Noise and artifacts

ECG 2A: Sinus rhythm with a clearly visible P-wave in lead II (slightly less distinctive in V5, V6); there is marked movement of the baseline which could be misdiagnosed as atrial arrhythmia; left bundle branch block;

ECG 2B:

Tracing recorded a few minutes later with careful re-application of the electrodes; disappearance of the baseline movements; tracing is now of much better quality and thus interpretable; confirmation that the rhythm is indeed sinus rhythm;

Patient: young female patient 23 years of age with no cardiac history;

ECG 2C:

Sinus rhythm with a clearly visible P-wave in lead III; the remainder of the tracing is extremely contaminated with rapid and chaotic waves superimposed over the normal tracing; this tracing is thus very difficult to interpret;

ECG 2D:

This tracing was recorded a few minutes after the first recording, the patient is now in a more comfortable position resulting in a tracing of much higher quality; normal pattern;

Comments:

A perfect recording quality is a necessary prerequisite for the correct interpretation of an ECG; it is therefore imperative to obtain a tracing with good definition of all 12 leads without noise or artifacts.

The quality of the recorded tracing can be altered for a number of reasons:

• movement of the baseline
• baseline artifacts caused by myopotentials (muscle tremor) or AC signal noise (electrical interference)

1. Baseline movement (baseline should be isoelectric) usually occurs when there is poor contact between the electrode and the skin or when the wire of the cable exerts a traction on the electrode; respiratory movements can induce a variation in contact pressure of the electrodes which explains why the baseline movement is generally predominant in precordial leads. When there is poor contact between the wire and the electrode, the ECG may show abrupt shifts in the baseline along with large oscillations varying in amplitude and frequency which make any attempt to interpret changes in ST segment and T-wave morphology impossible. From the analysis of the tracing, it is possible to determine which electrode is responsible for the undesired effects. If the tracing is for example altered in leads l, III and aVL but not in lead II, it means that the electrode on the right arm is not at fault; to reduce the occurrence of such problems, it may be useful to shave chest hair, clean an overly oily or dry skin with alcohol, check ground connections, place the limb electrodes close to the shoulders/hips to reduce patient motion, ask the patient to reduce his or her respiratory movements during the recording;
2. The most commonly observed artifactual noise is secondary to muscle tremors, which usually occurs when the recording environment is too cold or when the patient is in an uncomfortable position; the recording is also often highly contaminated in patients with resting tremor (Parkinson’s disease), such movements are very difficult to control; the electrocardiogram exhibits a characteristic pattern with evidence of well-individualized, narrow, irregular, overlapping oscillations, which are superimposed on the tracing, distorting the baseline and predominant in precordial leads; this type of artifact makes any interpretation very difficult especially when the physiological cardiac signals are of low voltage and can lead to a misdiagnosis of atrial arrhythmia (atrial fibrillation or atrial flutter) or ventricular arrhythmia (polymorphic ventricular tachycardia); in patients with hemi-Parkinson, a motion artifact can sometimes be observed only in certain leads; in order to reduce these tremors, simple corrective measures (change in position, heating of the room, attaining better muscle relaxation) generally allow solving the problem; conversely, tremors associated with Parkinson’s disease are nearly impossible to remove and it is advisable to place the limb electrodes as close to the torso as possible where movements are often less pronounced;
3. the presence of an interference corresponding to the frequency of the AC power source (50 Hz in Europe, 60 Hz in the USA) is also a frequent cause of artifacts; the ECG exhibits a characteristic and easily identifiable pattern with rapid, regular oscillations, well separated sinus waves, in the form of small artifacts with a 20 ms cycle (50 Hz) superimposed on the tracing; if the interference is only visible on 2 limb leads, the interference is generated by the electrode common to these two leads; this may be due to a poor contact between the skin and the electrode, a poor attachment of the electrode, a faulty connection of the electrode to the cable linked to the device, a partial rupture of the cable, or contact of the limb with the metallic component of the patient’s bed; when the interference is visible on all leads, a cause must be sought in the surrounding electrical equipment; the devices are equipped with selective filters enabling to significantly reduce or eliminate this type of interference; to correct this problem if occurring nonetheless, careful effort must be made to properly apply the electrodes, verify the quality of the electrical socket and change the latter if necessary, avoid contact between the wires and the metal components of the bed and that the grounding of the device is ensured by a proper connection;

Take-home message: the presence of baseline undulation or significant noise artifact in the tracing renders any interpretation perilous and can lead to misdiagnosis (arrhythmias, coronary syndrome with STEMI, etc.)