Mode of operation induced by rapid external chest wall stimulation in patients with normally functioning QRS-inhibited (VVI) pacemakers.

The effects of rapid external chest wall stimulation were evaluated in 10 patients with normally-operating unipolar, lithium-powered, QRS-inhibited pacemakers functioning in their control VVI (QRS-inhibited) mode. Stimuli delivered at slow rates resulted in the expected pacemaker inhibition. On the other hand, during rapid (greater than 900/min) external chest wall stimulation, 8 pacemakers reverted to a VOO mode, 1 to a VVI mode and 1 to either a VVI or VOO mode. No pacemaker was totally inhibited by rapid chest wall stimulation. In accordance with the manufacturer's specifications, the rate to which the implanted pacemakers reverted during the VOO modes produced by the fast external stimuli were the same as those of the VOO modes induced by proper application of an external magnet. Although rapid external chest wall stimulation proved to be a safe procedure, more studies are necessary to determine its usefulness in the follow-up of patients with implanted QRS-inhibited (VVI) pacemakers. At present, conclusions cannot be drawn regarding whether these pacemakers will revert to a mode of operation similar to that induced by rapid external chest wall stimulation when subjected to extrinsic sources of strong electromagnetic interference.