ECG Challenge

ECG Section Editors

Authors: Andrés F. Miranda-Arboleda, MD, Cengiz Burak, MD, Adrian Baranchuk, MD

Non-prodromal syncope and frequent premature ventricular contractions

An 81-year-old male with a previous history of coronary artery disease (CAD), hypertension, and frequent premature ventricular contractions (PVC) presented to the emergency department with 2 episodes of non-prodromal syncope lasting less than 1 minute and recurrent presyncopal episodes. He denied chest pain, angina, palpitations, or heart failure symptoms. His recent cardiovascular work-up showed a normal left ventricle ejection fraction of 58% without other relevant abnormalities on the echocardiogram. A 48-hours Holter monitor showed frequent PVC with a burden of 26%, 2 predominant morphologies, and episodes of ventricular bigeminy. An angiogram performed 4 months before his presentation showed a 99% lesion in the mid-left anterior descending artery that was treated with percutaneous coronary intervention.

His vital signs on arrival showed mild bradycardia with a heart rate of 56 bpm, but they were otherwise normal. Laboratory work-up was unremarkable and did not reveal any potential explanation of syncope. The electrocardiogram (ECG) obtained in the emergency department is shown in figure 1A.

The patient was on bisoprolol 10 mg once daily which was discontinued, 72 hours after his admission he continued presenting similar episodes on telemetry (figure 1B).

figure 1 600


The correct answer is D: Intrinsic paroxysmal - Pause dependent AV block (Phase IV)

Paroxysmal atrioventricular block (PAVB) is a clinical condition characterized by sudden changes in the electrophysiological properties of the conduction system resulting in advanced heart block, syncope and sudden cardiac death. PAVB can be classified into three groups, intrinsic PAVB (I-PAVB) due to underlying disease in the atrioventricular (AV) conduction system, extrinsic vagal PAVB (EV-PAVB) related to parasympathetic influence in the conduction system, and extrinsic idiopathic PAVB (EI-PAVB) which has been associated with low plasmatic levels of adenosine (1).

The I-PAVB, also known as phase 4 block or pause dependent,classically presents in patients with abnormalities in the His-Purkinje system (HPS) (2, 3). As in the presented case whohad baseline left anterior fascicular and right bundle branch block (Figure 1A).

Themechanisms of I-PAVB are related to a partial depolarization of a diseased HPS during the final part of the depolarization (phase 4), triggered by premature ventricular contractions (PVCs) that lead to concealed retrograde AV node conduction modifying the refractory periods and causing conduction delay (Figures1A and 1B) (2, 3).

The diagnosis of PAVB is often overlooked because of no clear evidence ofAV disease during 1:1 conduction (1). Our patient presented with non-prodromal syncope and the displayed ECG (Figures 1A and 1B) revealed I-PAVB triggered by a compensatory pause after PVC. Other forms of AV block were ruled out based on the absence of gradual slowing of the sinus rate and prolonging PR interval as inEV-PAVB and second-degree Mobitz I AV block. In the present case,AV block was always triggered by PVCs allowing us to rule out pseudo atrioventricular (AV) block due to concealed His bundle extrasystole and extrinsic idiopathic paroxysmal AV block.

The only available therapy for patients with syncope related to I-PAVB is permanent pacing (1). The discussed patient underwent a dual chamber pacemaker implantation with complete resolution of his symptoms during follow-up.

The diagnosis of PAVB is challenging, the current classification facilitates the identificationof patients at higher risk of intrinsic HPS disease, in order to define therapeutic interventions, especially in patients presenting with syncope.

Key teaching points:

  • ECG showing 1:1 conduction does not exclude advance block in patients presenting with syncope
  • Premature ventricular contractions can trigger paroxysmal AV block in patients with underlying His Purkinje System disease (I-PAVB - phase 4).
  • Permanent pacing is the only available treatment option in patients with intrinsic paroxysmal AV block.