Assoc Prof Harry Mond
February 23, 2026
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This ECG was reported as sinus rhythm with a short run of atrial fibrillation.
What do you think?
The first part is correct. There are sinus P waves present (red vertical arrows).
The short irregular run(red highlight) has rapid regular P waves (blue arrows) rate about 170 bpm and is an atrial tachycardia. There is 2:1 AV block with alternating non-conducting P waves (blue stippled arrows). The conducted P waves have a Wenckebach AV sequence (red vertical arrows) and the tachycardia is terminated before a dropped beat. These two AV blocks are called alternating Wenckebach.
How often do these bizarre blocks occur with atrial tachyarrhythmias?
I went through my file on atrial tachyarrhythmias and found the following:
Sinus rhythm (red arrows) with a short run of atrial tachycardia, rate 170 bpm (blue arrows). The blue stippled arrows do not conduct, but those that do have an AV Wenckebach PR prolongation appearance, which ignores the dropped beats.
As the atrial rate and block increases, so do the bizarre AV block patterns:
The atrial rate is now over200 bpm and the block increases. There is a pattern of 2:1, 3:1, 4:1 block with solid arrows conducted and stippled, not conducted. A single sequence (red highlight) once again shows a Wenckebach AV sequence with an increasing PR interval before a dropped beat (green stippled arrow).
There are consistent patterns of non-conducted P waves.
Rapid atrial tachycardia or flutter (vertical arrows) with variable conduction and non-conducted beats(stippled arrows) which fall into a pattern. When two sequential waves are not conducted (yellow highlight), a new sequence commences. A long sequence (red highlight), demonstrates Wenckebach AV block with conducted beats (red horizontal arrows). The second dropped beat (green stippled arrow) terminates the sequence, which starts again, but now without an intervening dropped beat. This termination of a Wenckebach sequence with two dropped beats and a new sequence commencing with a conducted beat is characteristic of alternating Wenckebach.
Don’t believe me! Here is another. Work it out yourself.
In the literature, clinical alternating Wenckebach is usually sinus rhythm and marked bradycardia.
See if you can work out what is going on. It looks like complete heart block, but the ventricular response is irregular.
This can also present with a bundle branch block, syncope, and sudden death.
Are the tachycardia and bradycardia sequences the same mechanism? I am aware that such blocks can occur with rapid atrial pacing at electrophysiology studies.
If someone would like to add any more information does please email me.
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.
September 18, 2026
Dr Thura Thant asked for an opinion on this confusing ECG. By using arrows and highlight, the ECG bared its soul and gave me an answer. See if you can work it out? What do you think?
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