Intermittent atrial sensing in a VDD pacemaker

This report deals with a patient with single-lead VDD pacemaker and intermittent atrial sensing failure. At first glance, the electrocardiogram suggested a VVI mechanism of pacing, being paced QRS complexes dissociated from sinus P waves: this revealed that intracardiac atrial electrograms were too small to reach the sensing threshold. At times, however, a sinus P wave triggered ventricular pacing. This only occurred when the spontaneous atrial activity fell within a critical interval ranging from 430 to 480 milliseconds from the previous paced QRS complex. Analysis of pulsed wave Doppler mitral flow recording demonstrated that atrial sensing and related ventricular pacing occurred whenever a sinus P wave coincided with the opening click of mitral valve. In this circumstance, the atrial volume decrease allowed the freely floating atrial lead to lie close to the right atrial wall, favoring sensing of a small intracardiac electrogram.