ECG Artefact

An ECG artefact is an inscription on an ECG tracing that does not originate from a cardiac source. Surface ECG electrodes record signals from a number of sources with cardiac electrical signals, the most important. Other signals include skeletal muscle, brain, respiratory interference, noise at the skin-electrode interface and external sources. For convenience, the appearance of ECG interference will be regarded as obvious or subtle and it is important to recognise interference to prevent ECG misdiagnosis.

ECG interference may be:

• Electrical – corporeal (skeletal myopotentials)

– extracorporeal (alternating current)

• Mechanical – loose lead/electrode or wandering baseline

ECG machines have four types of filters:

• Two types of bandpass filters, which limit the frequency band.
• High-pass filter – removes low frequency signals (motion artefact, respiration and wandering baseline).
• Low-pass filter – removes high frequency signals (muscle artefact and external interference) but reduces the QRS amplitude.
• AC (Notch) filter – removes 50 Hz line interference.
• Common mode rejection – filters via the earth electrode.

An example of high-pass filtering:

What are the causes of electrical interference?

• Alternating current (AC) 50 Hz (USA 60 Hz)
• Radiofrequency (RF) or electromagnetic (EM) – power lines, electrical equipment, mobile phones, fluorescent lights, electrical diathermy.
• Static electricity (humidity)
• Dry ECG electrodes
• Loose ECG connections (mixed)
• Dislodged ECG electrode/connection (mixed)
• Fractured ECG cables (mixed)
• Inappropriate filter settings

What are the causes of mechanical interference?

• Movement/tremor/shivering
• Respiration
• Dry ECG electrodes
• Loose or dislodged ECG connections
• Fractured ECG cables
• Creams and oils on skin

How do we recognise artefact on the ECG?

Electrical artefact; constant repetitive “fuzz” and more common with a 12-lead ECG

whereas,

mechanical artefact; non-constant with a variable amplitude and more common with ambulatory Holter monitoring recordings.

Obvious ECG artefact:

With electrical artefact, the P, QRS and T waves are usually easy to identify.

Here are some examples:

Electrical: High frequency alternating current (AC):

Electrical: Regular, high frequency electromagnetic (EM).

On occasion, the P, QRS and T waves may be obscured.

Electrical: High frequency alternating current (AC).

With mechanical artefact, the P, QRS and T waves may be hard to identify.

Here are some examples:

Mechanical: ECG; dry electrodes, fractured cable, or connection issue

Mechanical: Dry ECG electrodes, skin preparation and marked respiration.

Wandering baseline.

With a single ECG lead or electrode, it is possible to determine which is at fault.

In this example, the artefact is in leads I (red highlight) and III (yellow highlight) and therefore the common lead is the left arm.

All leads are involved. There is a serious ECG cable issue.

Frequently, we can’t tell if it is electrical or mechanical.

Other times, it is mixed.

Here is an example of gross Parkinson tremor.

The rhythm appears very irregular and a short period of absent tremor (red highlight) suggests it is atrial fibrillation, despite the possible artefactual P waves.

Another medical cause of obvious artefact is neuromodulation artefact. These are due to non-cardiac implanted pulse generators and lead I gives a clue as to what part of the body is being stimulated.

Brain: Involves lead I (red highlight) which is upper torso.

Bladder: Lower torso and lead I (red highlight) not involved.

Bowel: Lower body and lead I (red highlight) not involved.

Subtle artefact will be covered in the next “Fun with ECGs”.

Harry Mond .