Tracing

At the beginning of the tracing, atrial pacing with intrinsic atrioventricular conduction; blocked atrial pacing; ventricular pause; after 2 seconds, the pause criterion is fulfilled (programmable parameter: pause of 2, 3 or 4 seconds); switching to DDD mode (vertical line);

Comments

The fourth switching criterion to DDD mode is the presence of a programmable ventricular pause of 2, 3 or 4 seconds. The duration of the ventricular pause may slightly exceed the programmed value such that atrioventricular activation can be synchronized at the time of the mode switch:

• the duration of the pause corresponds to the programmed value + the AV delay at rest: this case occurs when no atrial depolarization has occurred within 500 ms preceding the end of the duration of the programmed pause; the device paces the atrium at the end of the programmed pause, followed by the ventricle at the end of the programmed AV delay and then switches to DDD mode.
• the duration of the pause corresponds to the programmed value + a shorter duration than the AV delay at rest: this case occurs when the intrinsic atrial depolarization occurs just before the end of the programmed pause (value less than the programmed AV delay); the device paces the ventricle at the end of the programmed AV delay and then switches to DDD mode.
• the duration of the pause corresponds to the programmed value: this occurs when an intrinsic atrial depolarization occurs within 500 ms preceding the end of the duration of the programmed pause; the device prolongs the AV delay over one cycle to pace exactly at the end of the programmed duration of the pause and then switches to DDD mode.

When the minimum rate is programmed in the conventional manner (in the order of 50 to 60 bpm), the pause and BAV III criteria can overlap or be exclusive. Indeed, in this case, if the pause is programmed at 4 seconds, the BAV III criterion (2 consecutive atrial activities) will be fulfilled before the pause criterion and the maximum duration of the pause will never be reached. In contrast, when the pause criterion is programmed at 3 seconds, the 2 criteria can be fulfilled on the same tracing.

The pause criterion was designed to prevent the occurrence of a prolonged ventricular pause exceeding 2, 3 or 4 seconds. When the minimum rate is set to 60 bpm, a pause time of 4 seconds cannot be reached, as the AVB III criterion will be met before.