Electrocardiogram (ECG)

An electrocardiogram (ECG) is a recording of the heart's electrical activity over time, picked up through electrodes placed on the skin of the arms, legs and chest, and it plots how electrical potential across the heart changes during a single cardiac cycle. Importantly, every wave on an ECG reflects depolarisation or repolarisation of heart muscle, not the mechanical contraction itself - the electrical wave always arrives just before the corresponding contraction begins. A normal ECG shows three distinct waves (see Figure 7.9 and, mapped stage by stage against the conduction pathway, Figure 7.10). The P wave is a small upward deflection representing atrial depolarisation - the time taken for the SA node's excitation to spread across both atria, a contraction that itself lasts about 0.8-1.0 seconds. The PQ interval, from the start of the P wave to the start of the QRS complex, times the delay as the impulse crosses the AV node on its way from atria to ventricles (normally 0.12-0.21 sec), and so is a direct measure of AV conduction time. The QRS complex represents ventricular depolarisation; atrial depolarisation has no separate wave of its own here because it happens at the same time as this ventricular event. The QRS complex is shorter than the P wave (lasting only 0.06-0.09 sec) because the signal spreads unusually fast through the Purkinje fibres; an abnormally prolonged QRS often signals delayed ventricular conduction, for instance from ventricular hypertrophy or a block somewhere in the bundle of His. The ST segment, between the QRS complex and the T wave, marks the period when the ventricles are completely depolarised and reflects the long plateau p …