MVP and atrioventricular block

ECG 5A: Onset is atrial pacing and ventricular pacing (DDD mode); programming of MVP mode (Managed Ventricul Pacing, specific to Medtronic devices); atrial pacing not followed by spontaneous QRS; on the following cycle, atrial pacing and ventricular pacing with a short AV delay (80 ms); this short AV delay on the cycle following the blocked P wave is the signature feature of the MVP mode; identical sequence (blocked atrial pacing, atrial pacing and ventricular pacing with short AV delay); return to DDD mode with programmed AV delay (switching when 2 out of 4 atrial activities are blocked);

ECG 5B: Confirmation on the EGMs with initially the DDD mode followed by programming of the MVP mode (programming successful); the intervals show the AV delay: after a blocked atrial pacing, AV delay at 80 ms on the following cycle;

Comments: This tracing shows the specificities of the MVP mode for Medtronic pacemakers. It is essential to understand its operating principles so as not to be surprised by the presence of blocked P waves which could suggest a dysfunction. It is not uncommon to be called by a nurse who considered a tracing with P wave as abnormal.

During MVP mode, if a single atrial activity (detected outside the refractory or pacing period) is not conducted, the device remains in ADI mode and provides an emergency ventricular pacing pulse 80 ms after the next atrial sensed event. A single blocked P wave therefore does not induce switching. On the other hand, if two atrial activities (detected outside the refractory or pacing period) out of four are blocked, the device switches to DDD mode. A ventricular safety pacing is delivered 80 ms after each atrial activity following the missed ventricular beat. Therefore, there cannot be two consecutive blocked atrial activities. The duration of the maximum ventricular pause depends on the sinus rate and the minimum programmed rate (maximum pause more or less equal to half the programmed minimum rate). There is thus only one mode switch criterion (no switching on a long PR for example).

After switching to DDD mode, the device regularly monitors AV conduction, awaiting to return to the ADI mode. The first AV conduction check occurs one minute after switching to DDD mode. During this check, the device switches to ADI pacing mode during one cycle. If the next A-A interval includes a sensed ventricular beat, the sensing control was successful. The device remains in ADI pacing mode. If the next A-A interval does not include a sensed ventricular beat (possibility of a blocked P wave), the conduction control failed and the device returns to DDD mode. The interval between each conduction control is then doubled (2, 4, 8 … minutes to a maximum of 16 hours). If the patient is in complete atrioventricular block, the device operates in persistent DDD mode. Every 16 hours, the device verifies the presence of AV conduction, which results in a missed ventricular beat. All parameters associated with DDD mode apply.

Take-home message: Each manufacturer proposes a specific mode to reduce the percentage of ventricular pacing, each with its specificities. A blocked P wave may correspond to a normal functioning for Medtronic, Biotronik, Boston Scientific and Sorin dual-chamber pacemakers but not for St. Jude Medical devices.