What Do You Think 24

23-year old fairy princess with dizzy turns.

Holter monitor tracings:

What do you think?

There is ventricular pacing(red arrow) with a P wave preceding each QRS (As Vp), together with a fixed AV delay(red highlight).

This suggest dual chamber pacing, but what mode?

The middle tracing shows a P wave without a paced QRS (No Vp).

The Vp-Vp pause is 1530 ms (40bpm) with no atrial pacing prior to Vp.

The pacing is:‍

V  Ventricular pacing only  

DD Atria land ventricular sensing and inhibition.

As well, atrial sensing (As) triggers ventricular pacing (Vp) after an AV delay.

This is VDD pacing.

Historically, this was one of the first dual chamber modes, without programming or  sensing, other than the AV trigger (VAT). Early programmable models were used in young patients with congenital complete heart block. In these patients, retrograde conduction was often retained, resulting in troublesome endless loop tachycardia.

Here is an example of VDD pacing with ventricular ectopics.

There is As Vp (red highlight). A ventricular ectopic (yellow highlight) results in a compensatory pause. Once again, there is As Vp, but with the next ventricular ectopic, there is no P wave and thus there is an escape Vp only (blue highlight).

Compare this to DDD pacing, where there is also atrial pacing.

The left tracing shows As Vp following the compensatory pause (820 ms). In the right tracing, the pause is longer (850 ms) and results in atrial pacing (Ap).There is also the commencement of a sinus P wave and thus this is a fusion beat (F). Remember, this is not pacemaker malfunction, as there has been insufficient time for the atrial lead to detect atrial depolarization.

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To summarize normal VDD pacing:

There is As Vp (red highlight).Carotid sinus massage (red arrow) results in sinus slowing and ventricular pacing (Vp)(yellow highlight) only. A P wave in the T wave of a Vp is sensed(blue highlight) and there is a prolonged AV conduction (blue arrow), because the upper rate limit of ventricular pacing (Vp to Vp) cannot be violated.  

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Now let us return to our case study:

The report said Wenckebach AV block. However, the PR intervals (AV delay, red highlight) are fixed and thefirst complex following the dropped beat has no P wave. Apart from the droppedbeat, no footprints of Wenckebach AV block are present.

The other report said complete heat block, as there was no AV synchrony.

The P waves are not sensed (blue arrows) and the programmed lower rate 40 bpm.

The diagnosis is VDD pacing with atrial undersensing.

Another interesting feature of this tracing is the varying sinus P to P cycle lengths. With an embedded Vs, the cycle length is 900 ms, whereas if there is no embedded Vs, the cycle length is1000 ms. This is called ventriculophasic sinusarrhythmia.

To summarize ventriculophasic sinus arrhythmia:

• Seen usually with second degree (<40%) or complete AV block (40%).
• The P-P interval containing the QRS complex are shorter than the P-P interval without the QRS complex.
• Reflex mediated: Intravascular pressures and volumes.

An example:

You work out the physiology. Think of ventricular filling and stroke volume.