THE development of the inhibitorydemand pacemaker was a significant step in the evolution of a "prosthetic" cardiac conduction system for man.1 This unit monitored the heart rate by sensing the R wave, recycling without firing when the rate was adequate, and firing at an appropriate rate when bradycardia occurred. The problem of unwanted competitive pacing from the older fixed-rate pacemaker was thus solved. The solution of one problem, however, often creates new ones. It was soon apparent that such pacemakers could sense electromagnetic potentials other than the intrinsic myocardial potentials.2 These extraneous signals are collectively referred to as electromagnetic interference (EMI). These signals from such sources as automobile ignition systems and small electric motors were able to produce, in a few cases, unwanted and potentially dangerous inhibition of the pulse generator. The phenomenon occurred more readily in unipolar than bipolar pacemakers because of the larger antenna area between