With dual chamber pacing, there are two programmable AV delays.

The paced AV delay is the interval between atrial (Ap) and ventricular pacing (Vp) (red highlight), whereas the sensed AV delay is from P wave sensing (As) to ventricular pacing (yellow highlight). The ECGs are from the same patient and demonstrate a 40 ms difference, which relates to the time required for the pacemaker to sense the P wave. This difference is usually considered when programming AV delays.The term “AV delay” is also loosely used with atrial pacing-ventricular sensing, although it must be emphasized that this is not a programmed time interval. In the literature, I have referred to the differences in the AV delay seen with ventricular sensing and pacing as “the law of AV conduction”.

To summarize: The programmed paced AV delay (Ap Vp, red highlight) is always longer than the sensed AV delay (Ap Vs, yellow highlight). If AV conduction results in ventricular sensing, it is intuitive that it will occur before completion of the AV delay unless it is a fusion beat. Why then is the law so important? Today, there are pacing algorithms that violate this law to encourage AV conduction. On inspection of a paced ECG, it can be deduced from the AV delay, whether this is normal pacemaker function or a programmed algorithm to minimize ventricular pacing.Another programmable AV delay feature is rate adaptive AV shortening.

With exercise, the paced AV delay (Ap Vp) shortens from 220 ms to 140 ms. The main reason is to extend the upper rate limit.A rate adaptive dual chamber system will allow atrial and consequently ventricular pacing to increase in response to physiologic demand. However, there is a rate that atrial pacing cannot exceed. This is called the upper rate limit. To determine this limit, the basic timing events of dual chamber pacing need to be understood.

The pacing cycle length is from Ap to Ap. This is divided into two periods; the “total atrial refractory time (TARP)” and the period of normal atrial sensing, the “atrial tracking interval (ATI, green highlight)” which is consumed with exercise as the upper rate is reached. The TARP is also divided into two periods, the AV delay and the “post ventricular atrial refractory period (PVARP)”, both of which can be programmed to shorten with physiologic demand. As shown, a programmed low rate of 60 bpm will have an upper rate of 120 bpm. Programming a shorter AV delay and PVARP will allow the upper rate to increase further.With atrial sensing (As Vp), the atrial rate may exceed the upper rate limit, and this will result in an atypical Wenckebach response. On the ECG, the rate increase can be abrupt with 2:1 block, which must be differentiated from alternating ventricular exit block.

At rest the sinus rate is 80 bpm. There is a rapid rise in the sinus rate to the upper rate limit of 150 bpm, when 2:1 block occurs with a ventricular heart rate of 75 bpm. With rest, the block resolves.On occasion, longer Wenckebach sequences can be seen.

As Vp with ECG at rest. P waves (red vertical arrows) are at the upper rate limit of 120 bpm (500 ms) and there is an increasing AV delay (red highlight) until a dropped P wave occurs (red stippled vertical arrow). The minor artefact was due to a fine tremor and the diagnosis was hyperthyroidism.A confirmatory way of diagnosing an upper rate limit Wenckebach block is to use interrogated atrial (A EGM) and ventricular (V EGM) electrograms and markers.

DDDR. 5:4 Wenckebach. There is an increasing AV delay (red highlight) until a dropped beat (blue highlight) which lies in the PVARP (black highlight). The next atrial complex is paced as the patient is exercising and the atrial paced rate is also high. The atrial and ventricular electrograms demonstrate a Wenckebach response.The consequences of a long AV delay have previously been discussed under atrial pacing. During exercise, a long programmed paced AV delay (red highlight) may also result in atrial contraction whilst the AV valves are closed.

Violation of the law of AV conduction in order to minimize ventricular pacing will be discussed in a future “fun with ECGs”.Harry Mond