Pacemaker-mediated tachycardia (PMT)

Pacemaker-mediated tachycardia may arise if the pacemaker records a rapid atrial rate (e.g. during a supraventricular tachycardia) and stimulates the ventricles with the same rate. This occurs if ventricular stimulation is set to be triggered by the atrial impulse. Another cause of pacemaker-mediated tachycardia is endless loop tachycardia, which also depends on the ventricular stimulation being triggered by atrial activity. In endless loop tachycardia, the ventricular impulse manages to propagate to the atria where the atrial electrode records the impulse (and assumes that it represents an atrial activation) and thus triggers another ventricular stimulation. This cycle may repeat itself so that a tachyarrhythmia is established.

Pacemaker mediated tachycardia on ECG

Pacemaker beats are mostly easy to recognize on the ECG. Pacemaker stimulation generates a stimulation artifact (“pacemaker spike”). Older pacemakers generate large and visible spikes, whereas newer models may generate very minute or even invisible spikes (at least in some leads). Pacemaker-stimulated beats display wide QRS complexes and a left bundle branch block pattern on ECG; this is explained by the fact that the pacemaker electrode is placed and stimulates the right ventricle. Hence, whenever a patient with a pacemaker presents with tachyarrhythmia and the QRS complexes appear with a left bundle branch block pattern, always suspect pacemaker-mediated tachycardia.

CRT devices (cardiac resynchronization therapy) may also cause tachycardias, and these devices produce slightly narrower QRS complexes (because they stimulate both ventricles simultaneously). A tachycardia with Q-wave or QS complex in lead I suggest that the tachycardia is caused by the CRT device.