Single lead dual-chamber pacing

Abstract

The patient was a 62-year-old man routinely ventilated and paced at 90 beats per minute following an uncomplicated triple coronary artery bypass graft. The temporary pacemaker was set in atrial pacing AAI mode in order to provide atrial transport and a narrow QRS complex. The atrioventricular (spike-to-QRS) interval was 0.20 s. The paced P waves were isoelectric in this lead; their presence was inferred by atrial pacing at different rates (not shown). The staff reported that at times the atrial lead was pacing the ventricle (Fig 1, upper strip). The ventricular pacing was associated with a drop in mean blood pressure of 10 to 12 mm Hg. While the pacemaker output was being decreased to determine the pacing threshold, atrial pacing was resumed (Fig 1, bottom strip), with improved blood pressure recorded on the readout. No movements or changes in the ventilator or patient status were observed apart from the deliberate reduction in the pacemaker output. This had to be accounted for. Whereas transvenous temporary pacing wires can “flip” between the right ventricle and the right atrium, this was not (and still is not) known to happen to epicardial leads inserted by the surgeon during the operation. The pacemaker was next set into a sequential (DDD) mode, proving that the lead pacing the ventricle was the original atrial lead (Fig 2, upper strip). The ventricular spike, as programmed, followed the atrial spike by 0.12 s and fell in the middle of the broad (paced) QRS complex. This was harmless, but looked vaguely alarming; the pulse generator was reset to the AAI mode. Next, repeated manipulation of the output showed that, at higher outputs, the lead behaved as a ventricular pacer and at lower outputs, as atrial From Canberra Hospital, ACT, Australia.