Pacemaker pacing rate is far below the lower rate and hence not functioning normal. The sensed AV delay must be less than 200ms if the onset of the P-wave is taken into consideration. Atrial oversensing would not have resulted in violation of lower rate. It would have caused rather unnecessary ventricular pacing. Most likely T-wave oversensing by the ventricular lead.

The tachycardia was noted during acute inferior - lateral posterior wall MI (ECGs are shown below). This is accelerated idioventricular rhythm driven by peri AV junctional region in the ventricle. The relatively wide QRS (particularly seen in lead aVR 110ms) should raise the suspicion of aberrant AV conduction (during sinus rhythm or supraventricular tachycardia) or ventricular tachycardia. The sharp QRS complexes however are against the diagnosis of ventricular tachycardia even though abnormal QRS axis in limb leads (North West axis) raises the suspicion of ventricular tachycardia. The ST segment elevation in inferior leads with loss of R-waves in these leads and also in the lateral precordial leads with preservation of the R-wave in V1 raise the suspicion of Inferior-Lateral posterior wall MI. The retrograde P-wave during the tachycardia are seen more clearly in the inferior leads at the end of the QRS.

The mode of tachycardia termination many a times help in making the most probable diagnosis. Among the supraventricular tachycardia, a specific pattern of termination – atrial electrogram followed by termination almost rules out atrial tachycardia. A premature ventricular electrogram without associated atrial electrogram terminating the tachycardia also most often rules out atrial tachycardia. It also rules out AVNRT if it coincides with the His bundle electrogram. The pattern of termination of the present tachycardia does not help in establishing or ruling out atrial tachycardia, AVNRT (atypical), AVRT.