appropriate ventricles using intra-atrial baffles (Mustard or Senning procedure).67 This atrial switch operation has met with great success in the early operative period, and survival rates at 10 and 20 years postoperatively of 90% and 80%, respectively.67 Enthusiasm for this approach has been dampened somewhat due to concerns that the right ventricle may be unable to function successfully as the systemic ventricle for long periods of time.

Anatomic correction of transposition is now commonly performed.68 The procedure consists of switching the great vessels to the proper ventricles and reimplantation of the coronary arteries to the newly formed aorta. The morphologic left ventricle then becomes the systemic pumping chamber for which it is better suited. Although technically demanding, the procedure is best performed during the neonatal period, prior to the regression of left ventricular mass as a result of the progressive decrease in pulmonary vascular resistance that occurs after birth. Alternatively, the left ventricle can be conditioned to accept the high systemic vascular resistance by performing pulmonary banding as an initial procedure. Concerns have been raised since the inception of the arterial switch procedure about the possibility that distortion or growth failure of the newly implanted coronary arteries may result in myocardial ischemia and possibly infarction.

Some studies suggest that coronary artery manipulation and reimplantation do not result in late myocardial perfusion abnormalities, at least in those infants with successful initial results.68 However, large perfusion abnormalities have been seen in the territories of stenosed coronary arteries following the switch procedure (eFig. 494.10).

Given the remarkable advances over the past decade and the expected advances over the next decade, it is an exciting time in cardiovascular imaging. The future of magnetic resonance imaging (MRI) holds parallel processing and navigator pulses, which will decrease scan times and decrease need for sedation or anesthesia. Real-time imaging, MRI-guided interventions, myocardial-tagging assessment of wall strain, and 7-dimension flow techniques will reach the clinical armamentarium in the near future, making MRI an even more useful tool in evaluating and managing this patient population. The evolution of higher-field strength scanners (3 tesla) may augment some MRI tools. The future of CT includes dual-source scanners, which will decrease scan times and radiation dose, further improve spatial resolution, and improve temporal resolution (necessary for functional assessment). Radionuclide methods often lack sufficient resolution to precisely characterize complex morphology in congenital heart lesions. However, these methods provide accurate and reproducible quantitative assessment of the physiological consequences of structural heart disease.

The field of congenital heart disease is advancing rapidly and, as a result, patients with complex disease are surviving and patients in general are living longer. Management of their complex residual cardiovascular disease requires close monitoring. The growth of the field has challenged and will continue to challenge the noninvasive imaging arena to advance.

Ultrasonography is a valuable and widely available technique for evaluating the patient with suspected heart disease. Standard 2-dimensional and Doppler echocardiography play a major role in defining cardiac anatomy, assessing ventricular function, and detecting abnormal flow patterns associated with cardiac disease. In many instances, the diagnostic accuracy of echocardiography is equivalent to that of cardiac catheterization,1 and has dramatically decreased the need for diagnostic catheterization preoperatively. In addition to transthoracic echocardiography, other modalities such as fetal, transesophageal, and stress echocardiography are widely used. It is the goal of this chapter to acquaint the reader with the basic concepts, as well as the indications for each type of echocardiographic examination.

Ultrasound was first used during World War II, when the technique of sending sound waves through water and observing the returning echoes to identify submarines was widely used. After the war, medical investigators transferred this technology for use in medical diagnosis. The major application of diagnostic ultrasound has been imaging of tissue structures. Imaging modalities of ultrasound include M-mode echocardiography and 2-dimensional echocardiography.

M-mode echocardiography provides an “ice-pick” view of the heart by emitting a narrow ultrasound beam. Structures encountered by the beam are reflected back and displayed as a dot, and as the image scrolls through to display time, the motion of this site over the cardiac cycle is displayed (Fig. 495-1). The frame rate in M-mode echocardiography is 1000 to 4000 frames per second, yielding excellent time resolution. Thus, M-mode provides the most precise display of events that occur rapidly within the cardiac cycle, such as the opening and closing of valves, the motion of the ventricular and atrial walls, and the changes in size of the cavities during contraction and relaxation. However, owing to the narrow area of interrogation used in M-mode echocardiography, anatomic relationships are best left to other modalities.

If the dots are scrolled over time and multiple parallel beams are emitted, the beginnings of a 2-dimensional echocardiogram are noted. In today’s echocardiogram, emitted beams of sound are steered, usually over 30 times a second, over an arc of approximately 90, producing a fan-shaped or sector image. As the sector is made up of a series of lines of information, the ultrasound information represented on each one of the lines within this sector fan produces an image. The replay of this ultrasound image from the sector fan in the time that is almost instantaneous with its generation has led to the technique being called real-time imaging.

Two-dimensional echocardiography shows a tomographic slice of the heart. These slices allow the reviewer to assess the spatial and anatomic relationships in the heart. Because each view is limited to a slice through the heart, multiple planes of the heart are investigated during a complete study. On a transthoracic study, these views are obtained from multiple sites on the chest, including parasternal position, cardiac apex, subcostal position in the midline below the subxiphoid process, and suprasternal position from the suprasternal notch. Only with complete display of the cardiac images from all of these transducer positions can a reliable 3-dimensional concept of cardiac anatomy be achieved (Fig. 495-2).

Two-dimensional imaging is useful for anatomic information, but does not assess flow patterns or hemodynamics. To do so, the Doppler principle is applied to ultrasound technology. Johann Christian Doppler (1803–1853), an Austrian physicist, first demonstrated that the relative motion between a sound source and an observer determines the observed frequency of the sound wave. If the observer is moving relative to the sound source, the detected frequency will be different from the transmitted frequency. If the observer is moving toward the sound source, a higher frequency tone is heard. This phenomenon, induced by the relative motion of the sound source and the observer, is referred to as the Doppler effect, and the resulting change in frequency of sound is called the Doppler shift.

Blood flow patterns within the heart and great vessels are evaluated by measuring blood velocity and displaying this information in various formats. Ultrasonic Doppler blood velocity measurement is performed by both spectral and color flow Doppler. Spectral Doppler displays blood velocity vertically on a scale as a function of time (horizontal axis) (Fig. 495-3). If the direction of flow is toward the transducer, the flow velocity information is displayed above the baseline (as positive velocity values), and if moving away from the transducer, is displayed below the baseline. Spectral Doppler assessments of flow can be performed using 1 of 2 modes of ultrasound, either pulsed wave (PW) or continuous wave (CW) mode. Each of these has specific benefits and limitations that are complementary in application. In the PW mode, measurements can be performed within a small range and at a precise site determined by the operator. However, in the PW mode the maximum velocity that can be measured is limited. In pediatric cardiology, PW Doppler velocimetry is used to define the origin of flow disturbances within the cardiovascular system, such as flow events through valve orifices and the direction of blood flow. The CW mode has no limit to the maximum measurable velocity; however, the site of velocity measurement cannot be controlled, and blood flow at all depths along the axis of the Doppler beam is measured and displayed. Thus in practice, the operator usually uses PW Doppler for precise determination of the site, direction, and pattern of flow, and CW to measure higher velocities accurately.

Color Doppler flow-mapping techniques have become an important aspect of the echocardiographic examination as its use improves the diagnostic accuracy of the study. In color Doppler echocardiography, information on blood flow velocity is computed, color encoded, and simultaneously superimposed on the 2-dimensional echocardiographic image. Flow traveling toward the transducer has red hues, ranging from a deep red at low velocities to a yellow hue at higher velocities, whereas blood traveling away from the transducer is assigned a deep blue hue at lower velocities and a light blue at higher velocities. Disturbed flow is displayed as a mosaic color that includes green. The information is updated several times a second, producing a real-time map of flow during the cardiac cycle. In this manner, color Doppler provides information regarding direction, velocity, and flow disturbances so that normal and abnormal flow patterns can be easily identified (Fig. 495-4).2 Color flow mapping techniques are qualitative at present and are not used for precise velocity measurements.3 Their main utility is in recognizing abnormal flow patterns and as an aid in aligning spectral Doppler beam for accurate flow-velocity measurement. Since M-mode echocardiography, 2-dimensional echocardiography, and spectral and color Doppler velocity measurements each assess a different aspect of heart anatomy, function, or flow information, the complete echocardiogram comprises all of them.

Echocardiography not only describes anatomy and flow aberrations but also is used to measure cardiac dimensions and function and to estimate pressure drop and shunt magnitude. Chamber and vessel dimensions as measured by either M-mode or 2-dimensional images are compared with normal data. This information aids in the assessment of the effects of a shunt lesion, such as assessing left ventricular dilation in a patient with aortic insufficiency. The dimension in question is related to normal data, often by calculating a z score. A z score is the number of standard deviations above or below the average indexed size of the structure in question on the basis of normative data.4 If z score data are not readily available, normal plots of ventricular dimensions; valve sizes; or great vessels size related to patient body surface area, patient weight, or age and height exist for the pediatric population and are useful for comparison.5-8

Left ventricular function is commonly assessed during a routine examination by M-mode measurement of the left ventricular shortening fraction, which is the percentage of change of the left ventricular chamber diameter during a cardiac cycle. Normal values for left ventricular shortening fraction remain unchanged throughout childhood, and range from 28% to 44%.9-11 Left ventricular shortening fraction is easily measured, but it does not take into account the loading conditions of the left ventricle. Thus, it estimates global left ventricular function but not its contractility. For example, a patient with severe systemic hypertension may have a depressed left ventricular shortening fraction because of the increased afterload to which it is exposed, but the contractility of the ventricular muscle is normal. Shortening fraction is limited in its ability to assess global ventricular function as well. It assumes that left ventricular geometry is normal. If this is not true, as occurs when septal wall motion is flattened or paradoxical, shortening fraction will not reliably reflect left ventricular systolic function. In such instances, a measure of change in volume rather than change in dimension should be used. Ejection fraction is the most common measure used and is usually determined using Simpson’s rule. This method is based on the principle that the volume of any object, regardless of its shape, equals the sum of the volumes of multiple individual slices of known thickness that compose the object.12,13 Using the 2-dimensional image, the cavity of the ventricle is traced in systole and diastole, and the ultrasound machine assigns a series of discs of a known thickness and a diameter determined by the left ventricular dimensions. The volumes of these discs are then summed, and the change in volume between systole and diastole is computed and reported as an ejection fraction. Normal values for ejection fraction range from 56% to 78%.14Although the Simpson’s rule method transcends most geometrical assumptions, it too is load dependent and thus provides a useful index of ventricular function under a wide variety of circumstances, but not of contractility.

The quantitative assessment of flow events in the heart and great vessels is performed using spectral Doppler velocity measurement of flow velocities. In many situations the blood velocity can be used to predict a pressure drop across the area of flow. Using the modified Bernoulli equation, the pressure drop is 4v2, where v is velocity in meters per second. Thus, a peak jet velocity of 3 m/s predicts a peak pressure drop of 4 × 32, or 36 mm Hg. High-velocity jets can be measured arising from a ventricular septal defect, patent ductus arteriosus, valvar stenosis, valve regurgitation, or from a pulmonary artery band or aortic coarctation (Fig. 495-3). This gradient can be used to predict a chamber or vessel pressure or may be used to assess stenosis severity.15-18 The flow volume across a region can also be estimated, such as the cardiac output. Volume per beat can be calculated by measuring the area under the curve of the velocity signal and the cross-sectional area of the valve estimated using 2-dimensional imaging. The volume of flow is determined as the product of flow cross-sectional area and the velocity-time integral of the Doppler signal. This product is the stroke volume across the area of interest, or volume per beat. The stroke volume across the aortic valve is converted to a cardiac output by multiplying the stroke volume by the heart rate.

Transesophageal echocardiography (TEE) has been used in children as small as 2.3 kg by inserting a phased-array ultrasound transducer mounted on the tip of a fiber-optic gastroscope as small as 6 mm in diameter. This device allows imaging of the heart from the esophagus and stomach when transthoracic images are of poor quality, when the chest is open, or when a transthoracic study is impractical because of extensive chest bandaging.19-22 The most common indication for TEE in children is during cardiac surgery. Intraoperative TEE has had a great impact by (1) providing additional information preoperatively that alters the planned surgical approach, (2) identifying postoperative structural abnormalities after the initial surgical procedure that require additional surgical intervention while the patient is still in the operating room, and (3) identifying postoperative abnormalities that alter medical management immediately after separation from cardiopulmonary bypass support.23 Transesophageal echocardiography also can play an integral role during cardiac catheterization during placement of devices and stents24,25 for monitoring of catheter placement during transeptal or ablation procedures.26,27

Stress echocardiography is more common in the adult population, yet indications for stress echocardiography are increasing in children. Whereas standard echocardiographic studies are performed in a resting state, the purpose of stress echocardiography is that the patient can be evaluated during exercise or during an enhanced inotropic state (accomplished by dobutamine infusion). Stress echocardiography may demonstrate abnormal ventricular function in asymptomatic patients with normal resting state assessments.28 Stress echocardiography is useful in assessing exercise intolerance after surgical intervention,29 and has been shown to be helpful in evaluating the patient with Kawasaki disease.30

Contrast echocardiography, using cross-sectional or M-mode modalities, is useful for detecting right-to-left intracardiac shunts as well as for validating cardiac structures. Microcavitations from agitated saline injected into a peripheral vessel or via catheter can be produced by injecting 1 to 2 mL of saline, dextrose, or other intravenous solution mixed with a small quantity of blood. As the capillary bed traps the microbubbles (less than 200 µm in diameter), when injected on the right side, they do not enter the systemic circulation, unless an intracardiac communication is present. The microbubbles will, however, follow the blood flow patterns so that when right-to-left shunts occur, even in degrees not detectable by other techniques such as angiography or oximetry, these shunts can be detected by the microbubbles in the left heart. The technique is particularly valuable for defining intra-atrial right-to-left shunting.

Transtelephonic echocardiography is useful in areas where a pediatric echocardiographer is geographically distant, yet immediate review of studies are needed to determine patient management. This method uses one of a variety of means to transmit echocardiographic images using either video transmission services or computer lines. The echocardiographic images are transferred to a regional tertiary care center, where a pediatric cardiologist can review the images while they are being performed or as soon as the study is completed. Often, real-time telephone connections between the sonographer performing the study and the pediatric cardiologist are established when the study begins, and the cardiologist assists the sonographer with verbal instructions. Thus far, transtelephonic echocardiography has been a reliable and cost-effective means for assessing patients in remote locations and has been especially effective in avoiding unnecessary neonatal transports.31

The costs of and indications for echocardiography remain misunderstood by many primary care physicians. In today’s environment, cost-effective means for diagnosis are a major concern to the primary care physician, yet patients are often referred for echocardiography instead of a cardiology consultation to evaluate murmurs. Bensky et al studied the causes of this practice and determined that the majority of referring physicians underestimated the cost of echocardiography drastically, believing it was cost-equivalent to cardiac consultation.32 Primary care physicians also believe a cardiologist will obtain an echocardiogram as part of their evaluation of a child with a murmur,32 despite ample published literature documenting cardiology consultation alone is often sufficiently diagnostic33-35 and is a much more cost-effective means36 for assessing a heart murmur. Despite these facts, the request for echocardiography amongst primary care physicians as a screening tool for heart disease is rising.37

The technical skill and impact of technician experience performing the echocardiogram is often underestimated. Echocardiography requires a multitude of decisions and actions by the performer: Appropriate machine settings and transducers must be chosen, correct windows with full sweeps must be performed, and the patient cooperation must often be coerced in the pediatric setting. Most often, cooperation is accomplished with reassurance, entertainment, and skill with children. The operator-dependent nature of echocardiography has a profound impact upon the accuracy of the test: One recent study demonstrated a high rate of clinically relevant diagnostic error in pediatric echocardiograms performed in adult laboratories.38 Given the above comments, we feel that the echocardiogram is best used when ordered by a cardiologist and performed in a pediatric echocardiography laboratory.

The role of cardiac catheterization for children with congenital heart disease has changed dramatically over the past 10 years. Improved anatomic imaging of complex congenital heart disease with echocardiography, computerized tomography (CT), and magnetic resonance imaging (MRI) has made catheter-based angiography nearly obsolete for many common conditions. Today, most surgical patients with intracardiac lesions are diagnosed by echocardiography and undergo surgery without cardiac catheterization. MRI or CT has supplanted cardiac catheterization as the preoperative imaging modality for aortic arch abnormalities and coronary anomalies in many institutions, and MRI is the primary modality for measuring ventricular volumes and semilunar valve insufficiency. Transesophageal echocardiography supplements direct visualization during surgery and identifies residual abnormalities that can be promptly addressed intraoperatively, reducing the need for postoperative cardiac catheterization. Cardiac catheterization remains the primary test for evaluating complex physiology in children with abnormal pulmonary vascular resistance and reactivity, complex single ventricle anatomy, multiple obstructions in the right or left heart, or lesions of peripheral pulmonary arteries not seen well with other imaging modalities. Patients may need cardiac catheterization to assess residual defects soon after surgery when surface echo-Doppler studies may be less accurate.

Cardiac catheterization can rarely lead to serious complications, including arrhythmias, arterial obstruction, reactions to contrast medium, hemorrhage, cardiac perforation, hypoxemic episodes, infections, and death. The mortality rate is 0.2%, with the highest risk occurring in premature infants, critically ill term neonates, and those patients undergoing complex interventional catheterization procedures. Older children at particular risk of death are those with a very high pulmonary vascular resistance and no means of shunting. Pulmonary hypertensive crises or vagal episodes may decrease systemic output and cause death during or soon after catheterization in such patients. About 3% of children may have significant but nonfatal complications. Radiation exposure is a concern, especially in children with complex disease requiring repeat catheterizations. However, the few longitudinal studies done in children after cardiac catheterization in childhood have not shown an increased rate of cancer in adult years.

The most commonly used vessels for cardiac catheterization are the femoral, internal jugular, and subclavian veins and the femoral artery. The catheters are inserted percutaneously. Local infections and arterial complications are extremely rare, and the same vessels may be used repeatedly. In patients with congenital heart disease, the femoral approach often permits passage of a venous catheter into the left side of the heart through a patent foramen ovale or atrial septal defect, avoiding the use of the artery. Patients who have many repeat catheterizations may develop obstruction of the femoral or internal jugular veins, prohibiting their future use. If needed, vascular access can be safely obtained through percutaneous cannulation of the hepatic veins, called a “transhepatic approach” (eFig. 496.1). Some centers use carotid artery cut down with direct sheath placement and surgical repair for access for balloon valvuloplasty for critical aortic valve stenosis or for stent placement in the patent ductus arteriosus in a neonate with pulmonary atresia.

Catheter manipulation is performed under fluoroscopy, with image intensification and pulsed radiation to reduce radiation exposure. Catheters, long thin plastic tubes with one or more holes at the end, come in many sizes, shapes, and materials. Angiographic catheters have multiple holes at their tip to allow rapid injection of contrast medium for angiography. Directional catheters come in many preformed shapes, have a single end hole, and are made out of stiff materials to provide excellent control (ie, “steerability”) to facilitate manipulation into specific chambers and vessels. Balloon-tipped catheters have soft shafts and, with the balloon inflated, tend to follow the course of blood flow (eFig. 496.2).

Measurements during the catheterization include oxygen saturations and pressures in the cardiac chambers and great vessels. This information is used to calculate blood flows, intracardiac shunts, and vascular resistances. Angiography in one or more chambers or vessels is then done to define the cardiac anatomy.

Oxygen Saturations

A small blood sample for oximetry is taken from each of the major vessels and cardiac chambers. Saturations are currently measured by spectrophotometry based on the difference between the maximal wavelength absorption of hemoglobin bound to oxygen (600 nm) versus deoxygenated hemoglobin (506.5 nm). Shunts are diagnosed by comparing oxygen saturations in great veins, chambers, and great vessels. This information must be interpreted with caution, as streaming may influence saturations, especially in the vena cavae and right atrium. For example, hepatic venous blood with medium saturation mixes poorly with highly saturated renal streams and less saturated femoral venous streams in the inferior vena cava. The lowest oxygen concentrations are found in coronary sinus blood, and because the coronary sinus enters just above the tricuspid valve, a sample in the right ventricle may be a little less saturated than one in the right atrium. Normally saturations should vary little through the right heart, less than 4%, so any increase in saturation of greater than 5% indicates a significant left to right shunt. An increase in oxygen saturation at a given level may be the result of a shunt at that level, a shunt at a more proximal level with streaming, or a shunt at a more distal level with regurgitation.

Although noninvasive methods, including echocardiography with color Doppler and nuclear medicine shunt studies, can be used to quantify cardiac shunts, calculation from oxygen saturation data remains the gold standard. Saturation data may be of limited use in quantifying multiple cardiac shunts. A patient with a large left-to-right atrial shunt has blood with very high oxygen saturation entering the right ventricle. A second shunt of equal magnitude at the ventricular level produces a relatively small additional increase in oxygen saturation. Consequently, multiple cardiac shunts are best delineated by using both oxygen saturation data and angiography.

Normal right-sided oxygen saturations range between 65% and 80%, depending on cardiac output and hemoglobin concentration. In patients breathing room air, left-sided saturations are usually 95% to 98%. This mild left-sided desaturation results from normal right-to-left shunting of desaturated pulmonary bronchial blood into the pulmonary veins.

Flows, Shunts, and Resistances

Flows and shunts may be calculated by the Fick (oxygen content) method or by indicator dilution techniques that today are done with room temperature saline and thermistor catheters. The thermodilution technique is accurate and reliable except in patients with significant pulmonary or tricuspid regurgitation, or a significant intracardiac shunt. This technique is therefore primarily used in heart transplant patients, those with isolated pulmonary hypertension or isolated obstructive lesions, such as aortic stenosis or coarctation. Because shunts and regurgitation are common in congenital heart disease, the Fick method, using oxygen saturation and blood hemoglobin concentration, is much more widely used. It makes use of the fact that flow through an organ can be estimated by measuring the concentration of a substance (indicator) in arterial blood flowing to that organ, the amount of that substance added to or removed from blood as it passes through that organ, and the concentration of the substance in venous blood leaving that organ. For example, if blood entering the lung has an oxygen content of 150 mL/L, and blood leaving the lung has an oxygen content of 200 mL/L, then each liter of blood passing through the lungs picks up 50 mL of oxygen. If the oxygen consumption is 200 mL/min, then each minute 200 mL of oxygen is taken up by the lungs and carried away by 200/50 = 4 L of blood. Hence the pulmonary flow (Q̇p) is 4 L/min, calculated as:

Where Q̇p is pulmonary flow (L/min), V̇O2 is oxygen consumption (mL/min), Cpv is pulmonary venous oxygen content (mL/L of blood), and Cpa is pulmonary arterial oxygen content (mL/L of blood).

Note that the calculation uses oxygen content, not oxygen saturation. Oxygen saturation refers to the percentage of hemoglobin in blood that is combined with oxygen, and is independent of hemoglobin concentration. In contrast, oxygen content refers to the total amount of oxygen in a volume of blood and includes physically dissolved oxygen in addition to that bound to hemoglobin. It is expressed as either milliliters of oxygen per deciliter (dL or 100 mL) or liter of blood or as volume percent. Clearly, the oxygen content of blood with 50% oxygen saturation and a hemoglobin concentration of 20 g/dL is nearly twice that of blood with 50% oxygen saturation and a hemoglobin concentration of 10 g/dL.

In patients breathing room air, the physically dissolved oxygen (0.3 mL/dL) is negligible in comparison with the oxygen bound to hemoglobin (13.6 mL/dL at 100% saturation, even in an anemic patient with a hemoglobin of 10g/dL). The oxygen capacity, or the total content of oxygen bound to hemoglobin when 100% saturated, is calculated by multiplying the patient’s hemoglobin concentration in gm/dL by the constant 1.36 mL O2/gm of hemoglobin times 10 to convert dL to liters. A close approximation of the arterial-venous (AV) difference in oxygen content may be obtained by multiplying the arterial-venous difference in oxygen saturation by the oxygen capacity. Hence,

where satPV is pulmonary venous oxygen saturation expressed as a fraction of 1, and satpa is pulmonary arterial oxygen saturation, and capacity = Hgb (in gm/dl) × 1.36 × 10.

With no right-to-left shunting, an aortic sample is preferable to a pulmonary venous sample because it is a mixture of blood from all four pulmonary veins. Similarly, with no left-to-right shunting, a pulmonary arterial sample is the best representative mixture of systemic venous blood.

In a steady state, the amount of oxygen utilized in the tissues is equal to the amount taken into the lungs, and systemic flow is calculated with the same equation and oxygen consumption. Without shunts the systemic arterial-venous (AV) oxygen difference is the same as the pulmonary AV oxygen difference, and the calculated pulmonary flow (Q̇p) equals the systemic flow (Q̇s) or cardiac output. If there are shunts, then pulmonary and systemic flows must be calculated separately, the former from oxygen contents in pulmonary veins and pulmonary artery, the latter from aortic and mixed systemic venous blood.

Flow and shunt calculations are shown in eFigures 496.2, 496.3, 496.4, and 496.5. In each, the oxygen consumption (V̇O2) is assumed to be 150 mL/min/m2, and the hemoglobin (Hb) concentration is assumed to be 14.7 g/dL blood or 147 g/L of blood. Because 1 g of Hb combines with 1.36 mL O2, then

In the equations, saturations are given as fractions of unity. Thus, 96% is given as 0.96.

Example 1: Left‐to-Right Shunt

The most distal saturation that is proximal to the left-to-right shunt is used in the calculation, as it is usually the best-mixed venous sample available. Left-to-right shunt = Q̇p – Q̇s = 3.75 L/min/m2.

Example 2: Right‐to-Left Shunt

Right-to-left shunt = Qs – Qp = 1.25 L/min/m2.

Example 3: Bidirectional Shunting

Calculation of bidirectional shunts requires the concept of effective pulmonary flow (Qep), defined as the volume of systemic venous blood per minute that flows through the lungs. This volume does not include blood that is shunted in either direction because shunted blood is not effective; it neither picks up oxygen nor delivers it. In a patient with only a left-to-right shunt, effective flow equals systemic flow (Q̇p – Q̇ep the left-to-right shunt), and in a patient with only a right-to-left shunt, effective flow equals pulmonary flow (Q̇s – Q̇ep is the right-to-left shunt). In patients with bidirectional shunting, the arteriovenous difference used to calculate effective pulmonary flow uses the mixed systemic venous and mixed pulmonary venous oxygen contents. So effective pulmonary flow is calculated by

Left-to-right shunt Q̇p– Q̇ep = 0.75 L/min/m2; right-to-left shunt = Q̇s – Q̇ep = 2.0 L/min/m2.

Pressures are recorded in each chamber and vessel entered and are monitored continuously during catheter manipulation to assist in determining catheter position. The volume of blood in a chamber or vessel, the size of the chamber or vessel, and the compliance of the walls will determine the pressure. Abnormalities in cardiac pressures can be used to diagnose congenital cardiac disease, so an understanding of normal physiology is key to recognizing abnormalities. Pressure signals from the transducers are sent to a recording device that displays them on a monitor and records them digitally for future reference. Normal pressures and common abnormalities are shown in Table 496-1.

Atrial Pressures

Atrial pressures are reported as an a wave, v wave, and mean pressure. Although there is a c wave and an X and Y descent, they are rarely used to detect clinical abnormalities. The a wave is an increase in pressure that occurs at the end of ventricular diastole generated by contraction of the atrium so it starts with the P wave of the EKG. The a wave peaks as the atrium completes contraction then the pressure decreases as the atrioventricular (AV) valves close with ventricular systole (eFig. 496.6). Atrial pressure continues to decrease during early ventricular systole, the X descent, then increases in late ventricular systole creating the v wave as forward flow fills the atrium against a closed AV valve. As the AV valves open at the start of ventricular diastole the atrial pressure falls, Y descent, resulting in early rapid filling of the ventricles in diastole.

Elevated atrial pressures signify increased intravascular volume, decreased compliance or hypoplasia of the atrium or decreased compliance of the downstream ventricle. Low atrial pressures typically indicate hypovolemia. Elevated a and v wave pressures indicate obstruction to emptying of the atria during diastole, either tricuspid or mitral stenosis or decreased ventricular compliance, a “stiff ventricle” takes more pressure to fill (eFig. 496.7). Mitral or tricuspid insufficiency will increase the v wave and shift the peak earlier in the cardiac cycle, sometimes termed a “regurgitant wave” (eFig. 496.8). All of these relationships are dependent on normal sinus rhythm. If there is AV block then this pattern will be disturbed and the ventricle will at times contract with an open AV valve resulting in a cannon a wave (eFig. 496.9).

Ventricular Pressures

Ventricular pressures are reported as end diastolic and systolic; no mean pressure is reported. In addition, the minimal ventricular pressure and the contour of the pressure tracing can be useful in detecting cardiac abnormalities. Normal ventricular pressure ranges are shown in Table 496-1. End diastolic pressure occurs just before systole begins and should be similar to the peak a wave atrial pressure. If the a wave is greater this indicates AV valve stenosis. Elevation of the end diastolic pressure indicates decreased compliance of the ventricle commonly due to muscular hypertrophy or diastolic dysfunction. It also occurs if there is restriction or constriction as with pericardial tamponade. The normal ventricular systolic pressure trace has a square shape. Elevation of ventricular systolic pressure with a late-peaked shape indicates dynamic ventricular outflow tract obstruction (sub-AS or sub-PS) (eFig. 496.10A,B). Although not typically reported, the minimal pressure the ventricle reaches during the end of isovolumetric relaxation should be below 4 mmHg. The ventricle actively relaxes against closed AV and semilunar (aortic or pulmonary) valves generating negative pressure until the AV valve opens and the ventricle begins to fill. Elevated minimal ventricular pressures and a rapid rise to end diastolic pressures (square root sign) indicate an abnormality with relaxation either due to diastolic dysfunction such as myocardial restriction or external limitation such as pericardial constriction (eFig. 496.11).

Arterial Pressures

Normal pulmonary artery and aortic systolic, diastolic, and mean pressures are reported in Table 496-1. At peak systole the pressure in the ventricle proximal to the artery should be identical to that in the artery. Elevation of the ventricular pressure suggests aortic or pulmonary valve stenosis (eFig. 496.12). Elevation of the arterial systolic pressure suggests downstream obstruction, either large vessel stenosis such as coarctation (eFig. 496.13A,B) or pulmonary artery stenosis or small vessel vascular resistance elevation (pulmonary or systemic hypertension). Elevated arterial diastolic pressure suggests limited distal runoff due to elevated vascular resistance. Low diastolic pressures on the other hand suggest brisk distal runoff from low vascular resistance or some abnormal run off such as aortic insufficiency, patent ductus arteriosus, aortic window, or systemic arteriovenous malformations in the systemic circulation or pulmonary insufficiency or pulmonary arteriovenous malformations in the pulmonary circulation (eFig. 496.14A,B).

Vascular Resistance

The resistance to blood flow in the pulmonary and systemic circulations is calculated from the equations:

where PA and PV are pulmonary artery and vein, respectively, LA and RA are left and right atria, respectively, Ao is aorta, RP is pulmonary vascular resistance (mm Hg/L per min/ m2commonly known as resistance units/m2 or Wood units), Q̇p is pulmonary flow (L/min/m2), RS is systemic vascular resistance (resistance units/m2), and Q̇s is systemic flow (L/min/m2). Most of the resistance to flow is at the arteriolar level, and the calculated resistance is inversely related to the cross-sectional area of the arteriolar lumen. In the systemic circulation, the calculated resistance is the combined value across several vascular beds in parallel. The normal systemic vascular resistance varies between 15 and 30 units/m2. Although high at birth, the pulmonary vascular resistance reaches values near low adult values after 2 to 4 weeks. So that these values can be compared at different ages, they are usually related to surface area. Normal values in children and adults are 1 to 3 units/m2. Elevation to 12 units/m2 suggests mild to moderate pulmonary vascular disease. Marked elevation to greater than 12 units/m2 often suggests significantly elevated pulmonary vascular resistance, which is unlikely to return to normal with vasodilators (“fixed” pulmonary vascular disease). When pulmonary vascular resistance is elevated patients are given inhaled pulmonary vasodilators (100 % oxygen, 40 ppm nitric oxide, or iloprost 2.5 to 5 mcg) in the catheterization laboratory and repeat pulmonary vascular resistance measured to determine vascular reactivity and the minimal pulmonary resistance. This information is often used to determine suitability for surgical repair or heart transplant, plan postoperative management, or to guide outpatient vasodilator therapy.

The cross-sectional area of a valve or vessel most accurately reflects the degree of obstruction. Although pressure gradient is commonly used as a measure of obstruction, it is determined by the cross-sectional area of the obstruction and the blood flow through the obstruction. In patients with valvar, aortic, or mitral stenosis, the effective valve area can be calculated from stroke volume and pressure measurements to more accurately assess the degree of obstruction. Gorlin and Gorlin in 19511 described a formula for calculating valve area that continues in use today. Flow through a valve only occurs during a portion of the cardiac cycle (for example, during systole for the aortic valve and during diastole for the mitral valve), so this must be considered when calculating valve areas. Systolic ejection time is measured from simultaneous left ventricular (LV) and aortic pressure tracings by determining the duration in seconds that the LV pressure exceeds that of the aortic trace in systole. Similarly, the diastolic filling time is measured from simultaneous left atrial (LA) (or pulmonary capillary wedge) and LV pressure tracings by determining the duration in seconds that the LV pressure exceeds that of the LA trace in diastole. The formulas for valve areas are listed below:

The mean gradient across a valve is most accurately determined by the area between the pressure tracings during ejection of blood through the valve. For the aortic valve this would be the area between the LV and aortic pressure tracings during systole, and for the mitral this would be the area between the LA and LV tracing during diastole. Current catheterization laboratory computerized hemodynamic systems easily, accurately, and automatically calculate these values (eFig. 496.15). Normal aortic and mitral valve areas are given in Table 496-1.

Positioning a catheter in a particular chamber and rapidly injecting iodinated contrast permits radiographic delineation of most cardiovascular structures. Multiple injection sites and angulation of the cameras are often necessary to obtain a complete anatomic diagnosis.

Angiograms are recorded digitally at 15, 30, or 60 frames per second. Biplane angiograms yield more information than single-plane angiograms while using the same volume of contrast material, although with greater radiation exposure. This is important for patients with complicated abnormalities who require multiple angiograms and in newborns who may have a low tolerance to contrast medium. A digital replay system permits immediate examination of angiograms, as well as a roadmap for further catheter manipulation. Figure 496-1A,B shows a right ventricular angiogram in the right anterior oblique (ROA) and lateral projections with Figure 496-2A,B shows a left ventricular injection in a RAO and LAO projection.

Additional Testing

Additional tests may be done during the procedure. Drug testing with isoproterenol or dobutamine infusion to simulate exercise can be useful in patients with mild valvar, subvalvar, or aortic obstruction who have symptoms or hypertrophy out of proportion to their resting gradients (eFig. 496.10A,B). Giving oxygen, nitric oxide, or iloprost to determine pulmonary vascular reactivity is common.

Endomyocardial biopsy is the gold standard for detecting rejection in cardiac transplant patients. Nearly all institutions screen their pediatric transplant patients with cardiac biopsies at regular intervals, although this is less common for neonates. Other, more controversial, indications for biopsy include new-onset cardiomyopathy and suspected myocarditis. Though in the past it was thought useful to biopsy both ventricles, right ventricular biopsy alone is now standard practice.1-10