We report an unusual cause of syncope in a 59-year-old woman with underlying third-degree heart block and VVI pacemaker insertion after aortic valve replacement.

Six months after initial pacemaker placement, the patient presented with palpitations and syncope. Electrocardiographic monitoring disclosed multiple asystolic episodes with pacemaker noncapture. Also noted were episodes of what seemed to be atrioventricular sequential pacing with two pacemaker spikes accompanying each QRS complex (Figure 1). In these tracings, both spikes occurred at a rate of 70 beats/min. The electrocardiogram also showed brief episodes of single spikes without ventricular capture, resulting in asystole. It became clear that there were two "populations" of pacemaker spikes: one programmable and the other not programmable. The nonprogrammable spikes did not result in ventricular capture and inappropriately suppressed the programmable spikes, leading to periods of asystole. When the patient's left pectoral pacemaker was programmed to asynchronous VOO mode, reliable 1:1 capture was obtained. It was postulated that a second pacemaker was providing a stimulus that was sensed by the pectoral pacemaker as ventricular electrical activity. This inhibited appropriate stimulus discharge from the functional pacemaker. Search for another unit resulted in its location deep in the epigastric region.

View larger version (28K): [in this window] [in a new window]   Figure 1. Electrocardiogram showing suppression of normally functioning bipolar pacemaker (pectoral unit) by malfunctioning unipolar pacemaker (epigastric unit). Small downward spikes originating from the functioning bipolar pacemaker are labeled by arrows, and large upward spikes from the nonsensing and noncapturing unipolar pacemaker are labeled by dots. There is a pause in the unipolar pacemaker between the fourth and fifth spikes, presumably caused by oversensing of the T wave. This resulted in the unipolar pacing artifact (the fifth unipolar spike) now landing outside the refractory period of the bipolar pacemaker. The bipolar pacemaker senses the unipolar pacing artifact as a QRS complex and thus is inhibited. The result is a string of large unipolar pacing spikes without ventricular capture, causing syncope.

Review of the patient's medical records from another hospital confirmed that a second permanent pacemaker had been implanted because of malfunction of the first unit. The events surrounding this event were not well documented, and the cardiac surgeon who had performed both procedures had died unexpectedly.

In general, the first step with pacemaker noncapture is to reprogram the pacemaker to higher output values [1]. If this fails, a second pacemaker may be placed; the first pacemaker is usually removed. Although inappropriate suppression of one permanent pacemaker by another has not previously been reported, a similar situation is relatively common in patients with a temporary pacemaker who undergo permanent pacemaker implantation [2]. In this situation, during pacing threshold evaluation of a newly implanted permanent electrode, subthreshold pacing stimuli may inappropriately inhibit the output of a temporary pacemaker. Our patient continues to do well.