Electrocardiogram Basics: Waves, Intervals, and Segments

Depolarization and Repolarization

In Chapter 1 , the term electrical activation (stimulation) was applied to the spread of electrical signals through the atria and ventricles. The more technical term for the cardiac activation process is depolarization. The return of heart muscle cells to their resting state after depolarization is termed repolarization.

These key designations derive from the basic electrophysiologic finding that normal “resting” myocardial cells are polarized; that is, they carry electrical charges on their surface. Fig. 2.1 A shows the resting polarized state of a normal atrial or ventricular heart muscle cell. Notice that the outside of the resting cell is positive and the inside is negative (about −90 mV [millivolt] gradient between them). ^a

a Membrane polarization is due to differences in the concentration of ions inside and outside the cell. A brief review of this important topic is presented in the online material, and also see the Bibliography for references that present the basic electrophysiology of the resting membrane potential and cellular depolarization and repolarization (the action potential) underlying the ECG waves recorded on the body surface.

When a heart muscle cell (or group of cells) is stimulated, it depolarizes. As a result, the outside of the cell, in the area where the stimulation has occurred, becomes negatively charged and the inside of the cell becomes positive. This produces a difference in electrical voltage on the outside surface of the cell between the stimulated depolarized area and the unstimulated polarized area ( Fig. 2.1 B). Consequently, a small electrical current is formed that spreads along the length of the cell as stimulation and depolarization occur until the entire cell is depolarized ( Fig. 2.1 C). The path of depolarization can be represented by an arrow, as shown in Fig. 2.1 B.

Note: For individual myocardial cells (fibers), depolarization and repolarization proceed in the same direction. However, for the entire myocardium, depolarization normally proceeds from innermost layer (endocardium) to outermost layer (epicardium), whereas repolarization proceeds in the opposite direction. The exact mechanisms of this well-established asymmetry are not fully understood.

The depolarizing electrical current is recorded on the ECG as a P wave (when the atria are stimulated) and as a QRS complex (when the ventricles are stimulated).

Repolarization starts when the fully stimulated, depolarized cell begins to return to the resting state. A small area on the outside of the cell becomes positive again ( Fig. 2.1 D), and the repolarization spreads along the length of the cell until the entire cell is once again fully repolarized. Ventricular repolarization is sequentially recorded on the ECG as the ST segment, T wave, and U wave.

In summary, whether the ECG is normal or abnormal, it records just two basic events: (1) depolarization, the spread of a stimulus (stimuli) through the heart muscle, and (2) repolarization, the return of the stimulated heart muscle to the resting state. The basic cellular processes of depolarization and repolarization are responsible for the waveforms, segments, and intervals seen on the body surface (standard) ECG.