30 A 12-lead ECG records changes in electrical activity of the heart during each part of the cardiac cycle that is observed from 12 different views (leads). The conduction of the heart’s electrical impulses is detected through the skin by 10 electrodes, six placed on the chest and four on the limbs (Figure 30.1). The trace is displayed on a cardiac monitor or recorded by the ECG machine. The heart muscle contracts and relaxes in response to the electrical impulse that arises in the sinoatrial node and spreads through the conduction system across the atria and ventricles. This electrical activity is recorded as the ECG waveform on graph paper. The paper is divided into small squares of 1 mm each. Standard ECG paper speed is 25 mm/s per one square (1 mm) which equals 0.04 s horizontally or 0.1 mV vertically. Each waveform deflection is related to the electrical events of depolarisation and repolarisation occurring in the cardiac tissue. A normal ECG waveform (Figures 30.2 and 30.3) consists of:
The 12-lead electrocardiogram
Electrocardiogram waveform
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