Assoc Prof Harry Mond
February 23, 2026
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Urgent referral to pacemaker clinic:
AV block with failure to pace in the ventricle and 1.7second pause.
What do you think?
There is atrial pacing with a prominent stimulus artefact (SA), a P wave (P) and normal AV conduction (QRS/T).
On the bottom tracing, leadII, the baseline is isoelectric and flat (red arrow) apart from the stimulus artefact. There is no T wave from the previous QRS (red highlight), no P wave (yellow highlight) and no QRS (blue highlight) with the next beat.
This is clearly an ECG artefact, where the recording has been wiped or maybe filtered. I do not understand the mechanism with modern ECG and Holter monitor machines. In past years, it was not unusual with tape-based Holter monitors and probably related to fluctuations in the power source.
Most early single channel ECG machines had an “instow” switch, which if activated would act as a filter for artefact. At that time, most pacing systems were unipolar, and the ventricular stimulus artefact would result in gross filtering for 400 ms.
Unipolar ventricular pacing.
Instow is ON, and the QRS/T waves are filtered for 400 ms after the stimulus artefact in leads I and II, but not in III. When turned OFF, theQRS/T waves are now seen.
Here is another example of filtering. Note it is not the stimulus artefact causing this.
The baseline is isoelectric from the end of the T wave to the next T wave (red arrow). In leads I, II, andIII there is no P wave and QRS (red highlight) for one beat.
With non-paced rhythms, it is most common just to see a single “lost” QRS and sometimes a T wave (red highlight).
When the T wave is included, it can be very difficult to differentiate the “lost” QRS/T from Wenckebach AV block.
Above: Lost QRS/T wave with isoelectric flat baseline (red arrow). The PR interval before and after the pause are the same (red highlight).
Below: Wenckebach AV block. The PR interval before the lost beat is longer than after the dropped beat.
The ECG glitches described are electronic and may be related to filtering. There is another type of ECG glitch which is mechanical and related to uneven paper delivery or “sticky” rollers.
ECG sinus rhythm with uneven paper delivery due to “sticky” rollers. Normal complex (red highlight). The first complex has a long PR interval and wide QRS (yellow highlight) and the second a miniscule P wave with a vertical line for the QRS (blue highlight).
Other bizarre confusing patterns can be seen.
When only the QRS/T waves are involved Wenckebach AV block can be mis-diagnosed (red highlight). The first complex has a P wave and a miniscule QRS, whereas the second last only a T wave which resembles a P wave.
The appearance of an extra QRS/T mimics an atrial ectopic (yellow highlight).
Harry Mond
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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