Emergency physicians are able to quickly decode the ciphers of ECGs into meaningful clinical data, at least until they are faced with a pediatric ECG. It breaks their pattern recognition, and they are forced to use the slow part of their brain (recommended reading: Daniel Kahneman's Thinking, Fast and Slow).
Even though the pediatric ECG looks vaguely similar to all of the other ECGs seen during a shift, this one turns into a mystery, and confidence drains. It doesn't look normal based on an adult tracing, but they are unsure if it's normal for the child. The best ECG readers can resort to folding the tracing underneath and relying on the computer read. But we might make the ECG a little less mysterious if we can decipher the differences of the pediatric heart.
Heart Rate: The heart rate increases during the first month of life because of increasing autonomic drive, and then it gradually decreases over the years to a normal adult rate driven by changes in intrinsic sinus node activity. Despite these changes, the cardiac output does not change much. The heart rate is slowing and the left ventricle is enlarging, but the product (stroke volume x heart rate) remains relatively constant.
P Waves: Atrial depolarization indicated by the P wave starts at the SA node and proceeds inferiorly and to the left toward the AV node, the same as in adults. This means the P wave should be positive in lead I and aVF. The infant heart is usually aligned more vertically in the chest, so the aVF component may be more prominent. As the child ages, the right atria enlarges and the P wave widens to show the additional time required to depolarize the larger atrium.
PR Interval: During the child's first month, an autonomic surge stimulates the AV node speeding conduction. Once this tapers off, the PR interval settles at about 90 ms, which is considerably shorter than the adult normal of 120 ms.
QRS Complex: The sequence of ventricular depolarization is the same in children and adults, and the amplitude and morphology of the QRS complex depends on the relative mass of the right and left ventricles, the cardiac axis, the position of the heart in the thorax, and overlying soft tissue. At birth the right ventricle is larger than the left, but they become equal by about 1 month of age. The adult ratio is reached by about 6 months, and remains the same during the rest of the cardiac growth. The early prominence of the right ventricle leads to prominent R waves in right precordium and deep S waves in the left precordium. The overall cardiac mass is smaller, though, so the QRS complex is generally narrower (53 ms at term). By adolescence, it has lengthened to 70, most of which can be attributed to larger ventricular mass rather than conduction changes.
Q waves are particularly meaningful in pediatric ECGs. They are normal in the inferior and left lateral precordial leads. Even though they may have a large amplitude, the duration should be less than 20 ms. Deep (>3mm) and wide (>30ms) Q waves in leads I and aVL, especially without other normal Q waves, can be a sign of anomalous origin of the left coronary artery. (http://bit.ly/1f8Aswk.) Q waves in the right precordium are always pathological and indicate right ventricular hypertrophy. Deep Q waves in the left lateral precordial leads are seen in left ventricular hypertrophy of various etiologies and should be investigated.
ST Segment: Because the P wave often overlaps with the previous T wave, identification of the ST segment isoelectric line can be difficult. This segment is preferred to the PR interval for establishing the tracing baseline, especially given the short PR interval. ST-segment elevation >1mm in children is rare in normal pediatric patients. If seen in the precordial leads, it should occur either where the T wave orientation is in transition or related to early depolarization, which should be accompanied by an ST angle greater than 20 degrees. Pathologic ischemic changes to the ST segment are more likely to be secondary to congenital or acquired disease, but the age for considering atherosclerotic disease is getting younger because of higher rates of obesity.
T Waves: The right precordial T waves (V1) start out upright like adults, but invert around seven days and remains inverted for six or seven years. In this age range, an upright T wave in these leads can indicate right ventricular hypertrophy, which should prompt further evaluation. If this persists into adulthood, it is labelled persistent juvenile T wave pattern.
QT Interval: Like in adults, exact measurement of the QT interval is difficult. In early ECG machines, each channel was recorded separately even if the resultant printout was on a single sheet of paper in standard 12-lead layout. Lead II was the preferred lead to measure the QT interval in these situations. Modern machines measure all leads simultaneously, and the QT interval can be measured from the earliest onset of the QRS complex to the end of the T wave where it rejoins the baseline. The longest QT interval should be used when there is a discrepancy between the leads. Similar to adults, teenage girls have longer corrected QT intervals, but this has not been confirmed in younger children.
Fast heart rates in young children may cause the P interval to be superimposed on the T wave. Identifying the QT interval in this situation may require extrapolation from the T wave using the PR segment as the isoelectric baseline.
Children also typically have pronounced sinus arrhythmia, which leads to RR variability. Because this measurement is part of the QT interval heart rate correction, you end up with a beat-to-beat changing of the QT interval. There is no real agreement of whether the shortest RR interval or an averaged value should be used to establish the QT interval, so just be mindful that the cut-points for normal values will depend on the method used.
The familiar Bazett rate-correction formula for the QT interval (which uses a square root of the RR interval) overcorrects for children. The Fridericia formula or nomograms are probably more accurate, though less familiar to emergency physicians. Fortunately, the short-cut method for estimating a prolonged QT interval if it exceeds half of the RR interval has been validated in children. Be aware that the automated QT calculations can be inaccurate and should be verified with manual calculations.
A normal corrected QT interval is less than 440 ms, borderline 440-60 ms, and >460 abnormal. Evaluation one minute into recovery after exercise may increase the discriminant ability of the ECG.
Indications: Outside of the emergency department, many pediatric ECGs are obtained to screen for congenital heart disease. Without the presence of a murmur, screening asymptomatic children has very low sensitive and specificity. Additionally, studies routinely demonstrate that they don't change management.
More clinically relevant to EPs is the evaluation of suspected arrhythmias, especially in identifying ventricular pre-excitation and prolonged QT intervals. An ECG has demonstrated value for evaluating presentations of syncope and seizure.
The short PR interval may make it difficult to recognize the ventricular pre-excitation of the QRS complex (delta waves). If the PR interval is less than 100 ms, the absence of Q waves in the left lateral precordial leads and left axis deviation may be a useful secondary indicator.
ECGs are also useful to monitor medication-related changes, such as treatment with pro-kinetic agents, antidepressants, atypical antipsychotics, stimulants, and antiarrhythmic medications. This is most easily accomplished by measuring the QT interval at similar baseline heart rates, say, 60 bpm.
By far the most common indication for pediatric ECG ordered in the ED is the evaluation of chest pain. They are often unrevealing, but may uncover ventricular ectopy, QT interval prolongation, or ventricular hypertrophy.
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