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Conduction of premature atrial complexes (PACs) is usually predictable in a patient with a relatively constant heart rate. A late (i.e., long atrial coupling interval) PAC will conduct to the ventricle and a relatively early (i.e., short atrial coupling interval) PAC often blocks in the atrioventricular (AV) conduction system. In some instances, however, apparent paradoxical conduction can occur in the AV conduction system. This happens when a late PAC conducts to the ventricle, an earlier PAC blocks, but—unexpectedly—a PAC with even a shorter coupling interval again conducts to the ventricle. Various electrophysiologic phenomena can explain these conduction patterns, but they are often very difficult to diagnose by surface electrocardiography only. Resumption of conduction may occur because of supernormal excitability and conduction, which is probably relatively rare, but more commonly can be explained by other mechanisms, especially the gap phenomenon. Specific issues related to the bundle branch conduction system are presented in detail in Chapter 3.
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Moe and colleagues1 noted in a series of canine experiments that a PAC that did not conduct to the ventricle could occur in timing between a later and earlier PAC that did conduct to the ventricle. The unexpected conduction of the earliest PAC was referred to as a gap in AV transmission. The gap phenomenon was studied by many investigators and several subdivisions of gap were identified.2-5 However, the basic underlying electrophysiologic principle for all varieties of gap is the same and can be described as follows. A relatively late-coupled PAC that conducts to the ventricle involves transmission through the atria, AV node, His bundle, and all or part of the bundle branch system (Figure 4-1). When the PAC occurs early enough, it may fail to conduct to the ventricle due to block either in the AV node or His-Purkinje system (areas B, C, and D in Figure 4-1). In essence, the effective refractory period of the tissue at the site of block is greater than the functional refractory period of tissue proximal to the site of block. In other words, an electrical impulse can be conducted through areas proximal to but not distal to the site of block because of the longer refractory period at that point. Gap occurs when a PAC with an even earlier premature atrial interval undergoes slowing of conduction in tissue proximal to the previous area of block such that this site of previous block has time to recover excitability and allow the electrical impulse to conduct to the ventricle.
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This seemingly complex electrophysiologic event may be simplified by considering an analogy that occurs during everyday driving of an automobile (Figure 4-2). In this figure, the red traffic light represents the distal site of ...