Physiology

The SA node has the highest rate of spontaneous depolarization (automaticity) in the specialized conduction system, and under ordinary circumstances, it is the major generator of impulses. Its unique location astride the large SA nodal artery provides an ideal milieu for continuous monitoring and instantaneous adjustment of heart rate to meet the body's changing metabolic needs. The SA node is connected to the AV node by several specialized fiber tracts, the function of which has not been fully elucidated. The AV node appears to have three major functions: It delays the passing impulse for approximately 0.04 seconds under normal circumstances, permitting complete atrial emptying with appropriate loading of the ventricle; it serves as a subsidiary impulse generator, as its concentration of P cells is second only to that of the SA node; and it acts as a type of filter, limiting ventricular rates in the event of an atrial tachyarrhythmia.

The His bundle arises from the convergence of Purkinje fibers from the AV node, although the exact point at which the AV node ends and the His bundle begins has not been delineated either anatomically or electrically. The separation of the His bundle into longitudinally distinct compartments by the collagenous framework allows for longitudinal dissociation of electrical impulses. Thus a localized lesion below the bifurcation of the His bundle (into the bundle branches) may cause a specific conduction defect (e.g., left anterior fascicular block). The bundle branches arise as a direct continuation of the His bundle fibers. Disease within any aspect of the His bundle branch system may cause conduction defects that can affect AV synchrony or prevent synchronous right and left ventricular activation. The accompanying hemodynamic consequences have considerable clinical relevance. These consequences have provided the impetus for some of the advances in pacemaker technology, which will be addressed in later chapters of this book.

Although a detailed discussion of the histopathology of the conduction system is beyond the scope of the present chapter, it is worth noting that conduction system disease is often diffuse. For example, normal AV conduction cannot necessarily be assumed when a pacemaker is implanted for a disorder seemingly localized to the sinus node. Similarly, normal sinus node function cannot be assumed when a pacemaker is implanted in a patient with AV block.

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