Josephson Clinical Cardiac Electrophysiology

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Chapter 5: Miscellaneous Phenomena Related to Atrioventricular Conduction ■

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FIGURE 5.10 Type I gap in atrioventricu lar (A-V) conduction. The basic atrial drive rate (A1-A1) in each panel is 700 msec, with the introduction of progressively pre mature atrial depolarization (A2). A. There is intact A-V conduction with a prolonged (120 msec) A2-H2 interval and an H1-H2 interval of 470 msec. B. Shortening A1-A2 to 380 msec results in an A2-H2 interval of 135 msec and an H1-H2 interval of 425 msec; the H1-H2 interval is shorter than the effective refractory period (ERP) of the His Purkinje system (HPS), and the atrial depo larization is blocked below the His bundle. C. Shortening A1-A2 to 380 msec results in a marked prolongation of the A2-H2 interval to 245 msec and subsequent prolongation of the H1-H2 interval to 515 msec, which exceeds the ERP of the HPS, and A-V con duction resumes.

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intact human heart has not been adequately demonstrated. Physiologic mechanisms can be invoked to explain virtually all episodes of apparent supernormal conduction observed in humans. Physiologic mechanisms explaining apparent super normal conduction include (a) the gap phenomenon, (b) peel ing back refractoriness, (c) the shortening of refractoriness by changing the preceding cycle length, (d) the Wenckebach phe nomenon in the bundle branches, (e) bradycardia-dependent blocks, (f) summation, and (g) dual A-V nodal pathways. Gap phenomena and changes in refractoriness, either di rectly by altering cycle length or by peeling back the refractory period by premature stimulation, are common mechanisms of apparent supernormal conduction. They have already been dis cussed. Each of these phenomena is not uncommonly seen at long basal cycle lengths, during which His-Purkinje refractori ness is prolonged and infra-His conduction disturbances are common. It should be emphasized that most of the cases of the so-called supernormal conduction described in humans have been associated with baseline disturbances of A-V conduction. Therefore, the term supernormal has referred to improved con duction, but not to conduction that is better than normal. 21 The gap phenomenon and all its variants are probably the most common mechanisms of pseudo-supernormality ( Figures 5.10 through 5.14 ). An example of how marked delay

time to recover again. Retrograde gaps can manifest initial delay in the A-V node or the His-Purkinje system, with proximal de lay in the distal His-Purkinje system ( Figure 5.14 ). Because the gap phenomenon depends on the relationship between the electrophysiologic properties of two sites, any in terventions that alter these relationships (eg, a change in cycle length or drug intervention) may eliminate or perpetuate the gap phenomenon and may also convert one type of gap to an other. 22 For example, atropine may convert a Type I gap, in which proximal delay in the A-V node allows distal recovery of the His-Purkinje system, to a Type II gap because the req uisite degree of A-V nodal delay can no longer be achieved. 22 SUPERNORMAL CONDUCTION Supernormal conduction implies conduction that is bet ter than anticipated or conduction that occurs when block is expected. 23-25 Hence, in much the same manner as con cealed conduction, what is considered supernormal depends on what is anticipated. When an alteration in conduction can be explained in terms of known physiologic events, true su pernormality need not be invoked. 26,27 Although supernor mal conduction and excitability have been demonstrated in vitro, 24 the existence of true supernormal conduction in the

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