The physiology, natural history, and treatment of arrhythmias in fetuses, infants, children, and teenagers are influenced by developmental changes in cardiac dimensions; the hemodynamics, pharmacokinetics, and pharmacodynamics of antiarrhythmic drugs; and specific electrophysiologic features of the maturing heart. Furthermore, a growing understanding of the developmental changes in myocyte ion channels, intercellular connections, and autonomic nervous system influences is helping to unravel our knowledge of the gross changes that occur during maturation. Finally, cardiac arrhythmias in young individuals are relatively more likely to be related to structural congenital heart disease compared with arrhythmias in adults.
This chapter focuses on arrhythmia substrates in patients having congenital heart disease, arrhythmias exclusive to the pediatric age range, and pharmacologic and nonpharmacologic treatment of these arrhythmias in children.
Recently supported by studies using immunohistochemical and molecular markers, the specialized conduction system of the human heart is most likely derived from specialized cardiomyocytes of the primary heart field along the four intersegmental zones of the primitive heart tube,1 the sinoatrial (SA) ring between the sinus venosus and primitive atrium (which gives rise to the sinus node and part of the atrioventricular [AV] node), the AV ring between the primitive atrium and primitive left ventricle (which contributes to the AV node), the primary ring between the primitive left ventricle and the bulbus cordis (which gives rise to the His bundle and bundle branches), and the ventriculoarterial ring between the bulbus cordis and the truncus arteriosus. According to this theory, during cardiac looping, these rings come together at the inner curvature of the heart tube, and portions of these rings lose their specialized character (Fig. 47–1). There are also contributions from neural crest cells to both the venous and arterial poles and from epicardium-derived cells (second heart field), especially for peripheral Purkinje fiber development and annulus fibrosis formation. This theory is supported by our recent appreciation for arrhythmogenic substrates that are localized to the proximal pulmonary veins, proximal great arteries, and AV annuli. In addition, and more germane to this chapter, reports of dissections of the specialized conduction systems from malformed hearts have proved complementary and have resulted in the accurate prediction of the anatomy of the conduction system in many forms of complex congenital heart disease.
Current theory of development of the specialized conduction system. Left. After initiation of normal rightward ("dextro") looping of the early heart tube, rings at transitional zones appear. Middle. During later development and early septation, these rings appear at the putative junctions between chambers—the sinoatrial ring (SAR; teal), atrioventricular ring (AVR; blue), primary ring at the bulboventricular foramen (PR; yellow), and ventriculoarterial ring (VAR; green). Right. Position of rings in the developed heart. AS, aortic sac; PA, primitive atrium; SV, sinus venosus; VIS, ventricular inlet segment; VOS, ventricular outlet segment. Adapted with permission from Jongbloed MR, Mahtab EA, Blom ...