Introduction Review ECG Basics to increase your knowledge of the electrocardiogram (ECG). In the PALS Provider Course you must be able to identify core rhythms during the case simulations and core case tests. Electrocardiogram Normal Cardiac Cycle The surface ECG is a graphic representation of the sequence of myocardial depolarization and repolarization. Each normal cardiac cycle (Figure 1) consists of a ? P wave ? QRS complex ? T wave Electrical depolarization begins in the sinoatrial node at the junction of the superior vena cava and right atrium and advances through atrial tissue to the atrioventricular (AV) node, where conduction velocity slows temporarily. It then progresses via the bundle of His and the Purkinje system to depolarize the ventricular myocardium (Figure 2). The first deflection on the surface ECG (P wave) represents depolarization of both atria. The time required for depolarization to pass through the atria, the AV node, and the His-Purkinje system is represented by the PR interval. The QRS complex represents depolarization of the ventricular myocardium. Ventricular repolarization is characterized on the surface ECG as the ST segment and T wave (Figure 3). Figure 1. The electrocardiogram