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Calculator · Corrected QT interval before you prescribe
QTc — Bazett and Fridericia
Both formulae, because they disagree and the disagreement is the point. Bazett over-corrects at fast heart rates and under-corrects at slow ones — which is how a tachycardic patient acquires a QTc that looks alarming and is not.
Before you use it
Measure the QT in the lead where it is longest, from the start of the QRS to the end of the T wave, and do NOT include a U wave. Unreliable in atrial fibrillation (average several beats) and in bundle branch block or paced rhythm, where the QRS itself is wide and the QT is prolonged for a reason that is not repolarisation. Above 500 ms the torsades risk is the thing that matters — stop the offending drug, check K+, Mg2+ and Ca2+, and ask for help rather than reaching for another antiemetic.
Source: Bazett 1920 / Fridericia 1920; AHA/ACCF/HRS 2009 standardisation
Checked, and shown
Every threshold on this page was checked against its primary source by a practising doctor. Here is what that involved, and what it found.