Sinus Rhythm - ECG Structure

ECG Structure

There are typically six distinct waves (identified by the letters P, Q, R, S, T, and U) in a single beat of the heart in sinus rhythm, and they occur in a specific order, over specific periods of time, with specific relative sizes. While there is a significant range within which variations in rhythm are considered normal, anything that deviates from sinus rhythm by more than a certain amount may be indicative of heart disease.

See also Heart rate variability.