Figure 1. The cardiac cycle starts when cells in the sinoatrial node discharges an action potential that spreads as an electrical impulse through the atria and – via the atrioventricular node – to the ventricles. As the impulse spreads through the myocardium, it activates the cells which respond by contracting. The action potential generates electrical currents which gives rise to the classical ECG waveforms presented here. Activation of the atria is reflected as the P-wave and activation of the ventricles results in the QRS complex. The T-wave reflects the recovery (repolarization) of the ventricles. Note that the ECG rarely shows atrial recovery (repolarization) since it coincides with ventricular depolarization (i.e QRS complex), which has much stronger electrical potentials.