A pacemaker-mediated tachycardia (PMT) can be defined as any condition in which a pacemaker paces the ventricles at rates that are inappropriately fast. This can be due to (1) a rate response setting that is too sensitive, (2) tracking of atrial noise (such as what may occur with electromagnetic interference), (3) inappropriate pacemaker manipulation with rate response turned on, or (4) tracking of an atrial tachyarrhythmia related to upper rate settings.
Traditionally, however, the term pacemaker-mediated tachycardia, also called endless-loop tachycardia, is used to refer to a form of a reentrant tachycardia and can occur in patients who have dual-chamber pacemakers. The pacemaker forms the anterograde (atrium to ventricle [A → V]) limb of the circuit and the atrioventricular (AV) node is the retrograde limb (ventricle to atrium [V → A]) of the circuit.[1] This classic form is referred to and discussed as PMT for the rest of this article.
ECG images below.
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. 
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. 
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 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. NextPathophysiologyThe following is the most common scenario causing PMT. A dual-chamber pacemaker programmed DDD or VAT, but not DDI, is implanted. The patient must have retrograde (V → A) conduction with an atrial activation time that is longer than the programmed postventricular atrial refractory period (PVARP). A ventricular-paced beat or a properly timed premature ventricular contraction (PVC) conducts retrograde via the AV node (or an accessory pathway, if present) to the atrium. If the atrial depolarization occurs after the set PVARP, but before the next timed atrial-paced beat, ventricular pacing will be triggered at the programmed AV interval.
PMT tends to occur at or near the programmed upper rate limit and depend upon the programmed AV delay and the PVARP. This generates an incessant reentrant arrhythmia circuit that persists as long as there is continuous VA conduction with atrial activation outside the PVARP. The pacemaker forms the antegrade limb of the circuit and VA conduction forms the retrograde limb as the essential critical components.[1] In many instances, the PVARP varies so that it shortens with rate. This can make PMT more likely. The AV interval can be programmed to change with the rate. This tends to reduce the risk of PMT. Depending on the pacemaker programming and the VA conduction, the rate of the PMT may vary, but it is bounded by (and generally occurs near or at) the programmed upper rate limit.
While PMT is commonly initiated by an isolated ventricular ectopic beat, it can also be initiated by failure to capture the atrium with a paced beat followed by a ventricular paced beat. If this occurs, the atrium is then amenable to depolarization by the impulse conducted retrograde from the ventricle. The tachycardia continues until retrograde conduction ends or the atrium becomes refractory. Specific pacemaker algorithms may also terminate PMT if tracked rates persist at the upper rate limit. The pacemaker can be programmed to lengthen the PVARP after a PVC or with an incessant tachycardia at the upper rate limit or prevent one atrial sensed event from being tracked. Up to one third of patients with antegrade complete AV block have intact (or intermittent) retrograde (V → A) conduction.
Other situations that can result in PMT include intermittent atrial undersensing. An atrial premature beat with a long A sense-V pace interval (due to a long programmed AV interval) in a patient with heart block can also initiate PMT as any antegrade concealed depolarization otherwise present cannot prevent VA conduction in this instance.[2]
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