In the evaluation of twenty-three patients with implanted QRS-inhibited lithium-powered pulse generators (Intermedics C-MOS-1 and ARCO L1-2D), we repeatedly observed the phenomenon of partial pacemaker recycling (PPR), in which the escape interval induced by a premature ventricular beat is shorter than the automatic interval (AI). In order to determine the sensing properties of these pulse generators, programmed chest wall stimulation (CWS) was systematically performed in all 23 patients and, in addition, intracardiac programmed ventricular stimulation (PVS) via temporary intravenous pacing catheters was performed in 9 of them. The AI of these pulse generators ranged from 820 to 860 msec and absolute refractory periods (ARP) from 220 to 330 msec. Precise correlation in determining pacemaker sensing properties could be demonstrated between CWS and PVS. The phenomenon of PPR occurred 110 to 240 msec following ARP with both CWS and PVW, and appeared to be time- rather than voltage-dependent. We conclude that: (1) CWS is a safe, simple and accurate method for studying pacemaker sensing properties, and (2) time-dependent PPR is a normal electrical feature of certain implanted QRS-inhibited lithium-powered pulse generators.
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