Assoc Prof Harry Mond
February 23, 2026
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I thought I would discuss bundle branch block.
Can I hear “Booorring”?
Maybe it isn’t so boring.
Look at this ECG.
The rhythm strip shows sinus rhythm with two QRS configurations; one narrow (yellow highlight) and one wide with a right bundle branch block configuration (red highlight). This is a bidirectional rhythm (or tachycardia if greater than 100 bpm).
Let us look at the tracings more closely.
Although not so obvious, the P waves are bidirectional (red arrows and magnified inserts) and PR intervals are identical.
This is an alternating right bundle branch block. Both alternating left and right bundle branch block are infrequent, and I believe that many of those reported as such are actually late (end diastolic), atrial (aberrant) or ventricular ectopic bigeminy.
Here is an example of supraventricular tachycardia with alternating right bundle branch block.
The rate is 120 bpm at rest and the P waves regular
Here is another tachycardia example. Although the rhythm is sinus 60 bpm (red arrows), the actual rate is much faster because of atrial bigeminy (blue arrows).
This example shows interpolated atrial bigeminy with intermittent aberration rather than true right bundle branch block.
The ectopic prematurity is subtle and more obvious when magnified.
Then there is first degree AV block and alternating right and left bundle branch block. This is only rarely seen as it is transitional to complete heart block.
The next one continues to puzzle me, and I don’t have the full answer.
The resting 12-lead ECG rhythm strip lead II shows a bidirectional rhythm.
Rhythm strip: P waves are regular and almost identical. Assume sinus rhythm! The QRS complexes show alternating right (red highlight) and left (yellow highlight) bundle branch block.
Let us look at more rhythm strips.
V1 shows an interesting QRS transition pattern (red highlight).
With the right bundle branch block, the PR interval gradually increases in Wenckebach sequences, the QRS narrows (less aberration).
Anyone think of a better explanation.
I couldn’t finish without this ECG, which is frequently misdiagnosed as end diastolic ventricular bigeminy (red highlight).
How would you diagnose this bidirectional rhythm?
The clue is in the rhythm strip.
Alternating pre-excitation; “Wolff-Parkinson-White syndrome”.
There is more with bundle branch block to be discussed next time.
Remember, it’s all in the timing
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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