48: Arrhythmia

An infant has an abnormal tracing on the heart rate monitor.

1. What is the heart rate? The heart rate in newborns varies from 70–190 beats/min. It is normally 120–140 beats/min but may decrease to 70–90 beats/min during sleep and increase to 170–190 beats/min with increased activity such as crying. See Table 48–1 for normal heart rate values.

2. Is the abnormality continuous or transient? Transient episodes of sinus bradycardia, tachycardia, or arrhythmias (usually lasting <15 seconds) are benign and do not require further workup. Episodes lasting >15 seconds usually require full electrocardiogram (ECG) assessment.

3. Is the infant symptomatic? A symptomatic infant may need immediate treatment. Signs and symptoms of some pathologic arrhythmias include tachypnea, poor skin perfusion, lethargy, hepatomegaly, and rales on pulmonary examination. All of these signs and symptoms may signify congestive heart failure (CHF), which may accompany arrhythmias. CHF resulting from rapid cardiac rhythms is unusual with heart rates <240 beats/min.

1. Heart rate abnormalities. Heart rates in the normal newborn vary dramatically. Some evidence, using computer programs to assess heart rate variability in the neonatal period, suggests that the lower heart rates early on are due to the inability of the infant's sympathetic system to inhibit the parasympathetic (or vagal) response.

   1. Tachycardia. Heart rate >2 standard deviations (SD) above the mean for age (see Table 48–1).

      1. Benign causes. Postdelivery, heat or cold stress, painful stimuli, medications (eg, atropine, caffeine, epinephrine, intravenous glucagon, pancuronium bromide, tolazoline, and isoproterenol).

      2. Pathologic causes

         1. Common. Fever, shock, hypoxia, anemia, sepsis, patent ductus arteriosus, and CHF.

         2. Uncommon. Hyperthyroidism, metabolic disorders, cardiac arrhythmias, and hyperammonemia.

   2. Bradycardia. Bradycardia is a heart rate >2 SD below the mean for age (see Table 48–1). Transient bradycardia is fairly common in newborns; rates range from 60–70 beats/min.

      1. Benign causes. During defecation, vomiting or micturition, gavage feedings, suctioning, medications (eg, propranolol, digitalis, atropine, and infusion of calcium, maternal long-acting β-blockers to treat hypertension given within 24 hours of delivery).

      2. Pathologic causes

         1. Common. Hypoxia, apnea, convulsions, airway obstruction, air leak (eg, pneumothorax), CHF, intracranial bleeding, severe acidosis, and severe hypothermia.

         2. Uncommon. Hyperkalemia, cardiac arrhythmias, pulmonary hemorrhage, diaphragmatic hernia, hypothyroidism, and hydrocephalus.

2. Arrhythmias

   1. Benign arrhythmias. Include any transient episode (<15 seconds) of sinus bradycardia and tachycardia and any of the benign causes of sinus tachycardia and bradycardia noted in Section III.A.1 and III.A.2. Sinus arrhythmia, a phasic variation in the heart rate often associated with respiration, is also benign.

      1. Premature atrial beats. These can occur in the newborn and are usually benign. The QRS is narrow, and the T waves are often discordant. (See the example in Figure 48–1C.) They tend to decrease in number or go away entirely in the first few months of life. A workup is usually not indicated unless the infant has the premature atrial beats in association with structural cardiac disease.

      2. Unifocal premature ventricular beats. Fairly frequent in the newborn. The QRS is wide, and the T wave is discordant with the sinus T wave. (See Figure 48–1D.) If seen in a newborn, obtain a 12-lead ECG. Do not treat unless the infant is symptomatic. Sometimes premature ventricular contractions (PVCs) become less frequent when the sinus rate increases. PVCs tend to decrease in number or go away entirely in the first few months of life.

      3. Benign bradycardia. Unusual.

   2. Pathologic arrhythmias

      1. Supraventricular tachycardia (SVT). The most common type of cardiac arrhythmia seen in the neonate (Figure 48–1A).

      2. Atrial flutter. Difficult to distinguish from other SVTs, unless the block is >2:1. Administration of adenosine may increase the block to 3:1 or 4:1, allowing the flutter waves to become more visible on the ECG tracing.

      3. Atrial fibrillation. Less common than SVT or atrial flutter.

      4. Wolff-Parkinson-White syndrome (WPW). (A short PR interval and delta wave and slow upstroke of the QRS complex.) Difficult to identify when the rate is fast (Figure 48–1B).

      5. Ectopic beats.

      6. Ventricular tachycardia.

      7. Atrioventricular (AV) block with symptoms. Occurs in newborns with complete block and ventricular rates <55 beats/min. Premature infants may even require a higher rate. Because cardiac output in premature and young term babies can increase only by rate increase (their stroke volume is fixed by the small size of the ventricles), fevers, sepsis, and other stressors are tolerated poorly by babies with complete heart block.

   3. Secondary to extracardiac disease

      1. Sepsis (usually tachycardia).

      2. Diseases of the central nervous system (usually bradycardia).

      3. Hypoglycemia.

      4. Drug toxicity. Digoxin (potentiated by hypokalemia, alkalosis, hypercalcemia, and hypomagnesemia); theophylline (less frequently used in neonatal intensive care units).

      5. Electrolyte abnormalities such as potassium, sodium, magnesium, or calcium abnormalities.

      6. Other. Metabolic acidosis or alkalosis, adrenal insufficiency.

1. Physical examination. Check for signs of CHF (ie, tachypnea, rales on pulmonary examination, enlarged liver, and cardiomegaly). Vomiting and lethargy may be seen in patients with digoxin toxicity. Hypokalemia can cause ileus.

2. Laboratory studies

   1. Electrolyte, calcium, and magnesium levels

   2. Blood gas may reveal acidosis or hypoxia

   3. Drug levels to evaluate for toxicity

      1. Digoxin. Normal 0.5–2.0 mcg/mL (sometimes up to 4 mcg/mL). Elevated levels of digoxin alone are not diagnostic of toxicity; clinical and ECG findings consistent with toxicity are also needed, and many neonates have naturally occurring substances that interfere with the radioimmunoassay test for digoxin.

      2. Caffeine. Toxicity is manifested by tachycardia and feeding intolerance. Reduce dose or skip dose.

3. Imaging and other studies

   1. ECG. Should be performed in all infants who have an abnormal rhythm that lasts >15 seconds or is not related to a benign condition. Diagnostic features of the common arrhythmias are listed next. Although PR interval varies with heart rate, a PR interval of >160 milliseconds is abnormal in any newborn.

      1. Supraventricular tachycardia (see Figure 48–1A)

         1. Ventricular rate of 180–300 beats/min

         2. No change in heart rate with activity or crying

         3. An abnormal P wave or PR interval

         4. A fixed R-R interval

      2. Atrial flutter

         1. Atrial rate is 220–400 beats/min.

         2. A sawtooth configuration seen best in leads V₁–V₃ but often difficult to identify when a 2:1 block or rapid ventricular rate is present.

         3. The QRS complex is usually normal.

      3. Atrial fibrillation

         1. Irregular atrial waves that vary in size and shape from beat to beat.

         2. The atrial rate is 350–600 beats/min.

         3. The QRS complex is normal, but ventricular response is irregular.

      4. Wolff-Parkinson-White syndrome (see Figure 48–1B)

         1. A short PR interval

         2. A widened QRS complex

         3. Presence of a delta wave

      5. Ventricular tachycardia

         1. Ventricular premature beats at a rate of 120–200 beats/min and a widened QRS complex

      6. Ectopic beats

         1. Abnormal P wave and widened QRS complex

      7. Atrioventricular block

         1. First-degree block (Figure 48–2A)

            • (a) A prolonged PR interval (normal range, 0.08–0.12 second)

            • (b) Normal sinus rhythm

            • (c) A normal QRS complex

         2. Second-degree block

            • (a) Mobitz type I. (Figure 48–2B)

              • A progressively prolonged PR interval until a ventricular beat is dropped (Wenckebach)

              • A normal QRS complex

            • (b) Mobitz type II. (Figure 48–2C) A constant PR interval with dropped ventricular beats or nonconducted P waves

         3. Third-degree block (Figure 48–2D)

            • (a) Regular atrial beat.

            • (b) Slower ventricular rate.

            • (c) Independent atrial and ventricular beats.

            • (d) Atrial rate increases with crying and level of activity. The ventricular rate usually stays the same.

      8. Hyperkalemia

         1. Tall, tented T waves

         2. A widened QRS complex

         3. A flat and wide P wave

         4. Ventricular fibrillation and late asystole

      9. Hypokalemia

         1. Prolonged QT and PR intervals

         2. Depressed ST segment

         3. Flat T wave

      10. Hypocalcemia. A prolonged QT interval. QT interval prolongation may also be due to myocardial stress at the time of delivery and may resolve. Persistent prolongation of the PR interval with normocalcemia mandates questions about family history of arrhythmia or sudden death. ECG on the parents may be indicated.

      11. Hypercalcemia. Shortened QT interval.

      12. Hypomagnesemia. Same as for hyperkalemia.

      13. Hyponatremia

          1. Short QT interval

          2. Increased duration of the QRS complex

      14. Hypernatremia

          1. Prolonged QT interval

          2. Decreased duration of the QRS complex

      15. Metabolic acidosis

          1. Prolonged PR and QRS intervals

          2. Increased amplitude of the P wave

          3. Tall, peaked T waves

      16. Metabolic alkalosis. Inverted T wave.

      17. Digoxin

          1. Therapeutic levels. Prolonged PR interval and a short QT interval.

          2. Toxic levels. Most common are sinoatrial block, second-degree AV block, and multiple ectopic beats; also seen are AV block and bradycardia.

      18. Caffeine

          1. Therapeutic levels. Desired effect is decreased frequency and duration of apneic spells; no significant changes on ECG.

          2. Toxic levels. Tachycardia with feeding intolerance.

   2. Chest radiograph. Perform in all infants with suspected heart failure or air leak.

1. General management. Decide whether the arrhythmia is benign or pathologic, as noted. If it is pathologic, full ECG evaluation must be performed. Any acid-base disorder, hypoxia, or electrolyte abnormality needs to be corrected.

2. Specific management

   1. Heart rate abnormalities

      1. Tachycardia

         1. Benign. No treatment is necessary because the tachycardia is usually secondary to a self-limited event.

         2. Medications. With certain medications, such as caffeine, observe infant for other signs of toxicity and decrease or skip the next dose.

         3. Pathologic conditions. The underlying disease should be treated.

      2. Bradycardia. Confirm it is sinus bradycardia and not complete heart block.

         1. Benign. No treatment is usually necessary.

         2. Drug related. Check the serum drug level if possible and then consider lowering the dosage or discontinuing the drug unless it is necessary.

         3. Pathologic. Treat the underlying disease.

            • (a) In severe hypotension or cardiac arrest, check the airway and initiate breathing and cardiac compressions.

            • (b) Administer atropine, epinephrine, or isoproterenol to restore normal rhythm. (See Chapter 148 for dosing.)

   2. Arrhythmias. For dosages and other pharmacologic information, see Chapter 148; for details on cardioversion, see Chapter 28.

      1. Benign. Observation.

      2. Pathologic. Treat any underlying acid-base disorders, hypoxia, or electrolyte abnormalities.

         1. Supraventricular tachycardia

            • (a) If the infant's condition is critical. Electrical cardioversion is indicated, with digoxin started for maintenance therapy, if sinus rhythm without preexcitation is present after cardioversion. Propranolol is drug of choice in babies with preexcitation (WPW).

            • (b) If the infant's condition is stable. Vagal stimulation (ice or an ice-cold washcloth applied to the infant's face for a few seconds) can be tried. Adenosine, 100 mcg/kg, IV push into a central vein, converts SVT to sinus rhythm. It may be necessary to double the dose to 200 mcg/kg (300 mcg/kg maximum). Never use verapamil in infants. Digoxin should be started as a maintenance drug, unless there is preexcitation. Another drug that may be used instead of or in addition to digoxin is propranolol. SVT refractory to digoxin and propranolol may be treated with flecainide or amiodarone.

         2. Atrial flutter

            • (a) If the infant's condition is critical (severe CHF or unstable hemodynamic state). Perform electrical cardioversion, with digoxin started for maintenance.

            • (b) If the infant is stable. Start digoxin, which slows the ventricular rate. A combination of digoxin and propranolol may be used instead of digoxin alone.

            • (c) If rate is rapid and 2:1 block present. Atrial flutter may be hard to identify on ECG. May give adenosine (see above) to increase block to 3 or 4:1.

         3. Recurrent atrial flutter management. Same as that for atrial flutter.

         4. Atrial fibrillation (unusual in infants). Management is the same as for atrial flutter.

         5. Wolff-Parkinson-White syndrome. Often accompanied by SVT. β-Blockers are preferred because digoxin may promote 1:1 conduction and demise.

         6. Ventricular tachycardia. Perform electrical cardioversion (except in digitalis toxicity), with lidocaine started for maintenance therapy. Although lidocaine is the drug of choice, other drugs that may be used are procainamide or phenytoin.

   3. Ectopic beats

      1. Asymptomatic. No treatment is necessary.

      2. Symptomatic. With underlying heart disease, in the unlikely event that ectopic beats compromise cardiac output, suppress with phenytoin, propranolol, or amiodarone.

   4. Atrioventricular block

      1. First degree. No specific treatment necessary.

      2. Second degree. Treat the underlying cause.

      3. Third degree (complete heart block)

         1. Rate >70 beats/min. If the infant is asymptomatic, observe. Generally at this rate, no problems develop.

         2. Rate <50 beats/min. The patient usually needs emergency transvenous pacing, with the need for subsequent permanent pacing.

         3. Rate between 50 and 70 beats/min. Gray zone. Monitor urine output as an index of end-organ perfusion and measure serum lactate. Check the mother for SAA or SSA antinuclear antibodies (association with complete heart block and cardiomyopathy). Infants with nonimmune complete heart block generally fare better than those whose heart block is caused by maternal antibodies. Mothers who have given birth to an infant with complete heart block because of autoimmune-related SSA or SAA antibodies have a 50–69% chance of subsequent infants having complete heart block. Standard of care for these mothers' subsequent pregnancies include maternal treatment with Plaquenil and following PR interval of the fetus weekly starting at 12 weeks. In an effort to increase fetal heart rate, maternal terbutaline is sometimes used if the infant's heart rate is slow enough to worry about fetal demise.

   5. Arrhythmias secondary to an extracardiac cause

      1. Pathologic conditions. Treat the underlying disease.

      2. Digoxin toxicity. Check the PR interval before each dose, obtain a stat serum digoxin level, and hold the dose. Consider digoxin immune Fab (Digibind).

      3. Theophylline toxicity. Reduce the dosage or discontinue medication.

   6. Electrolyte abnormalities

      1. Check serum electrolyte levels with repeat determinations.

      2. Treat electrolyte abnormalities accordingly.

Presented in Chapter 28.

A multidisciplinary approach in any fetus with any arrhythmia serious enough to cause fetal hydrops is mandatory. Perinatologists, neonatologists, and cardiologists must be prepared to exert efforts in concert if such infants are to survive. Usually there is at least some warning that such an infant is being delivered, but in the event that specialists are not all in attendance, the physician assuming responsibility for care must be ready to intubate, do pleural and pericardial taps and a paracentesis if necessary, and treat the arrhythmia responsible for fetal CHF.