Chapter 21

### DEFINITION

Arrhythmias, alterations in the heartbeat rhythm, are a common problem in the newborn period. Many are now diagnosed in utero. We consider the tachycardias (rapid heart rhythms) first, then the bradycardias (slowed heart rhythms).

### TACHYCARDIAS

The most commonly diagnosed fetal arrhythmias are premature atrial contractions (PACs), which often resolve soon after birth. The other tachycardias frequently seen in the fetus and the newborn period are atrial tachycardia, atrial flutter, and the more usual reentrant-type supraventricular tachycardia (SVT).1, 2, and 3

#### Differential Diagnosis

It is often necessary to distinguish sinus tachycardia from a true tachyarrhythmia. The normal resting heart rate for infants varies with gestational age, with term infants at 130–150 beats per minute (bpm). Preterm infants have slightly faster rates, with an average resting heart rate of 160 bpm at 32 weeks. The average heart rate4 of a newborn is 145 with a range of 110–150 bpm.

It can be difficult to tell the difference between sinus tachycardia and an atrial tachycardia that originates close to the sinoatrial (SA) node based on the electrocardiographic appearance. If the clinical evidence is consistent with sepsis, dehydration, or blood loss, then it is likely that the rhythm is a reactive sinus tachycardia. Another helpful clue to sinus rhythm is the morphology of the P waves. With normal sinus rhythm, P waves are upright in leads I and aVF. In addition, one should see a biphasic P wave in V1. This is caused by the normal electrical depolarization of the atria that begins in the right atrium, spreads to the left atrium, then proceeds to the AV node.

If the baby is being monitored on telemetry, there may be a clear change in P-wave morphology or axis that will suggest PACs or an ectopic atrial rhythm. The PR interval may also prolong.5 Figure 21-1 shows a comparison of sinus tachycardia and atrial tachycardia.

###### FIGURE 21-1

A, Sinus tachycardia vs B, ectopic atrial tachycardia (EAT). In sinus tachycardia, there is 1 P-wave morphology; in EAT there are at least 2 different P-wave morphologies.

The PACs look similar to sinus beats with a P wave preceding each QRS complex, but these occur earlier than expected. The P wave may be buried in the preceding T wave if the heart rate is increased. If they occur in a pattern of every other beat with sinus beats, they are called atrial bigeminy. Both of these rhythms are usually benign and self-limited but can occur if there is a central line within the right atrium or electrolyte disturbances.

If the PAC is conducted aberrantly, there is often a question regarding whether it is a premature ventricular contraction (PVC) or PAC. It is helpful to ...

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