Electrocardiogram An ECG may be recorded by detecting and amplifying the electrical activity generated by the myocardium, as detected on the surface of the body. In order for muscle to contract it must depolarize. After contraction the muscle will repolarize, prior to the next depolarization. P waves are generated by atrial depolarization and so represent atrial contraction, QRS complexes are generated by ventricular myocardial depolarization so represent ventricular contraction. T waves are generated by ventricular myocardial repolarization and so do not represent active muscular contraction. As each QRS complex represents ventricular contraction, each is associated with one arterial pulsation. A three lead ECG is commonly used to monitor the heart rhythm. However, if a 12 lead ECG is used the heart may be 'viewed' from 12 different positions. This allows any pathology in the myocardium to be localised.