Every heartbeat is controlled by electrical waves passing through the heart. The summation of this electrical activity results in the tracing seen on an electrocardiogram (ECG). The first ECG was performed in the 1880s through the work of August Waller and Willem Einthoven. The ECG has since been used as a simple and noninvasive method to screen patients for a variety of anatomic and electrical disorders. This chapter provides an overview for interpreting pediatric ECGs and identifying important rhythm disturbances in the pediatric population.
A standard ECG is made up of 12 leads, consisting of 6 limb leads and 6 precordial leads. This divides the heart into a frontal plane (leads I, II, III, aVR, aVL, aVF) and a transverse plane (leads V1–V6). Due to the predominance of the right ventricle during fetal life and in many types of congenital heart disease, a complete pediatric ECG consists of three additional precordial leads: V7 on the left, and V3r, V4r on the right. The combination of these leads creates a three-dimensional model on which the electrical waves of the heart can be plotted and tracked.
The first step in ECG reading is to check the standards by which it is obtained. Since ECG interpretation depends on pattern recognition, diagnostic accuracy requires that all tracings be obtained in standard fashion. The paper speed should be 25 mm/s and the amplitude 10 mm/mV. If the QRS deflection is large, the ECG machine may decrease the amplitude automatically. Failure to recognize this may lead to a missed diagnosis of ventricular hypertrophy (Figure 52-1).
Patient with hypertrophic cardiomyopathy. The leads in the precordial plane are at ¼ standard, masking the prominent left-sided forces.
Beyond this, the ECG should be analyzed systematically. The following can be used as a model for ECG interpretation.
APPROACH TO INTERPRETING ELECTROCARDIOGRAMS
Normal cardiac depolarization starts in the sinus node, high in the right atrium. This results in atrial depolarization, inscribed as the P wave on an ECG. Electricity then passes from the atria into the atrioventricular (AV) node, which relays the impulse to the ventricles via the right and left bundle branches. Ventricular depolarization results in the QRS complex on ECG. As the ventricles transition from depolarization to repolarization, there is an isoelectric ST segment. Repolarization is represented by the T wave.
The heart rate can be calculated in milliseconds (ms) by counting the squares between consecutive QRS complexes, and in beats per minute (bpm) by dividing milliseconds into 60,000. Each small square is 40 ms; each large square is 200 ms. As an example, three large squares between beats is 60,000/600 ms = 100 bpm. The normal heart rate range varies ...