A “drop” in the atrioventricular conduction curve and dual-pathway electrophysiology

Figures

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  Figure 1. Atrioventricular (AV) conduction curve showing a drop and the transition from fast pathway (FP) to slow pathway (SP) conduction seen in 3 preparations (A, B, C). The AV conduction curve was plotted using the atrial premature coupling interval (A1A2) in the abscissa and the atrial-His interval (A2H2), representing the AV conduction time, in the ordinate. The transition from FP to SP conduction judged by the His electrogram (HE) alternans is indicated, as well as the drop (red arrow). Note that a drop always occurred later (at shorter A1A2) than the FP to SP transition in all 3 preparations.

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  Figure 2. Beats with a drop and His electrogram (HE) alternans seen in 3 preparations (A, B, C). Atrial electrograms from interatrial septum (IAS), crista terminalis (CrT) together with the inferior His electrogram (IHE) recording at the premature beat preceding the drop (black traces), with the drop (red traces), and the overlapped traces of the 2 beats are shown in each preparation (A, B, C). The atrial echo beat induced by the premature beat is marked with “e”. There were 2 preparations with a drop resulted in a fast pathway (FP) pattern (A, B) and 1 preparation showing a slow pathway (SP) pattern (C).

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  Figure 3. Panel A shows simultaneously recorded atrial electrograms from interatrial septum (IAS), crista terminalis (CrT), and nodal cellular action potentials (AP) from a fiber in the superior nodal domain, together with the inferior His electrogram (IHE) during shortening of atrial prematurity A1A2. The numbers at AP traces indicate the corresponding A1A2 (Color matched). Note the presence of an early, decremental wavefront (1, arrow) and a late wavefront (2) at A2 beat as A1A2 gradually shortened. At the beat resulting in a drop (A1A2 = 75 ms, green traces), the first wavefront decreased further as expected, however, the second wavefront occurred much earlier, together with the atrial echo beat on IAS trace, no atrial activation on CrT trace. The His bundle activation showed a low IHE (FP pattern). To better illustrate these changes, panel B shows only 2 episodes: the episode before the drop (black traces) and the episode resulting in a drop (red traces), and the overlapped traces of these 2 episodes. Note that at the beat before the drop (A1A2 = 80 ms), the timing of HE and the second wavefront were almost at the same time, but at the beat resulting in a drop (A1A2 = 75 ms), the…

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  Figure 4. An example that formation of intranodal reentry likely resulted in His bundle activation. Note that in this case, superior His electrogram (SHE) was recorded. The recordings were taken at the same A1A2 = 130 ms. The A2 beat resulted in AV block in 1 episode (black traces). The action potential (AP) recording was from a fiber in the center of the node near the inferior nodal domain and showed a small local response, without His bundle activation. In another episode with the same A1A2 (red traces), however, there was His bundle activation (low SHE, slow pathway pattern). The AP recording showed an identical initial local response but followed by an additional activation (arrow), indicating an intranodal reentry occurred, but without atrial echo beat. The overlapped signals of these 2 beats are also shown.

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  Figure 5. Schematic illustrations showing hypothetical mechanisms that formation of intranodal/nodal–atrial reentry could result in a drop in the AV conduction curve. The electrical excitation sequence and functional dissociation (superior nodal–His bundle domain in blue, and inferior nodal–His bundle domain in red) are shown. For a detailed illustration of fast pathway (FP) conduction and slow pathway (SP) conduction, please refer to our recent article by Ma S and colleagues.17 Here we only illustrate the potential reentries that could lead to a drop in the AV conduction curve. A. at short A1A2, the normal FP conduction was blocked (curved green arrow in the superior nodal domain). The SP also failed to directly conduct to the His bundle (curved yellow arrow) but instead formed a nodal–atrial reentry and activated interatrial septum (IAS). The IAS excitation could then be conducted to the superior His bundle domain (blue arrow) through the “shortcut” located at the anterior/superior nodal region, resulting in an FP pattern of His bundle activation. B. another example of reentry formation resulting in a drop in the AV conduction curve. At short A1A2, the normal FP conduction was blocked…