Intervals and Segments

PR interval

– Time from the onset of the P wave (atrial depolarization) to the start of the QRS complex (ventricular depolarization)

– Represents: conduction through the AV node
– Normal PR interval: 120 – 200 ms (0.12-0.20s or 3 – 5 small squares)

– lntervals are measured in units of time

– Shorten with faster heart rates

PR Segment

– Isoelectric line between end of the P wave and the start of the QRS complex

– PR segment depression can be seen in acute pericarditis or atrial infarction

– Segments are analyzed based on change from the isoelectric line such as elevation or depression

QT Interval

– Time from start of Q wave to the end of the T wave

– Represents: Ventricular depolarisation and repolarisation

– Duration: Normal QTc < 440ms in men or < 460ms in women

– QT interval shortens at faster heart rates

– QT interval gets prolonged at slower heart rates

– Measured as the longest interval either in lead II or V5-6

– Corrected QT or QTc estimates the QT interval at a standard heart rate of 60 bpm. Measured with Bazett formula: QTC = QT / square root of RR

ST-T Segment

– Isoelectric line between the end of the S wave (the J point) and the beginning of the T wave

– Represents: Interval between ventricular depolarization and repolarization

– ST segment changes can be seen in ischemia, electrolyte abnormalities and multiple other conditions

J Point

– Junction between the termination of the QRS complex and the beginning of the ST segment

– Represents end of depolarization

– Elevation or depression of the J point is usually seen with ST segments changes such as in ischemia

– J point elevation can also be seen in early repolarization