Clinical skills: the physiological basis and interpretation of the ECG.

Electrocardiograph (ECG) interpretation is a complex subject that requires considerable experience. However, an understanding of the principles underlying generation of the ECG can make this learning process easier. This knowledge is necessary if interpretation is to be founded on understanding rather than being a pattern recognition-based process. The aim of this article is to introduce the practitioner to the basic processes and mechanisms that govern formation of the normal ECG. These principles will largely be applied to the normal ECG and include cardiac anatomy applied to the ECG, how the ECG leads look at the heart and how the normal ECG waveform is formed. Examples of how the ECG changes when the underlying mechanisms are disturbed will also be given. Together, this knowledge should help the practitioner to have a clearer understanding of interpreting the abnormalities seen on an ECG.