Most contemporary demand pacemakers contain a magnetic reed switch easily actuated by the application of an external magnet over the pulse generator. This report describes the unusual response of two implanted pulse generators to the testing magnet. The magnetic reed switch of one pulse generator was found to 'stick' in the open position making it impossible to revert the pulse generator to its fixed-rate mode with the magnet. The other pulse generator exhibited erratic pacing while the magnet was being held completely still over it, probably because the additionalforces imposed by the magnet upon the already swollen batteries unmasked imminent pacemaker failure.These observations constitute new additions to the growing list of demand pacemaker problems.Every technological innovation in pacing systems creates its own set of diagnostic problems. The widespread use of the testing magnet to actuate the reed switch mechanism of contemporary demand pacemakers is no exception to this statement, as illustrated by the two examples described in this report. EE interval lengthened momentarily upon application of the magnet, the automatic interval subsequently remained absolutely constant at 894-6-896-o msec. The demand automatic interval failed to change despite careful exploration of a large area overlying the testing magnet. The pulse generator was easily suppressed by chest wall stimulation (Fig. i, strip B) (Barold et al., I970). Repeated application of the magnet at various sites during chest wall stimulation did not actuate the reed switch so that fixed-rate pacing could not be achieved (Barold and Gaidula, I97Ia).Two months later, the demand automatic interval measured 895 4-895 7 msec and while the magnet again caused temporary prolongation of the EE interval, reversion to fixed-rate pacing did not occur during chest wall stimulation. The patient has remained well and when last seen in early 1972, the automatic interval was constant at 895-o-895-5 msec. On this occasion, repeated manipulation of the magnet toward and away from the pacemaker could no longer prolong the EE interval. The automatic intervals with and without the magnet over the generator were identical and unchanged. Chest wall stimulation easily suppressed the pulse generator but application of the magnet in various positions over the pulse generator could not actuate the reed switch. The procedure was repeated with two and later three magnets on top of one another, but reversion to fixed-rate pacing was found impossible.Case 2 A transvenous 5842 Medtronic demand pacemaker was inserted transvenously in an 84-year-old man with complete AV block and congestive heart failure. The patient remained well and 6 months later the demand automatic

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