Look at this ECG closely.Does it look normal?The AV delay timings don’t make sense. The AV delay for atrial sensing/ventricular pacing (As Vp) is always longer than with ventricular sensing (Vs).In this example the AV delay is longer with As Vs.Let us review the fundamental rule of AV conduction.

A dual chamber circuit is designed with a physiologic interval after atrial contraction (As/Ap) to allow ventricular filling. To accomplish this, there is a programmable AV delay, which terminates in ventricular pacing as shown above with Ap Vp. However, if normal AV conduction occurs earlier, then there is ventricular sensing or Ap Vs and ventricular pacing is inhibited.The AV delay can also shorten in a physiologic manner with exertion.

There is clear violation of the fundamental rule of AV conduction on the first ECG! How do we explain this?For many years, we have been told that right ventricular pacing at least from the apex may result in left ventricular dysfunction. Therefore, in patients who do not require ventricular pacing, it should be avoided where possible. One way to avoid ventricular pacing is to use single chamber AAI(R) pacing. The incidence of progression to high degree atrioventricular block is small and between 0.6% and 1.8% per annum, although, not unexpectantly, considerably higher in patients with pre-existing bundle branch or bifascicular block.Another way is to use dual chamber pacing with a prolonged AV delay as shown.

This is at rest. If rate adaptive pacing is programmed ON, then with exertion, the atrial pacing rate will increase, and atrial contraction will lie at the end of systole during closed AV valves. These patients may become very symptomatic with a form of pacemaker syndrome.Pacemaker manufacturers have now developed a range of programmable algorithms to avoid ventricular pacing, when the pacemaker is programmed in the dual chamber mode.There are seven algorithms from five manufacturers:

Those in black are conversion algorithms, where in the event of AV block and a dropped beat, the pacemaker is programmed from AAI(R) to DDD(R).Those in red are AV hysteresis algorithms. They remain in DDD(R) all the time, but simply extend the AV delay to encourage AV conduction. The programs are ALL different with characteristic appearances on the ECG which may be misinterpreted as pacemaker malfunction.Once in DDD(R)-VP, there are scheduled conduction tests where if AV conduction is recognised, convert to the appearance of Ap/As-Vs.A feature common to all these algorithms is the ECG appearance of the AV delay being longer for As/Ap – Vs than for As/Ap – Vp. Let us return to the first ECG:

This is the generic appearance of a successful scheduled conduction test common to all algorithms, except Boston RHYTHMIQ, (no scheduled conduction test).Remember: An apparent violation of the fundamental rule of the AV delay is due to a ventricular minimisation algorithm.In future Fun with ECGs I will review a number of these algorithms which result in bizarre ECGs.Harry Mond