Assoc Prof Harry Mond
February 23, 2026
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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.
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.
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:
An example:
You work out the physiology. Think of ventricular filling and stroke volume.
October 28, 2026
As its name implies, AVNRT is a reentrant junctional or supraventricular tachyarrhythmia which involves the fast and slow pathways within the AV node creating a self-perpetuating circus movement. It can be divided into typical (slow-fast) or atypical (fast-slow) as well as several rarer forms related to more than one slow or fast pathway with similar electrophysiological properties. On the ECG, they can be identified and differentiated by the positioning of the P wave in relation to the QRS complex, which in turn is related to the direction of the re-entrant AV nodal circuit. However, in about 60% of cases, the P wave is buried in the QRS/T and cannot be identified on the surface ECG.
October 16, 2026
Retrograde conduction is defined as conduction from an ectopic site to the atrium via the AV node or accessory pathway and referred to as VA conduction. II results in a P wave configuration which differs from an anterograde P wave, and in most ECG leads is inverted. The site from where the impulse originates can be the AV node, Bundle of His, distal conducting system or the body of the ventricles including ventricular pacing. On the ECG, the QRS complex usually precedes the inverted retrograde P wave but on occasion may follow it.
October 1, 2026
In 1970, I commenced work as a “novice” cardiologist at The Royal Melbourne Hospital. Training at that time was learning how to recognise the diastolic murmur of mitral stenosis and making sure the ECG and chest x-rays were the correct way up. As this was prior to coronary artery surgery, stenting and echocardiography, the ECG was a highly respected tool for the diagnosis of many conditions. I was fortunate to befriend a number of the gurus of ECG interpretation who taught me to look into the soul of an ECG. Today, even though the ECG is the first investigation performed by a cardiologist, the art of ECG interpretation has been lost because we depend on the new tools for cardiac diagnosis.
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