Unusual Electrophysiologic Phenomena



Unusual Electrophysiologic Phenomena








SUPERNORMALITY

The supernormal period is a short window at the end of repolarization during which an impulse finds otherwise refractory tissue capable of conduction and/or excitability.1,2 Because it occurs during repolarization, it behaves similarly to other repolarization phenomena showing cycle length dependency (restitution).3,4 It is, therefore, not a static but rather a dynamic time window widening and shifting rightward with longer cycle lengths (analogous to the QT interval and QT dispersion). Its timing corresponds to the end or shortly after the T wave (the ECG marker of global ventricular myocardial repolarization), but it is repolarization of the tissue in question that determines the true supernormal period. Supernormality has been described in “all or none” (fast response) tissues with prolonged refractoriness (His-Purkinje, ventricular myocardium, accessory pathways [APs]) and not the AV node. Its most common manifestation is momentary resolution of BBB or AV block following a premature impulse (supernormal period of His-Purkinje conduction) (Figs. 22-1, 22-2, and 22-8).5,6,7,8,9 While episodes of supernormality during AV block result in “better than expected” conduction, sometimes “faster than expected” conduction occurs with paradoxical shortening of the PR interval (see Fig. 1-35). Spontaneous episodes of supernormality in APs are rare and require the unusual combination of AV block and a poorly conducting AP (Fig. 22-9; see also Fig. 9-20).3,4,10,11,12 Supernormality in the ventricle occurs when otherwise subthreshold pacing stimuli capture the ventricle only during a critical period at the end of ventricular repolarization (supernormal period of ventricular excitability) (Fig. 22-10).13 Supernormality might be due to a transient window of increased voltage in the transmembrane action potential at the end of repolarization.2 However, the cellular basis of supernormality is unknown, and alternative mechanisms can also explain the phenomenon of unexpected conduction with prematurity.14,15


GAP PHENOMENA

During premature beats, the gap phenomenon refers to temporary loss of conduction over a structure at intermediate coupling intervals when longer and shorter coupling intervals are








able to conduct (i.e., “gap” in conduction).16,17 A premature impulse reaching a site in its absolute refractory period will fail to conduct (phase 3 block). A more premature impulse, however, might encounter relative refractoriness in tissue proximal to this site resulting in conduction delay. Enough delay proximally allows the previously blocked site sufficient time to recover excitability and conduct distally (“proximal delay allowing distal conduction”). Gap phenomena have been described both antegradely (six types) and retrogradely (two types) along the conduction system, particularly in the setting of dual AV node physiology (the slow pathway [SP] providing delay proximal to the His-Purkinje system) (Figs. 22-3, 22-4, and 22-11, 22-12 and 22-13).18,19,20,21






FIGURE 22-1 Supernormality in the right (top) and left (bottom) bundle. In both cases, the underlying rhythm is atrial fibrillation. QRS complexes normalize only during a short window at the end of repolarization when impulses fall into the supernormal period of the right and left bundle branch. Supernormality occurring later after the T wave results from concealed transeptal retrograde conduction from “unblocked” into “blocked” bundle—the late retrograde concealment into the “blocked” bundle shifts its supernormal period rightward.






FIGURE 22-2 Supernormality in the right bundle. The underlying rhythm is atrial fibrillation with right bundle branch block (RBBB). Loss of RBBB occurs at a critical time at the end of repolarization during the supernormal period of the right bundle. Lack of HV prolongation indicates that QRS normalization is not due to simultaneous delay in the left bundle (equal bilateral bundle branch delay).






FIGURE 22-3 Gap phenomenon in the right bundle. During atrial extrastimulation at a coupling interval of 280 ms, conduction occurs over the FP (AH = 123), which encroaches on right bundle refractoriness causing right bundle branch block (RBBB). At 270 ms (FP ERP), conduction occurs over the SP (AH = 273 ms), allowing complete recovery of the right bundle, normalization of the QRS complex, and initiation of typical AVNRT.






FIGURE 22-4 Gap phenomenon in the left bundle. A single atrial extrastimulus (coupling interval = 454 ms) conducts over the FP (AH = 135 ms) and encroaches on left bundle refractoriness causing left bundle branch block (LBBB). LBBB facilitates induction of orthodromic reciprocating tachycardia (ORT) using a left posterolateral AP. Note that the VA interval shortens by 41 ms with loss of LBBB. A shorter coupled extrastimulus (coupling interval = 416 ms) encounters FP refractoriness and conducts over the SP (AH = 255 ms) allowing complete recovery of the left bundle and normalization of the QRS complex. SP conduction facilitates induction of ORT.






FIGURE 22-5 Phase 4 left bundle branch block (LBBB). The underlying rhythm is counterclockwise (CCW) cavotricuspid isthmus-dependent atrial flutter with phase 4 LBBB and infrahisian AV block. LBBB only occurs after long His-His intervals with paradoxical QRS normalization following short His-His intervals. Exposure of the His-Purkinje system to long-short sequences also causes phase 3 block in both bundle branches resulting in infrahisian AV block.






FIGURE 22-6 Phase 4 right bundle branch block (RBBB). A single atrial extrastimulus delivered during sinus rhythm encounters FP refractoriness, conducts over the SP (AH = 294 ms), and induces typical AVNRT. Phase 4 RBBB during sinus rhythm paradoxically disappears during AVNRT.






FIGURE 22-7 Bilateral bundle branch delay. The telemetry strip shows sinus rhythm (asterisk) with 3:2 Wenckebach AV block. Paradoxically, the first QRS complex shows left bundle branch block (LBBB), while the second QRS narrows. During HRA pacing, His bundle recordings show that QRS narrowing is preceded by an 85 ms prolongation of the HV interval, which in the setting of LBBB indicates simultaneous delay over the right bundle. Two His bundle extrasystoles (H′) cause pseudo AV block.






FIGURE 22-8 Supernormality in the right bundle. The underlying rhythm is sinus with complete AV block and a left ventricular escape rhythm. Critically timed P waves falling into the supernormal period of the right bundle (downslope of the T wave of escape complexes) conduct with left bundle branch block (LBBB). Note retrograde P waves (arrows) only follow escape complexes occurring in mid-diastole of the atrium.

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Oct 13, 2019 | Posted by in CARDIOLOGY | Comments Off on Unusual Electrophysiologic Phenomena
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