A week ago, I presented a case on a patient with atrial pacing [AAI(R)] and a very prolonged AV delay which deteriorated to Wenckebach AV block with increasing atrial pacing rates. The low rate was 60 bpm. The same patient had a very interesting Holter 24-hour rate histogram. (Seen above).The histogram shows double counting overnight as the atrial stimulus artefacts are recognised as QRS complexes.What is more important is the lower rate of 60 bpm. There is intermittently a second lower rate of 45 bpm, which violates one of the fundamental rules of cardiac pacing; a rate lower than the programmed lower rate.Here is the ECG with atrial pacing at two lower rates

Think of the ways this can happen.

• Rate hysteresis. This is a programmable algorithm that refers to an escape interval or sensing period longer than the pacing cycle.

In the above figure, the patient has underlying atrial fibrillation with a slow ventricular response. Although there is ventricular pacing at 50 beats per minute (bpm), pacing will not occur until a sensing period of 1500 millisecond (ms) or 40 bpm has expired. The patient or physician could detect a pause in the pulse, confirmed on the ECG and suspect pacemaker malfunction. This can be also used with atrial pacing. In our case there was no atrial sensing.

• Rest, sleep rate or night mode are programmable algorithms available in single and dual chamber pacemakers designed primarily to lower the heart rate to a more physiological value when the patient is at rest. In this way, the function may encourage the patient’s intrinsic rhythm.

The programmed low rate is 70 bpm. However, when the patient goes to sleep, the rate falls to 60 bpm. The algorithm may be based on an accelerometer, so that at rest, the rate falls. Other algorithms use a timer and sleep times may be automatic or need to be programmed. In all cases, activity will cancel the rest rate. In our case the pacing rate slowing is not nocturnal.These algorithms are clearly not the cause of the intermittent daytime nocturnal violation of the lower rate.Let us review the ECG tracing again.

Following the atrial stimulus artefact (or atrial sensing), there is an atrial refractory period during which there is no atrial sensing. The length of this period varies considerably between manufacturers and may be automatic depending on atrial sensing or pacing and may be, but not always, programmable.In the above case, the programmable default atrial refractory period was 350 ms. The AV delay was 340 ms and deteriorated further with increasing pacing rates. Consequently, there was a very narrow window, where the QRS could be sensed by the atrial lead. This is referred to as far-field R wave sensing and is very important with dual chamber pacing where double counting by the atrial channel can lead to mode switching.So, we have two very unusual and critical factors that together lead to atrial oversensing and “apparent” violation of the lower rate limit; a very narrow zone of open atrial sensing and far-field R wave sensing. When the R waves are detected, they are logged as atrial sensing and the pacing rate slows. This is the first time I have seen this as atrial pacing, and is not commonly used. I could find only one old reference in the literature.The issue can be very easily solved by lengthening the atrial refractory period or programming atrial sensitivity to a less sensitive (higher number) value.Remember, normally there is no R wave sensing with AAI(R) pacing. Here is an example of unipolar atrial pacing and a multifocal ventricular couplet.

Whenever I show this illustration, I am told that it is pacemaker malfunction. However, this is NORMAL atrial pacing. With bipolar pacing we usually don’t see the stimulus artefacts.Harry Mond