This study evaluates the effect of nuclear magnetic resonance (NMR) scanning on pacemaker function. It must be emphasized that each manufacturer's pulse generators and each pacing modality may behave differently and, therefore, require individual evaluation. According to our results, patients with pacemakers should have their pacing activity monitored continuously during scanning with the NMR 1500 gauss imaging system. External pulse generators should be set to the asynchronous mode and placed outside the NMR image volume but within the radiofrequency (RF) shield. Implanted pacemakers should be verified for type and mode of operation. All implantable pulse generators evaluated reverted from the demand to the asynchronous mode within the magnetic field of the scanner. There was no observable damage to the discrete pacemaker components that were tested. In vivo testing of implantable single-chamber pulse generators did not significantly alter the pacemaker's operating parameters. Changes in stimulation rate analogous to the RF field pulse rate were seen. In single-chamber devices the resultant rate was a multiple of the RF frequency, changing to a value less than the normal asynchronous magnetic rate. With more sophisticated dual-chamber devices the results varied. With VDD pacing during RF scanning, the cardiac stimulation rate increased to a value analogous to the RF field modulation period. More extensive in vivo testing using different models of pulse generators of various manufacturers is needed to identify specific device susceptibility to the RF, time variance and steady-state magnet fields. From these data a comprehensive statement about NMR scanning of patients with implanted pacemakers can be made.

This study evaluated effects of extracorporeal shock wave lithotripsy on four models of Medtronic implantable cardiac pacemakers. In vitro testing consisted of: (1) unsynchronized pacemaker strapped on the patient with extracorporeal shock synchronized to the patient's native heart rate; and (2) pacemakers suspended alone in water 6 inches from the focal point, synchronizing the extracorporeal shock to pacemaker output. Unsynchronized shocks affected each model of pacemaker differently, i.e., single chamber constant rate pacemakers experienced extended periods of inhibition for more than three pacing cycles while activity-triggered rate response pacemakers exhibited rate increases to the upper rate setting. Dual chamber synchronous pacemakers exhibited intermittently a 59% decrease and a 20% increase in ventricular rate due to inhibition and triggering, respectively, from shock oversensing. Synchronized shocks did not alter the rate of single chamber constant rate pacemakers, but did cause the rate to increase to the upper rate setting for activity-triggered rate response pacemakers. The shock was synchronized to the initial atrial output from the dual chamber pacemaker and caused frequent inhibition of the ventricular stimulus when the ventricular-safety-pace (VSP) feature was programmed off. Programming VSP on reduced the incidence of ventricular inhibition resulting in near normal pacemaker operation. There was neither observable damage to pacemaker components nor spurious reprogramming of pacemaker parameters during the tests. Our studies with one manufacturer's pacemakers suggests that lithotripsy shock effects on implantable pacemakers can be tolerated provided: (1) the single chamber pacemaker is programmed to the demand constant rate modality; (2) the dual chamber pacemaker is programmed to VSP on or to the VVI mode; and (3) the pacemaker distance to the focal point is greater than 6 inches.(ABSTRACT TRUNCATED AT 250 WORDS)