What is the Risk of Sudden Cardiac Arrest With a Decremental Atrio-fasicular Pathway?

We describe the case of a 20-year-old male with known asymptomatic manifest pre-excitation who suffered from a ventricular fibrillation (VF) arrest. The patient was sitting in class when he suddenly collapsed. He was quickly resuscitated by his classmates, and was defibrillated at the scene three times by an automated external defibrillator (AED). The patient was transferred to our emergency room by EMS. He was awake and talking on arrival. He suffered no end-organ damage except transient short-term memory loss that resolved within a few days. 

His history revealed known manifest pre-excitation, as can be seen on the ECG in Figure 1. This was diagnosed at age 13 when he complained of two episodes of pre-syncope with heavy exertion. This led to risk stratification (at another facility) with a transesophageal atrial pacing study. At baseline, pre-excited 1:1 conduction was lost while pacing at cycle lengths (CL) shorter than 320 ms. The accessory pathway (AP) effective refractory period (ERP) following a pacing drive train at 600 ms was 330 ms. With isoproterenol infusion, pre-excited 1:1 conduction was observed while pacing at a cycle length of 220 ms, and the AP ERP following a drive train of atrial pacing at 500 ms was 220 ms. Neither atrial fibrillation (AF) nor supraventricular tachycardia could be induced with programmed atrial stimulation with or without isoproterenol. The clinical scenario was deemed consistent with dehydration, and his family was reassured that his risk of future arrhythmic events was low. 

Our work-up included an echocardiogram and a cardiac MRI. These showed a structurally normal heart with preserved left ventricular size and function. There were no anomalous coronary arteries. The right ventricle was normal in size and function as well. There was no myocardial gadolinium delayed enhancement to suggest scarring. 

The AED was interrogated. The initial rhythm was confirmed to be VF. The first shock defibrillated the patient into a slow ventricular escape rhythm with PVCs. A long-short sequence induced VF again (Figure 2). This happened a second time requiring a third shock that converted the rhythm to AF with rapid conduction to the ventricles. During AF, the shortest observed pre-excited R-R interval was 240 ms (Figure 3). AF then spontaneously converted to sinus rhythm.

An invasive electrophysiology study was pursued. This revealed a decremental atrio-fasicular (Mahaim) pathway. The atrio-fasicular pathway (AFP) could only conduct in an antegrade fashion. At baseline, pre-excited 1:1 conduction was lost while pacing at cycle lengths shorter than 320 ms, as can be seen in Figure 4. The AP ERP following a pacing drive train at 600 ms was 340 ms (Figure 5). Antidromic atrio-ventricular reciprocating tachycardia (AVRT) utilizing the AFP was easily inducible (Figure 6). With isoproterenol infusion, pre-excited 1:1 conduction was observed while pacing at 220 ms CL (Figure 7). During isoproterenol infusion, rapid pacing could only induce non-sustained AF. Conduction during non-sustained AF, however, was not rapid nor did it utilize the AFP despite isoproterenol, as can be seen in Figure 8. The AFP pathway was then successfully mapped and ablated on the lateral right atrial free wall at the 10 o’clock position on the tricuspid annulus. 

Discussion

WPW syndrome is defined as the presence of manifest pre-excitation on the surface ECG, and its association with AVRT or other atrial arrhythmias with rapid conduction. Asymptomatic WPW is the presence of a pre-excitation pattern on the ECG without associated symptoms of palpitations or pre-syncope. WPW, symptomatic or asymptomatic, is associated with a small but real increase in sudden cardiac death (SCD) incidence.¹ Even though VF is rare, it can be the first manifestation of WPW.^2-4 The incidence of SCD is very low in multiple natural history studies ranging from zero to 0.015 events per patient-year.¹ Pappone et al reported an incidence of 4.5 episodes of SCD per 1,000 patient-years in a prospective recent study of asymptomatic adults with WPW during a 38-month mean follow-up.⁵ 

Risk stratification of WPW is recommended even for young asymptomatic patients.¹ This should start with a non-invasive study such as an exercise test to look for sudden loss of pre-excitation with higher heart rates. If a non-invasive study is not reassuring, utilizing an invasive electrophysiology study for risk stratification is reasonable. Our best evidence for increased risk of sudden cardiac death is when the shortest pre-excited RR interval (SPERRI) during induced atrial fibrillation measures at ≤250 ms. Other measures for increased risk, such as AP ERP ≤240 ms or conduction properties during isoproterenol infusion, are less specific.¹ In clinical practice, one must always weigh the risks and benefits of treating WPW with ablation. This is particularly challenging in young and asymptomatic patients. Even with a low complication rate of ablation, the incidence of SCD is extremely low. The risk/benefit ratio must be considered for every WPW patient, particularly younger patients who have higher complication rates.⁶

In our patient, due diligence was pursued to risk stratify him at a young age with a transesophegeal pacing study. His accessory pathway could not conduct at baseline at cycle lengths shorter than 320 ms. Neither atrial fibrillation nor AVRT could be induced. His risk of future arrhythmic events was deemed low. Yet he presented with a VF cardiac arrest. His accessory pathway proved to be a decremental AFP. This is usually deemed to represent a lower risk for SCD.^7,8 His pathway at baseline in our study had poor conduction. This, however, improved drastically with isoproterenol infusion. The interrogation of his AED also added to the uncertainty of his presentation. He had a second and third VF episode related to long-short PVC-induced VF. This could be attributed to the patient being down for several minutes with global hypoperfusion and ischemia. Rapidly conducting AF was seen on the AED interrogation after the third shock, but no sustained AF could be induced in the lab. 

Although the patient’s AFP was successfully ablated, we remained uncertain about his risk of future arrhythmic events. Why would AF while sitting in class conduct rapidly enough to induce VF in a pathway that has poor baseline conduction? Our understanding of SCD with WPW is that rapidly conducting AF degenerates to VF. Are there other mechanisms that can induce VF? This uncertainty was discussed with the patient and his family, and after much deliberation, an ICD implant was recommended and pursued. 

Summary

This case exemplifies a major challenge to all electrophysiologists. We have to risk stratify young patients for lethal and often unforgiving arrhythmias. Our clinical data remains lacking, and we still have much to learn. We do not have long-term prospective data to help us establish evidence-based guidelines. Recall bias of similar cases persuades physicians to be more aggressive in pursuing ablation, when one has to remain careful and individually weigh the risks and benefits of each case. 

Disclosure: The authors have no conflicts of interest to report. 

References

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