Diagnostic Considerations
Any rapid atrial rhythm, such as atrial tachycardia, atrial flutter, or atrial fibrillation, sensed by the atrial lead, can then be tracked by the pacemaker to the upper rate limit. [8] This is confirmed when intracardiac electrograms are examined or by evaluation of the underlying rhythm. In unipolar pacemakers, myopotentials from the chest wall muscles or electromechanical signals from an electrocautery device in the operating room may be oversensed and also may drive the pacemaker to the upper rate limit. [9]
Extremely rare, but of concern, in older pacemakers, the pacemaker circuitry may malfunction, resulting in a runaway pacemaker in which the pacemaker paces the heart at a very rapid rate and this cannot be programmed off. [10] All the above problems require active involvement of the pacemaker to maintain the tachycardia but are not reentrant in nature and so not considered classic pacemaker-mediated tachycardia.
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Pacemaker-Mediated Tachycardia. Telemetered ECG tracing with surface lead II (top) and intracardiac electrograms (atrial electrogram [center] and ventricular electrogram [lower]) and marker channel (bottom) showing pacemaker-mediated tachycardia (PMT). The intracardiac markers indicate that the retrograde P waves, labeled AS for atrial-sensed event, occur 280 milliseconds after the ventricular-paced beats, labeled VP.
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Pacemaker-Mediated Tachycardia. Telemetered ECG tracing showing atrioventricular (AV)–paced rhythm at 60/min after termination of the pacemaker-mediated tachycardia (PMT). The tracing, from top to bottom, shows lead II, atrial electrogram, ventricular electrogram, and marker channels. The intracardiac markers indicate the rhythm is atrial paced (AP) and ventricular paced (VP). Note that the VP beats are ventricular pseudofusion beats.
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Pacemaker-Mediated Tachycardia. This is a typical example of PMT with ventricular pacing at maximum tracking rate (VP-MT) and then termination of the tachycardia as the atrial sensing (AS) is in the postventricular atrial refractory period (PVARP). This is due to PVARP extension, which is a feature of this particular pacemaker. The solid line indicates where PMT is detected and this is the point at which PVARP extension occurs. As this electrogram was detected, but not sensed to be acted upon, the ventricular tracking stopped and the tachycardia terminated. In some cases, pacemakers have a program to lengthen the PVARP after PMT detection to potentially stop the tachycardia. Alternatively, prevention of one ventricular paced beat can also stop the tachycardia. Some pacemakers use this algorithm.
