Intervals and Segments
PR interval
– 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
– 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
– 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
– Represents: Interval between ventricular depolarization and repolarization
– ST segment changes can be seen in ischemia, electrolyte abnormalities and multiple other conditions
J Point
– 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