7.12—Cardiac muscle electrical activity and ECGs

The sinoatrial node initiates a myogenic wave; the atria depolarise and contract, the atrioventricular node delays transmission, and Purkyne tissue spreads excitation through the ventricles from the apex.

On an ECG, the P wave reflects atrial depolarisation, the QRS complex ventricular depolarisation and the T wave ventricular repolarisation. Wave timing and rhythm reveal coordination, not blood pressure directly.

A prolonged interval or irregular rhythm can suggest conduction or fibrillation problems, while a low resting rate in a trained athlete may be normal bradycardia.

An ECG is an electrical trace, not a direct image of contraction force or a diagnosis by itself. Interpret the wave, interval and clinical context together.