Assoc Prof Harry Mond
February 23, 2026
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This is an old pacing ECG, which has a number of interesting features.
It was performed two hours after the implant.
Suggest you look at it carefully before going on.
Firstly, there is high voltage (5V) unipolar pacing. The stimulus artefacts are tall. Remember the stimulus artefact represents the electrical energy delivered to the myocardium via the cathode.
It is a voltage for a set period of time called the pulse duration or width. During this period, there is a voltage drop or exponential decay curve.
There are two ways of delivering this energy via the cathode; unipolar or bipolar.
All electrical circuits must have a negative cathode and a positive anode. With unipolar pacing, there is only one pole on the lead and the anode is on the can of the pulse generator. For bipolar pacing, both are on the lead. The current flow is across the chest for unipolar pacing, but local for bipolar. Hence unipolar pacing gives a tall stimulus artefact.
It is important to remember that a tall stimulus artefact will distort the ECG baseline with a decay curve, which is the after potential.
Consequently, the first 40 to 60 ms may be all artefact, which is not seen with bipolar pacing.The decay curve can be seen with failure to capture or exit block and can confuse the ECG reporter into thinking it is an Q or R wave.
The simulus artefact size is voltage dependent!
Now let us return to the original ECG:
All the ECG leads are distorted by the stimulus artefact. Note there are no R waves from V2 to V6, as all are artefact. The chest leads suggest pacing from the apex of the right ventricle, but leads I to III don’t make sense.
I sent the tech back to repeat the ECG.
The new ECG limb leads (below) are compared with the old ones on the original ECG, which are now very different: Lead I is flipped, leads II and III are reversed, aVL and aVR are reversed and aVF is the same.
The arm leads (left arm and right arm) are reversed.
Here is a summary of reversed arm leads.
October 29, 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.Â
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