In the previous “fun with ECGs”, we learnt how to recognise obvious artefact and, in many cases, determine the origin of the interference. There is another form of ECG interference, which I call subtle artefact, which can mimic cardiac arrhythmias and, in some instances, leads to serious misdiagnoses and inappropriate treatment.The list is extensive, and we will only cover the more common situations.Here is a list:

• The lost QRS
• Mimics an atrial tachyarrhythmia
• Mimics a ventricular tachyarrhythmia
• Mimics ventricular ectopy
• Mimics asystole

The lost QRS.On occasion there is a QRS missing in the tracing, which may lead to further unnecessary investigations. Let us look at some examples:

This was reported as ventricular pacing (red highlight) with intermittent exit block. The cycling period or spike to spike intervals all vary, which cannot occur with VVI pacing. The red open circle shows a unipolar stimulus artefact without a QRS, but there is a T wave. You cannot have repolarization without depolarization! The last paced complex (yellow highlight) is narrow and bizarre. The cause is irregular delivery of ECG paper due to sticky rollers. “Having lunch on the ECG machine”.

Regular atrial pacing (Ap) with AV conduction (Vs). Absent T wave (red highlight) followed by an atrial stimulus artefact, no P or QRS waves but a T wave (yellow highlight). The baseline is isoelectric. I have no idea what caused this, but the appearance excludes a pacemaker abnormality. Most likely an issue with filters and not a loose electrode as all leads are involved.

This one (red highlight) is most likely due to the filters. Mimics an atrial tachyarrhythmia.This ECG was reported as atrial fibrillation.

The rhythm is regular, which makes you suspicious! There is almost always one lead that gives a clue that it is sinus rhythm and mechanical interference. In this case the clue is in lead II (red highlight).With mechanical interference, look at the leads involved.

The tremor is in the limb leads and this was from a patient with Parkinson’s disease.It gets confusing if the combination is Parkinson’s disease and atrial fibrillation.

With mechanical interference, follow the QRS complexes through the artefact (red arrows). There is no change in rate from sinus rhythm (red highlight).

This was reported as artefact but there is an arrhythmia and the rate changes dramatically (red arrows).

This is a run of focal atrial tachycardia with block (red highlight).This was called an atrial triplet!

The QRS and T waves are too narrow to be real. “Sticky rollers”.Another case of sticky rollers called “paroxysmal supraventricular tachycardia” (yellow highlight).

Mimics a ventricular tachyarrhythmia.Despite being in only one lead this was called ventricular tachycardia.

To confuse ECG interference with a ventricular tachyarrhythmia is a very serious error with potentially grave consequences. One study (Am J Cardiol. 2001;110:35-8) provided an ECG with artefact to 766 physicians. Almost 50% made an incorrect diagnosis and 88% of the electrophysiologists recommended an invasive procedure. In another retrospective study (NEJM 1999;341:1270-4), physicians who diagnosed ECG artefact as ventricular tachycardia went onto recommend intravenous lidocaine, precordial thump, coronary angiography, and an ICD.What are the clues than can help prevent this? The “timed spike and notch”Here are examples that mimic ventricular tachycardia or flutter.

Sinus rhythm (red highlight) and artefact (yellow highlight). The QRS complexes (red arrows) continue through the tracings as regular “timed spikes” (blue arrows) confirming underlying sinus rhythm.Sometimes instead of spikes, there are “timed notches” (blue arrows) which are hard to diagnose.

Mimics ventricular ectopyIt is easy to confuse artefact with ventricular ectopy. The clues are the compensatory pause and the appearance in multiple leads.

Ventricular ectopic (red highlight) which looks like artefact but there is a compensatory pause and an ectopic in the bottom lead. The other (yellow highlight) is artefact with no compensatory pause and artefact in the bottom lead.Mimics asystole.This is another of the more serious errors with potentially grave consequences. Once again, we need clues if it occurs in more than one lead.

Sinus rhythm (red highlight) with an apparent asystolic period (yellow highlight). The QRS complexes before and after the pause are attenuated confirming a lead issue. Note the gradual increase in the QRS size (blue highlight).On occasion, there is QRS attenuation for a number of beats (yellow highlight) and they may be called P waves and a diagnosis of high degree AV block made.

Now you know all about ECG interference and artefactsHarry Mond