Assoc Prof Harry Mond
February 23, 2026
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I was asked for an opinion regarding this ECG.
The reporting doctor had requested a repeat ECG.
Patient has a pacemaker.
What do you think?
Should the ECG be repeated?
Let us start by looking at V1.
The background rhythm is atrial fibrillation (AF).
There is ventricular pacing (Vp).
There are 4 other “spikes” or pulses with a similar voltage (red highlight), but not as intense (pulse width) as the Vp stimulus artefact.
These pulses are due to subthreshold (below voltage required to pace) stimuli delivered into the heart during systole and once delivered, the “resistance” is measured between the lead cathode and the pulse generator can. The measurement is called intracardiac impedance and changes in this impedance can be used to indirectly determine cardiac contractility which in turn can be used to change the pacemaker rate, according to physiologic demand. The pulses are unipolar (cathode to pulse generator can) and thus prominent on the surface ECG.
The rate adaptive pacing system is called closed loop stimulation (CLS) and is sold by Biotronik, a German manufacturer of cardiac implantable electronic devices, including pacemakers. In situations where the heart rate cannot change with emotion or exertion, such as sick sinus syndrome and atrial fibrillation with a slow ventricular response, the system allows the pacing rate to rise and fall physiologically.
The CLS system has 16 subthreshold biphasic pulses delivered each 46 ms for 250 ms(yellow highlight). The pulses commence50 ms after the onset of a paced or sensed QRS. (blue lines)
The ECG shown earlier only had 4 pulses. The difference lies in the filters in the ECG machine that reduce or block the pulses. With this ECG, there is no filtering of the pulses but following each pulse there is a blanking zone where the next oneor two pulses are blocked from being detected on the ECG.
Let us review a number of these ECGs to highlight the way ECG machines deal with such artefact.
Atrial pacing (Ap) and ventricular sensing. There is a 50 ms initial delay (red highlight) followed by a bunch of artefacts for 250 ms finishing in the T wave (yellow highlight). The artefactis seen differently in each lead. The filter deals differently with each pulse and there is no blanking.
Here are examples of marked blanking with and without atrial (blue highlight) and ventricular paced rhythm.
How can you be sure this is CLS artefact?
On occasion a patient with the CLS sensor programmed ON will demonstrate no artefact on an ECG. This is because of the excellent work of the filters.
In other situations, the pulses continually change.
Maybe filter response is related to the size of the pulses which alter with respiration.
Although the atrial pacing stimulus artefact can be seen throughout the tracing (red arrows, Ap), at the end of the strip the atrial stimulus artefacts increase in size and the CLS artefact is pronounced. The filters have changed, or respiration allows recognition on the ECG.
The minute ventilation sensor also uses impedance measurements but this time measures transthoracic impedance. Minute ventilation, the product of respiratory rate and tidal volume is a highly physiologic variable, which closely reflects the metabolic demands of exercise, including cardiac output and heart rate.
Transthoracic impedance varies with respiration, falling with inspiration and rising with expiration. The system uses a conventional bipolar pacing lead.
A: Low energy, subthreshold pulses every 50 ms. The pulse generator recognizes the changes in impedance (green arrow) as changes in tidal volume and the oscillations as respiratory rate, the product of which is minute ventilation. B: The ECG demonstrates the pulses as continual regular interference (red highlight).
The 12-lead ECG can be very dramatic and is usually diagnosed as extra-corporeal interference with the ECG repeated several times.
Note the absence of artefact in leadI (red highlight). The arm leads lie at a distance from the current and do not detect the pulses. The ECG machine also recognizes the pulses as interference(yellow highlight). The interference is present with both paced and sensed rhythms.
Again, because of filtering, a variety of appearances can be seen.
Hyperventilation will result in pacing at the programmed upper sensor rate, which may result in the diagnosis of ventricular tachycardia.
The patient suffered from severe anxiety. Following sedation, the rate fell to 90 bpm. The underlying rhythm is atrial fibrillation and there are no interference pulses.
The pulse generator may also detect extra-corporeal pulses resulting in inappropriate sensor upper rate pacing.
Patient in emergency room, awake and alert. On attachment of ECG monitoring electrodes, the heart rate rose to sensor upper rate. With this monitor, a small current is passed through the chest electrodes and the skin impedance measured. A marked rise occurs if a monitoring lead is detached from the skin. Despite the pacemaker stimulus artefacts (V5) on a 12-lead ECG (red highlight), ventricular tachycardia was diagnosed, and cardioversion unsuccessfully attempted many times.
It is important to recognize these artefacts to prevent continually repeating the ECG.
The answer to the initial question is don’t repeat the ECG.
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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