Abstract
Case Summary
A 73-year-old man with severe aortic stenosis and preexisting right bundle branch block (RBBB) underwent transcatheter aortic valve replacement. Postprocedural day 4 mobile cardiac outpatient telemetry revealed intermittent atrioventricular block. A 12-lead electrocardiogram obtained in the emergency department revealed PR interval alternans with RBBB, with longer PR following longer preceding RP intervals, and shorter PR following shorter RP intervals. He suddenly experienced cardiac arrest due to proximal atrioventricular block, requiring cardiopulmonary resuscitation.
Take-Home Messages
Inverse decremental conduction, known as Yan conduction, demonstrates an inverse relationship between conduction time and frequency of upstream stimulation. Inverse decremental conduction within the left bundle branch in the setting of RBBB may manifest as PR alternans, heralding proximal atrioventricular block.
Key words: inverse decremental conduction, left bundle branch block, paroxysmal AV block, right bundle branch block, Yan conduction
Graphical Abstract
A 73-year-old man with severe aortic stenosis and preexisting right bundle branch block (RBBB) underwent balloon-expandable transcatheter aortic valve replacement (TAVR). On postprocedural day 1, 12-lead electrocardiogram (ECG) demonstrated sinus rhythm, normal PR interval, and unchanged RBBB with left anterior fascicular block. He was discharged with mobile cardiac outpatient telemetry (MCOT). Although asymptomatic, he was instructed to present to the emergency department (ED) after the monitor detected intermittent nocturnal atrioventricular block (Figure 1A). The 12-lead ECG obtained on arrival to the ED is shown in Figure 1B.
Figure 1.
Inverse Decremental Conduction Causing Alternating PR Intervals and Apparent AV Block
(A) Post-transcatheter aortic valve replacement MCOT rhythm strip recorded during nocturnal sleep demonstrates alternating prolonged PR intervals with intermittent nonconducted beats. The PP interval remains constant. (B) Twelve-lead ECG obtained on arrival to the emergency department reveals right bundle branch block (RBBB) with alternating PR intervals, 12 indicated. (C) Ladder diagram illustrating IDC (Yan conduction) within the left bundle branch (LBB). In the presence of RBBB, IDC in the LBB produces alternating PR intervals, as constant PP intervals minimize AV nodal influence and renders LBB conduction time the sole determinant of the PR interval. A = atria; A–V = atrioventricular node; AV = atrioventricular; BB = bundle branches; ECG = electrocardiogram; IDC = inverse decremental conduction; L = left bundle branch; MCOT = mobile cardiac outpatient telemetry; R = right bundle branch; V = ventricles.
Question:
Based on the MCOT and electrocardiographic findings, what is the underlying mechanism and subsequent management?
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A.
Vagally mediated atrioventricular block; continue MCOT monitoring
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B.
Dual atrioventricular nodal physiology; continue MCOT monitoring
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C.
Phase 4 block at the His bundle; admit for inpatient monitoring
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D.
Inverse decremental conduction (IDC) in the left bundle branch (LBB); emergent permanent pacemaker implantation indicated
The correct answer is D.
Discussion
The MCOT tracing demonstrates nonconducted P waves consistent with atrioventricular block. Given stable PP intervals (no sinus slowing) and the PR alternating between long and short before the onset of atrioventricular block, a vagally mediated mechanism is unlikely (answer A). The 12-lead ECG demonstrates sinus rhythm with alternating long and short PR intervals in the presence of RBBB. In contrast to dual atrioventricular nodal physiology, in which a longer preceding RP is typically associated with a shorter PR interval, the observed pattern here is PR varying in the opposite direction of the preceding RP interval, making dual atrioventricular nodal physiology unlikely (answer B).
Although phase 4 block can theoretically occur at the His bundle after TAVR, it would typically result in only 2 outcomes: normal conduction or block, which does not explain the broad range of PR intervals here. In contrast, a continuous correlation between the PR interval and the preceding RP interval within a specific rate range has been reported in patients with RBBB after TAVR.1,2 Moreover, TAVR-related conduction injury more commonly affects the LBB than the His bundle,3 ruling out phase 4 block at the His bundle (answer C).
Answer D is correct. IDC in the LBB (Figure 1C) in the setting of RBBB accounts for the alternating PR intervals, noting a positive correlation between the PR interval and the preceding RP interval.1,2 Shortly after the 12-lead ECG was obtained in the ED, the patient experienced 2 episodes of sudden cardiac arrest due to paroxysmal atrioventricular block with asystole, requiring cardiopulmonary resuscitation and transcutaneous pacing. He subsequently underwent emergent implantation of a dual-chamber permanent pacemaker.
IDC, also known as Yan conduction,2, 3, 4 was first described by Yan in 2021.1 It commonly occurs in the LBB after aortic valve surgery and is characterized by an inverse relationship between conduction time and the frequency of upstream stimulation within a certain range. This differs from phase 4 block, which typically manifests in 2 forms: normal conduction or block. Furthermore, phase 4 block may be chronically present, whereas our observation suggests that IDC occurred transiently shortly after the aortic valve surgery.
We previously hypothesized that spontaneous phase 4 depolarization during the resting phase of the action potential in injured Purkinje-fibers might underlie IDC.1 However, this mechanism appears unlikely. Enhanced phase 4 depolarization typically increases automaticity within the His-Purkinje system, predisposing to ectopic activity. However, patients with IDC in the LBB rarely exhibit arrhythmias attributable to increased automaticity.
A more plausible explanation for IDC comes from the Jalife et al5 animal experiments: in Purkinje fibers distal to a segment exposed to elevated potassium concentration, phase 0 upstroke velocity and amplitude, and therefore impulse propagation, decreased as the pacing rate in the proximal Purkinje fibers slowed, despite the absence of spontaneous phase 4 depolarization. In patients undergoing TAVR, a similar condition may occur, in which local injury to the LBB creates a region with elevated extracellular potassium. However, the precise mechanism underlying bradycardia leading to reduced sodium channel availability for phase 0 depolarization, thereby resulting in IDC, remains unclear.
Funding Support and Author Disclosures
Drs Yan and Keramati were supported by the Sharpe-Strumia Research Foundation. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose.
Footnotes
The authors attest they are in compliance with human studies committees and animal welfare regulations of the authors’ institutions and Food and Drug Administration guidelines, including patient consent where appropriate. For more information, visit the Author Center.
Contributor Information
Ali Keramati, Email: KeramatiA@mlhs.org.
Gan-Xin Yan, Email: yang@mlhs.org.
References
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