QT Interval

QT should be measured in lead II or V5/V6. There should be several beats measured and the QT is the longest taken of the 3. The QT is measured from the beginning of the QRS to the end of the T wave. If a patient has a U wave, that is included in the measurement.

The QTc is the QT corrected for the heart rate. The QT prolongs with a slower heart rate and shortens with a quicker heart rate. The preceding RR interval gives feedback to the conduction system to shorten its refractory period, shortening the QT interval. Because of this, refractory torsades des pointes may be treated with overdrive pacing or medications to increase the heart rate (ie terbutaline); the main exception of this is congenital long QT syndrome.

There are multiple possible formulas to use to calculate the QTc, but the Bazett formula is what is most commonly used.

QTC = QT / √ RR

The RR interval measurement is seconds. This is often given with the computer read section, both the uncorrected and corrected version. One should always at least 'eyeball' the uncorrected one to make sure it is measuring correctly, as it does not always do so. The QT interval should be less than 1/2 of the RR interval.