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Assoc Prof Harry Mond
December 2, 2024
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ECG artefact. What does this mean?I was handed this ECG and asked if it was an example of a rate adaptive pacing, which uses changes in transthoracic impedance to increase the pacemaker rate in response to physiologic demand.
Firstly, note that this is not a paced rhythm. However, these pacing algorithms create regular artefact on the ECG tracing, whether they are pacing or not in the ventricle. In the above illustration, the artefact initially appears regular at about every 50 msec. It is seen best in the limb leads, but with the rhythm strip of lead II, they “deteriorate” later in the strip.I then reprinted the ECG as rhythm strips.
Quite clearly the this is not regular in all leads and is therefore extracorporeal artefact.In 1987, the Australian pacemaker company Telectronics, created a rate adaptive sensor, requiring a standard bipolar lead. The rate response was achieved by passing a very small subthreshold unipolar current of 1mA for 15 microseconds every 46 ms from the anode to the pulse generator can across the thorax.
Sensing of this current was between the cathode and the pulse generator can. With each respiratory cycle, the impedance of the current changed reflecting changes in respiratory rate and tidal volume i.e. minute ventilation. The ECG shows the small very regular spikes each 46 msec.Boston Scientific still uses minute ventilation as a dual sensor with activity.Here are two 12-lead ECG examples, showing the sensor artefact in patients who are ventricular paced.
The size of the spikes is very dependent on the ECG filters and not infrequently very small in leads I, V5 and V6. This must be the way the ECG leads are orientated.Another impedance sensor is “Closed Loop Stimulation or CLS” by Biotronik. This is not transthoracic, but rather biphasic intraventricular stimulation and the energy is delivered into the paced or sensed QRS, 50 msec after the commencement of depolarization and continues for 250 msec.
The ECG features are very characteristic and cannot be mistaken for the minute ventilation sensor. Note that in this example, there is no ventricular pacing.
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 26, 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
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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