The electrocardiogram picks up the heart’s electrical impulses transmitted through the skin of the chest. Figure 2 gives a simplified concept of ion exchange — the polarized, depolarized, and repolarized state of the myocardial cell; and the action potential. An electrical current arriving at the cell causes positively charged irons to cross the cell membrane (depolarization), followed by repolarization which generates an action potential: phase 0, 1, 2, 3, and 4. This electrical event traverses the heart and initiates mechanical systole or a heartbeat.
Figure 3 gives a diagrammatic representation of the electrocardiogram and its relationship to the potassium and sodium exchange across the cardiac cell membrane with the generation of an action potential. The heart is initially activated by an infinitesimal current generated by the sinoatrial (SA) node, a natural pacemaker. The current of activation spreads radially from the SA node across the atria to the atrioventricular (AV) node and down the bundle branches to the ventricular muscle and Purkinje network (see Fig. 4). The SA node tracing shows no steady resting potential and is characterized by spontaneous depolarization. Figure 5 shows a normal ECG tracing.