The QT interval is the time from the start of the Q wave to the end of the T wave. It represents ventricular depolarisation and repolarisation.
The QT is longer with slower heart rates and shorter when the rate is fast. The Corrected QT interval is adjusted for rate. There are multiple formulae for this, but the Bazett formula is among the most common:
Accurate measurement of the QT interval is challenging, because the exact end of the T wave can be difficult to find. Machine interpretation is not good at this either. The most widely accepted definition of the T wave end is the point where the steepest tangent intersects with the baseline. This should be measured using lead II, or V5 if II is unreadable. If the T has multiple peaks or is biphasic, the tallest part is used. If the QRS is >120 msec, any extra width beyond 120 msec is subtracted from the QT interval.
A useful rule of thumb is that the QT interval should not be more than half the distance between two QRS complexes. This only works at normal rates, not with tachycardia.
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