Wide QRS Complex Arrhythmia with Alternating QRS Morphology: What Is the Mechanism?

Area of Science:

• Cardiology
• Electrophysiology
• Cardiac Pacing

Background:

• A 75-year-old male presented with an electrical storm and recurrent implantable cardioverter defibrillator (ICD) discharges.
• The patient had a history of extensive anterolateral myocardial infarction with severe left ventricular dysfunction (ejection fraction 25%) and prior ICD placement for primary prevention.
• Previous endocardial substrate ablation was unsuccessful in managing recurrent ventricular tachycardia (VT).

Observation:

• Despite antiarrhythmic therapy, the patient experienced persistent slow VT with a right bundle branch block pattern.
• Subsequent invasive studies, including mapping during induced VT and endocardial ablation, failed to identify critical arrhythmogenic substrate.
• A percutaneous epicardial approach was planned, and epicardial ventricular pacing readily induced wide QRS complex rhythms with alternating morphologies and prolonged stimulus-to-QRS intervals.

Findings:

• Epicardial pacing induced complex ventricular rhythms mimicking the clinical VT, suggesting a potential mechanism related to abnormal ventricular activation or reentry.
• The prolonged stimulus-to-QRS intervals during pacing indicate significant conduction delays or block within the ventricular myocardium.
• The inability to localize critical sites via endocardial mapping and the success of epicardial pacing suggest a substrate predominantly located on the epicardial surface or within transmural scar.

Implications:

• This case highlights the potential utility of epicardial pacing and mapping in complex ventricular tachycardia cases refractory to endocardial ablation.
• The findings suggest that epicardial approaches may be crucial for identifying and ablating substrates in patients with extensive myocardial scarring and dysfunction.
• Further investigation into pacing maneuvers during induced arrhythmias is necessary to precisely localize critical sites for successful ablation, potentially improving outcomes for patients with electrical storms.