Figure 35-4 A pacemaker-mediated tachycardia (PMT) follows magnet removal from a pacemaker. This patient had a dual chamber pacemaker implanted for atrioventricular (AV) nodal disease, and she was pacemaker dependent for ventricular activity. She had a sinus rate of 75 beats/min before the magnet application with appropriate ventricular pacing (strip not shown). Her programmed AV delay is 200 msec. With magnet application, her pacemaker produced asynchronous AV sequential pacing (DOO mode) at a rate of 60 beats/min. This strip is from an electrocardiographic recorder that enhances the pacemaker artifact with small, downward arrows. Because the asynchronous magnet rate of this device is lower than her intrinsic atrial rate, many of the atrial pacing stimuli were applied during an atrial refractory period (i.e., functional noncapture). A consequence of atrial noncapture can be retrograde AV nodal conduction, with depolarization of the atria after the depolarization of the ventricles. The retrograde P waves are shown with the upward arrows. While the magnet is applied, this retrograde depolarization of the atria is ignored. Shortly after the magnet was removed (open arrow), there was a paced ventricular event, followed by retrograde AV nodal conduction. With the ensuing depolarization of the atria from this retrograde conduction, the pacemaker sensed an atrial event and responded by pacing the ventricle 200 msec later. Another retrograde P wave appears, and each pace in response to a retrograde P wave created yet another ventricular pace. The result is a PMT at the upper tracking rate (programmed here to 130 beats/min) of the pacemaker. PMT from retrograde AV nodal conduction can occur in any DDD or VDD device with magnet removal, with a premature ventricular contraction, or with a noncaptured atrial pace. Treatment of this PMT entails reapplication of the magnet. Some pacemakers can be programmed to eventually break PMT by delaying one AV cycle when pacing at the upper tracking limit.