Segments and intervals in an electrocardiogram (ECG)

A segment in an electrocardiogram is the region between two waves. PR segment begins at the end of the P wave and ends at the onset of the QRS complex. ST segment starts from the end of the QRS and terminates at the onset of the T wave. TP segment is between the end of the T wave and the beginning of the next P wave. It is the true isoelectric interval in the electrocardiogram. Ideally elevation and depression of other segments have to be measured in relation to the TP segment. But this method may need to be modified when there is tachycardia as during a treadmill exercise test. TP segment may not be identified well as it shortens with tachycardia during a treadmill test.

An interval in an electrocardiogram includes one segment and one or more waves. PR interval starts at the beginning of the P wave and ends at the onset of the QRS. It denotes the conduction of the impulse from the upper part of the atrium to the ventricle. QT interval starts at the onset of the QRS and ends at the end of the T wave. It denotes the electrical systole of the heart. PP interval is the interval from the onset of one P wave to the onset of the next P wave in the next cardiac cycle. Atrial rate can be calculated from the PP interval. RR interval is the interval from the onset of one QRS to complex to the onset of the next QRS complex in the next cardiac cycle. Ventricular rate can be calculated from the RR interval.