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Although certain arrhythmias are more common in neonates and young infants compared to older children and adults, all types of arrhythmias can occur. Many are benign and do not cause hemodynamic compromise. Others may compromise cardiac output and cause decreased blood pressure and decreased perfusion. Sustained tachyarrhythmias may eventually cause myocardial dysfunction, which is known as tachycardia-induced cardiomyopathy. The purpose of this chapter is to review diagnosis and management of common arrhythmias in neonates and young infants.

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Normally the electrical impulse originates in the sinoatrial (SA) node. The atrioventricular (AV) node, His bundle, and bundle branches provide the only normal pathway for transmission of impulses between the atria and ventricles. Generation of impulses from the SA node is modulated by many factors including body temperature, blood pressure, autonomic nervous system, and circulating catecholamines. Conduction through the AV node is slowed so that atrial contraction is complete before ventricular contraction occurs.

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Abnormal Impulse Formation

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Abnormalities in impulse formation result in sinus bradycardia and tachycardia, premature atrial and ventricular contractions, and ectopic or automatic rhythms from the atria, AV node, or ventricles. Automatic tachycardias are usually incessant meaning that they are almost always present. Increased automaticity occurs when atrial, nodal, or ventricular cells display autonomous repetitive depolarization at a higher rate than is normal. Sinus tachycardia, atrial ectopic tachycardia, junctional ectopic tachycardia, and the automatic form of ventricular tachycardia are all forms of automatic tachycardia. Onset and termination are often gradual rather than abrupt. The rate of automatic tachycardias is often sensitive to changes in autonomic tone. Therapies that produce only transient effects, for example, direct current (DC) cardioversion and administration of adenosine, do not terminate automatic tachycardias.

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Abnormal Impulse Conduction

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Block within the normal conduction system is the most obvious form of abnormal impulse conduction. Block can occur at any point but atrioventricular block is most commonly seen.

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Reentry, the other form of abnormal impulse conduction, is an important mechanism underlying supraventricular tachycardia (SVT) in infants. The reentrant circuit involves two functionally distinct pathways that have different conduction velocities and refractory periods. Unidirectional block is present in one pathway, an electrical impulse traverses the other pathway and conduction is delayed enough so that the impulse is able to “reenter” the blocked pathway from the other direction thus completing the reentrant circuit. Reentry mechanisms usually cause paroxysmal tachycardias, which may start and stop multiple times in the course of the day. Reentrant tachycardias start and stop abruptly and they often terminate in response to interventions that produce only transient effects (eg, adenosine) because interruption of the reentrant circuit usually terminates the tachycardia.

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Sinus arrhythmia is a normal phasic variation in impulse formation from the SA node that is often in cycle with respiration (Figure 10-1). This is the most common cause of an irregular heart rate, especially in older infants. The P-wave ...

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