Pacemaker-mediated tachycardia

ECG 9A: Wide QRS tachycardia; a careful analysis of the tracing allows demonstrating a small-amplitude pacing artifact from V3 to V5; ventricular pacing; different hypotheses may be considered: supraventricular arrhythmia followed at the maximum rate, pacemaker-mediated tachycardia, sinus tachycardia; atrial activity is difficult to identify in order to refine the diagnosis;

ECG 9B: EGM analysis confirms the diagnosis of pacemaker-mediated tachycardia: uninterrupted succession of VP-AS cycles (ventricular pacing, atrial sensing) with automatic prolongation of the AV delay so as not to exceed the maximum follow-up rate;

Comments: Pacemaker-mediated tachycardia (PMT) is the term used to define pacemaker-driven tachycardia, most often at the maximal pacing rate with repeated ventricular pacing – retrograde atrial conduction sequences. The maintenance of a PMT results from the sensing of a retrograde P’ wave, outside of refractory periods, leading to the triggering of an often prolonged AV delay, which again promotes retrograde conduction after ventricular pacing, thus a continuous cycle ensues. A PMT is therefore a repetitive sequence in which the pacemaker reacts to each retrograde P’ wave by pacing the ventricle at a high rate which in turn generates a retrograde P’ wave. The cycle is thus repeated indefinitely unless there is appearance of a retrograde block or the intervention of a specific pacemaker algorithm.

The presence of retrograde conduction is therefore a prerequisite for the onset and maintenance of a PMT. Retrograde conduction is present on average in 40% of paced patients at rest, all indications combined. It is much more common in patients implanted for sinus node dysfunction than for atrioventricular block. These numbers are sufficiently elevated to justify systematically programming effective protection. In addition to the permeability of retrograde conduction, the onset of a PMT involves the programming of an atrial tracking mode (DDD or VDD) and a momentary or permanent loss of atrioventricular synchronization. Indeed, if ventricular activity is properly synchronized with that of the atrium, the retrograde conduction is blocked. The following events can contribute to a temporary atrioventricular dissociation, a retrograde conduction and the triggering of a PMT: a ventricular extrasystole (most common cause), an atrial extrasystole with prolongation of the AV delay to meet the programmed maximum heart rate, an excessively-long programmed AV delay (the nodo-Hisian pathway is outside of its refractory period at the time of ventricular pacing), external interference or myopotentials sensed by the atrial chain, an atrial sensing or pacing defect, etc.

The first pacemakers were not protected against this type of runaway and the tachycardia could last indefinitely, generating an occasionally very disabling symptomatology. A PMT of prolonged duration may indeed be poorly supported with a symptomatology ranging from simple sensation of malaise or palpitations to cardiac decompensation in patients with an underlying heart disease.

Take-home message: The electrocardiographic diagnosis of pacemaker-mediated tachycardia is based on the demonstration of a tachycardia with ventricular pacing (wide QRS with pacing artifact) and retrograde atrial conduction (negative P’ waves in the inferior leads). This diagnosis can sometimes be difficult (artifacts and P’ waves difficult to identify) but should be systematically evoked in the presence of a regular tachycardia with wide QRS in an implanted patient.