In a dual-chamber pacemaker, the AV delay determines the maximum time interval between an atrial event and the expected delivery of ventricular pacing. This interval corresponds to the electronic equivalent of the PR interval.

There are separate AV delays for paced and sensed atrial events.

These are programmed to ensure perfect mechanical coordination between atria and ventricles, whether the atrium is sensed or stimulated. The length of these intervals can be programmed over a wide range, fixed or adaptive, and is influenced by a large number of algorithms.

Stimulated AV delay

The stimulated AV delay is applied following atrial stimulation when the stimulator is operating in DDD, DDI, DVI and DOO modes.

Detected AV delay

Detected AV delay is applied following a detected atrial event when the stimulator is operating in atrial synchronized pacing mode (DDD and VDD).

Setting the AV delay

The AV delay that results in optimal hemodynamics varies considerably from patient to patient. Precise adjustment of the AV delay should ensure that the temporal relationship between left atrium and left ventricle remains constant, and that left atrial systole is completed before the onset of left ventricular systole.

Permanent programming of long AV delays (increased risk of PMT and pacing in a vulnerable period of undetected ESV in post-atrial ventricular blanking, poor hemodynamic results with E-wave and A-wave fusion) in non-dependent patients to promote spontaneous conduction can be avoided today thanks to the use of specific modes previously described (MVP, AAI-SafeR…).

If the patient is dependent (complete atrioventricular block), the mean value of the optimal detected AV delay at rest is around 110 ms.

Difference between stimulated and detected AV delays

The optimal detected AV delay is shorter than the optimal stimulated AV delay for a number of reasons:

• the P wave is never detected at the very start of the surface P wave, but when the atrial depolarization wave passes under the electrode. This is often later than the onset of the P wave on the surface ECG.
• when the P wave is stimulated, the conduction time between the right and left atria is lengthened.
• the difference between detected and stimulated AV delays depends on the position of the probe in the right atrium: on average 30 ms if the probe is inter-atrial septal, 50 ms if it is in the atrium, 70 ms at the high part of the right atrial border and 90 ms if it is lateral low. These values are averages, and in theory programming should be adapted to each individual. These differences are often longer if there is an intra- and/or inter-atrial conductive disorder.
• this difference changes little with effort, with a tendency to shortening under the influence of catecholamines, which reduce inter-atrial conduction time. If there is a major inter-atrial conductive disorder, this difference may increase with exercise. In practice, this value can remain fixed throughout the programmed frequency variation range.