QT interval
In cardiology, the QT interval is a measure of the time between the start of the Q wave and the end of the T wave in the heart's electrical cycle. The QT interval represents electrical depolarization and repolarization of the ventricles. A lengthened QT interval is a marker for the potential of ventricular tachyarrhythmias like torsades de pointes and a risk factor for sudden death.
Like the R–R interval, the QT interval is dependent on the heart rate in an obvious way (i.e., the faster the heart rate, the shorter the R–R interval and QT interval) and may be adjusted to improve the detection of patients at increased risk of ventricular arrhythmia. Modern computer-based ECG machines can easily calculate a corrected QT (QTc), but this correction may not aid in the detection of patients at increased risk of arrhythmia, as there are a number of different correction formulas.
The standard clinical correction is to use Bazett's formula, named after physiologist Henry Cuthbert Bazett, calculating the heart rate-corrected QT interval (QTcB).
Bazett's formula is based on observations of only 12 patients in 1920 and does not meet current scientific quality standards. A better method may be the Framingham correction based on the Framingham Heart Study, which used long-term cohort data of over 5,000 subjects.
Bazett's formula is:
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