Syncope and a short pause on event monitor

Written by Willy Frick

A middle aged man was inspecting the food in his (closed) oven when he felt a few moments of light-headedness. He subsequently woke up on the ground, unsure how long he had been unconscious. He presented for evaluation where inpatient workup including monitoring on telemetry was unremarkable. He was discharged with an event monitor and electrophysiology follow up.

The event monitor lasted for four weeks and showed almost no abnormalities, but the following strip was recorded.

What do you think?

Is the finding of just 3 blocked P waves on a 4 week event monitor enough to warrant permanent pacing?

Answer: YES. Remember that even a single episode of cardiac syncope at the wrong time (e.g. while driving) is enough to kill.

If the block is not vagal in nature, the patient should receive a pacemaker. Why do I make this distinction?

There was a randomized trial to determine whether pacemakers could benefit patients with vasovagal syncope, aptly named The North American Vasovagal Pacemaker Study (VPS). Patients with 6+ episodes of syncope and positive tilt table test were randomized 1:1 to pacemaker versus no pacemaker. What did they find?

(From VPS — Connolly et al — JACC, 1999 )

Major benefit to pacing! The relative risk of syncope was reduced by over 85%! The number needed to treat was 2. We almost never see benefit this profound! So...Why don't we pace patients with vasovagal syncope?

Answer: There was no sham control. Half the patients in VPS knew they had a fancy implanted computer to help the heart beat and prevent passing out. And half the patients knew they did NOT get such a device. Fortunately, this study was repeated with sham control. In the repeat study (VPS II), EVERY patient had an implanted pacemaker. Half of the pacemakers had pacing enabled, and half the pacemakers were set to sensing only without pacing. Most importantly, neither the patients nor the doctors knew which patients got which treatment, maintaining appropriate blinding. What did the authors find in VPS II?

(From VPS II — Connolly et al — JAMA, 2003 )

Answer: No significant reduction in syncope!! A stunning result. One of many examples in medical history that remind us of the importance of blinding in clinical trials. Why was there no benefit? Perhaps because the bradycardia in vasovagal syncope is only one part of the autonomic response. There is also systemic vasodilation resulting in transient hypotension and cerebral hypoperfusion. This is why we do not routinely pace patients with vasovagal syncope.

Back to the case:

Can we say any more about the cause of block here? Could it be vagal?

This is known as "paroxysmal atrioventricular block" due to a phenomenon called "phase 4 aberrancy." The short explanation is that a premature beat resets the His. The ensuing (very slight) prolongation in cycle length prolongs the duration of spontaneous diastolic depolarization in phase 4 of the action potential, rendering it refractory. Phase 4 block is also referred to as "bradycardia dependent block." It remains refractory until another extrasystole (escape beat, PVC, PAC, etc.) resets the His, allowing it to resume conduction. If you wish to read more about this, check out the references section at the bottom, especially the articles by Lee and Pillai. 

In the example above, the first PVC (boxed in red below) depolarizes the His at an unfavorable time rendering it refractory. Then three sinus P waves (black arrows) are blocked. Finally, a second ventricular extrasystole (boxed in blue) resets the His, and the next sinus P wave (orange arrow) marks the resumption of 1:1 AV conduction.

The way to recognize this type of block is that it both initiates and terminates with an extrasystole, and there is typically very little change in the underlying sinus rate. By contrast, vagal blocks are accompanied by slowing of the sinus rate.

The patient was given strict guidance not to drive, operate machinery, etc. and was scheduled for pacemaker implantation.

Learning points:

• Diagnose phase 4 block, an indication for permanent pacing

• Explain why we do not routinely pace patients with vasovagal syncope

• Review VPS and VPS II, a historical lesson in the importance of sham control

References:

Connolly, S. J., Sheldon, R., Thorpe, K. E., Roberts, R. S., Ellenbogen, K. A., Wilkoff, B. L., Morillo, C., & Gent, M. (2003). Pacemaker therapy for prevention of syncope in patients with recurrent severe vasovagal syncope: Second Vasovagal Pacemaker Study (VPS II): a randomized trial. JAMA, 289(17), 2224. https://doi.org/10.1001/jama.289.17.2224 

Connolly, S., Sheldon, R., Roberts, R., & Gent, M. (1999). The North American Vasovagal Pacemaker Study (VPS). Journal of the American College of Cardiology, 33(1), 16–20. https://doi.org/10.1016/s0735-1097(98)00549-x 

Lee, S., Wellens, H. J. J., & Josephson, M. E. (2009). Paroxysmal atrioventricular block. Heart Rhythm, 6(8), 1229–1234. https://doi.org/10.1016/j.hrthm.2009.04.001 

Pillai, A., Ellenbogen, K. A., & Padala, S. K. (2021). A tale of 2 blocks. Circulation, 143(10), 1062–1065. https://doi.org/10.1161/circulationaha.120.052981 

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MY Comment, by KEN GRAUER, MD (2/4/2025): 

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Enlightening case by Dr. Frick — with lifesaving chance occurrence of a single short pause over a 4-week period of monitoring, without which the probable result would have been a preventable sudden (and unexplained) death of the patient.

Looking Closer:  The Single Short Pause ...

It is indeed uncanny that an otherwise negative 4-week period of monitoring captured such a "textbook picture" of PAVB (Paroxysmal AtrioVentricular Block) — in which we see both the onset of the several second ventricular pause precipitated by a slightly early ventricular beat (labeled "X" in my Figure-1) — as well as the offset of the pause prompted by a 2nd earlier-than-expected ventricular beat (labeled "Y" in my Figure-1).

• As per Dr. Frick — the primary cause of one or more abrupt pauses with PAVB is a result of a diseased His-Purkinje system. That today's patient may be predisposed to this disorder is suggested in Figure-1 by: i) Obvious QRS widening on this single-lead ECG monitoring strip (suggesting some form of bundle branch block); — and, ii) 1st-Degree AV Block of conducted beats (PR interval ~0.26 second).
• Although the continuous rhythm strip in Figure-1 does not manifest slowing of the atrial rate prior to onset of the ventricular pause (ie, the atrial rate is fairly constant at ~66/minute in the first 2 lines of the rhythm strip) — there is a change in the atrial rate after the return of 1:1 AV conduction (in the form of slight acceleration of the atrial rate to ~81/minute for 6 beats — until resumption of the slower atrial rate at the end of the 3rd line). So, perhaps a component of vagal influence may be operative after all? (ie, 1st-degree AV block — and some rate variability).
• "Devil's Advocate" — Beat "Y" actually looks like a fusion beat (intermediate in morphology between sinus-conducted beats and early ventricular beat "X" ) — which if true, would make for an immediate "reset" of His-Purkinje conduction.

Figure-1: I've labeled today's continuous rhythm strip.

Paroxysmal AtrioVentricular Block (PAVB):

As suggested by Dr. Frick's discussion — and described by Bansal et al (J Arrhythmia 35:870-872, 2019) and Bosah et al (Cureus 14[7]: e27092, 2022) — the entity known as PAVB is a potential cause of syncope that is easily overlooked, and which is potentially lethal (probably more often than is commonly realized).

• PAVB is characterized by the sudden, unexpected onset of complete AV block with delayed ventricular escape — therefore resulting in a potentially prolonged period without any QRS on ECG. Prior to the prolonged pause — the patient manifests 1:1 AV conduction without other evidence of AV block (which is why onset of PAVB is typically so unsuspected! ).
• Because of its totally unexpected onset and propensity to result in sudden death — PAVB is difficult to document and significantly underdiagnosed.
• Three mechanisms for producing PAVB have been described: i) Vagally mediated (ie, Vagotonic Block — as described in the January 31, 2019 post, — and in more detail HERE — with the references listed at the end this Blog post citing instances of transient asystole from excessive vagal tone!); ii) Intrinsic (Phase 4 = pause- or bradycardic-dependent) PAVB; — and, iii) Idiopathic.

i) Vagotonic AV Block:

This form of PAVB is potentially benign when it results from a transient profound surge of parasympathetic tone in an otherwise healthy individual (as might occur with an episode of severe vomiting; a fit of severe coughing; vasovagal reaction from a blood draw).

• The problem with vagotonic PAVB is localized to within the AV Node.
• There will often be a "prodome" of diaphoresis, nausea, dizziness — with the patient aware of imminent fainting.
• Characteristic ECG findings of vagotonic PAVB include progressive sinus rate slowing — often associated with an increasing PR interval and a narrow-QRS escape focus — followed by recovery with progressive return to a normal sinus rate and normal PR interval.

ii) Intrinsic PAVB:

Several names have been attached to this mechanism of PAVB — including most commonly "Phase 4 AV block" and/or PD-PAVB (Pause-Dependent PAVB).

• PD-PAVB appears to be the primary mechanism for the cardiac rhythm in today's case (albeit potential for some vagal contribution given variability in the atrial rate in Figure-1). As per Dr. Frick — the underlying pathology of PD-PAVB is severe His-Purkinje System disease (supported by the finding of QRS widening with conducted beats in today's rhythm strip). This form of PD-PAVB is likely to be fatal unless the patient receives a permanent pacemaker.
• As is superbly shown by Dr. Frick's case — the interesting pathophysiology of PD-PAVB results from chance occurrence of an "appropriately-timed" PAC or PVC that partially depolarizes the diseased HPS (His-Purkinje System) at a specific point in the cycle that renders the poorly-functioning HPS unable to complete depolarization. The resultant prolonged pause in ventricular depolarization may only resolve if another "appropriately-timed" PAC or PVC occurs at the precise point needed to "reset" the HPS depolarization cycle (which presumably explains why the patient in today's case spontaneously recovered).
• Of note — although QRS widening and 1st-degree AV block is seen for conducted beats in today's case — up to 1/3 of patients with PD-PAVB do not show evidence of conduction defects on ECG. This complicates documentation and explains why it is so easy to miss the diagnosis.

iii) Idiopathic PAVB: 

This is the most recently described form of PAVB — in which findings are not fully consistent with either of the other 2 forms.

• The baseline ECG before idiopathic PAVB tends to be normal.
• No "trigger" for PAVB is evident (ie, no source of excessive vagal tone — and no precipitating PACs/PVCs are seen).

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BOTTOM Line: It's important to be aware of the various forms of PAVB — that up until recently were all-too-often undetected, especially in patients without evidence of conduction defect on their baseline ECG.

• It is fortunate that this patient was monitored over an extended period of time ( = 4 weeks). It is humbling for me to see how easily the diagnosis could have been missed in today's patient.