Electrical currents generated by the heart during the cardiac cycle can be detected on the surface of the body by the electrodes of an electrocardiograph. A recording of these currents, called an electrocardiogram (ECG or EKG), represents a sum of all the concurrent action potentials produced by the heart as detected by the 12 electrodes of the electrocardiograph. A single cardiac cycle produces a distinctive wave pattern, where peaks and valleys are indicated by the letters P, Q, R, S, and T (see Figure 1). An interpretation of the major characteristics of the ECG follows:
  • The P wave is a small wave that represents the depolarization of the atria. During this wave, the muscles of the atria are contracting.

  • The QRS complex is a rapid down‐up‐down movement. The upward movement produces a tall peak, indicated by R. The QRS complex represents the depolarization of the ventricles.

  • The T wave represents the repolarization of the ventricles. Electrical activity generated by the repolarization of the atria is concealed by the QRS complex.

Figure 1. Different periods of the cardiac cycle: (a) electrocardiogram, (b) heart sounds, (c) valves, (d) pressure, and (e) volume of left ventricle.