Mechanism of an unusual QRS pattern associated with right ventricular pacing

Abstract

Right ventricular pacing is conventionally accompanied by a QRS complex with a left bundle branch block (LBBB) morphology. Occasionally, right ventricular stimulation has been reported to result in a right bundle branch block (RBBB) QRS morphology.’ In this report, widely differing QRS complex morphologies associated with different right ventricular endocardial stimulation sites are described. J.P., a 65-year-old man with coronary artery disease and left ventricular aneurysm was admitted for recurrent ventricular tachycardia. A resting ECG showed sinus rhythm with an old inferior wall myocardial infarction and persistent ST-T changes of an inferior ventricular aneurysm. Multiple antiarrhythmic drugs, including quinidine, disopyramide, procainamide, and propranolol, were unsuccessful. Electrophysiologic studies were performed for management of this tachyarrhythmia. All antiarrhythmic therapy was withdrawn. Programmed right ventricular stimulation at multiple pacing cycle lengths demonstrated no retrograde ventriculoatrial conduction. Retrograde His bundle deflections or the V, phenomenon were not noted. A bipolar electrode catheter placed in the right ventricular apex showed LBBB paced QRS patterns. The following morning intermittent right ventricular capture was noted with an increase in threshold (7 mA). The paced QRS pattern was interpreted as extreme right axis (-240 degrees) with a RBBB morphology (Fig. 1). Fluoroscopic examination showed the catheter was within the right ventricular cavity pointing posteriorly; a free-floating catheter tip was noted on cineradiographic examination. Unipolar intracardiac electrograms demonstrated no injury currents with the catheter in situ or during its subsequent withdrawal. Repositioning in the right ventricular apex anteriorly showed normal pacing patterns for this site (Fig. 2). G.K., a 57-year-old man with known congestive cardiomyopathy was hospitalized for near syncopal episodes and congestive heart failure. A resting ECG revealed sinus tachycardia with LBBB. Holter monitoring revealed complex ventricular arrhythmias. After withdrawal of all cardioactive drugs for 36 hours, pacing during electrophysiologic studies was performed from various locations in the right ventricle. Right ventricular apical pacing