Description

An ECG can be described by anyone with the most basic knowledge, and an accurate description is needed as a basis for the later steps. The description starts with the heart rate and regularity, as measured by the intervals between the QRS complexes. The P waves must be identified; and if there are none, a clear statement of their absence is necessary. The relationship of the P waves to the QRS complexes is the next logical step, and the PR interval must be measured. The shape of the P wave needs to be recorded if it is abnormally peaked or bifid.

The QRS complexes need to be described in terms of their width and height, and also their shape: whether Q waves are present; whether there is more than one R wave in the QRS complex; and whether there are S waves in the leads where they would be expected. If there are Q waves, are they small and narrow, and are they only seen in the lateral leads, where they may be due to septal depolarization? If there are pathological Q waves, in which leads are they present, and do they suggest a possible inferior or anterior myocardial infarction? The cardiac axis should be defined.

Elevation or depression of the ST segment must be noted. If the ST segment is elevated, does it follow an S wave, so indicating high take-off? The T waves must be inspected in each lead, and while inversion in VR and V₁ is always normal, inversion in any other leads should be recorded. The QT interval should be measured, and if it appears long, it should be corrected for heart rate.

All these features can be identified without any knowledge of the patient, or indeed much knowledge of cardiology. The description of an ECG is reasonably well done by the automatic ‘interpretation’ function built into most modern ECG recorders, but it is important to remember that these are far from perfect. Automatic recorders tend to over-interpret ECGs so that nothing of importance is missed, and their descriptions are not always totally accurate. They can be poor at identifying P waves and they often miss ST-segment changes, and sometimes T-wave inversion. Therefore, you should never depend solely on a description provided by the ECG recorder itself.