The electrocardiogram (ECG) is composed of a P-wave, a QRS complex, and a T-wave. These three components are measured:
-once during each cardiac cycle.
-three times during each cardiac cycle: during a contraction of the atria, a contraction of the ventricles, and when the heart is at rest.
-twice during each cardiac cycle: during a contraction of the atria and a contraction of the ventricles.
-independently of the cardiac cycle.