[EXP.RP5]

Tracing

Initial atrial sensing and ventricular sensing (PR cycles); atrial undersensing and atrial pacing with QRS complex sensed in the safety window (Vr); this cycle is considered as a blocked paced atrial beat; atrial undersensing on 2 consecutive cycles with atrial pacing; on the first cycle, the QRS complex falls in the safety window (Vr), on the second cycle it falls in the post-atrial ventricular blanking; these 2 consecutive atrial paced beats are considered as blocked; AVB III criterion and switching to DDD mode (vertical line); continuation of atrial undersensing with 2 atrial paced beats; on the first cycle, the QRS is sensed after the safety window and is marked Vs; on the second cycle, the QRS complex is sensed in the safety window causing forced ventricular pacing after a 95ms AV delay (cycle Ap, Vr, Vn);

Comments

This tracing allows highlighting the specificities of the post-atrial ventricular refractory periods. In SafeR mode, even if the functioning of the device is designed to be that of an AAI or ADI mode, it is essential to protect the ventricular channel after atrial pacing to avoid crosstalk which could lead to the occurrence of an asystole during an AV block episode. Indeed, atrial pacing is delivered in Volts while ventricular sensing functions in mV with a ratio of 1 per 1000; the atrial stimulus will therefore necessarily be sensed by the ventricular channel in the absence of protection, hence the need for a refractory period.

The ventricular blanking is triggered after each atrial pulse (Ap).

• the post-atrial pacing ventricular blanking is non programmable (30 ms)

The safety window follows ventricular blanking and lasts 95 ms minus the programmed length of the ventricular blanking.

• in DDD mode, when a ventricular signal is sensed, a ventricular pulse is emitted at the end of the safety window
• in SafeR mode, when a ventricular signal is sensed, no ventricular pulse is emitted at the end of the safety window

CLICK HERE FOR MORE INFORMATION ON VENTRICULAR REFRACTORY PERIODS

Memory interrogation of a pacemaker operating in SafeR mode allows viewing the EGMs corresponding to the switching related to ventricular sensing in the safety window. Most often, these are late premature ventricular contractions or an atrial sensing failure as in the present example.