In more detail
QT is measured from the start of the QRS complex to the end of the T wave, covering the electrical activation and recovery of the ventricles. It shortens as heart rate rises, so QTc adjusts it to what it would be at 60 beats a minute. With RR in seconds (60 / heart rate), Bazett's formula is QT / √RR, Fridericia's QT / ∛RR, Framingham's QT + 154 × (1 − RR) and Hodges' QT + 1.75 × (HR − 60). Bazett over-corrects at fast heart rates and under-corrects at slow ones, which is why Fridericia is often preferred when monitoring medicines.
Why it matters
A long QTc is linked to a dangerous heart rhythm called torsades de pointes. Commonly cited upper limits are about 450 ms in men and 460 ms in women, and values above 500 ms are treated as high risk. Many medicines lengthen the QT, and some people have an inherited long-QT syndrome. Because the formula chosen can move a value across a threshold, interpretation belongs to a clinician.
Example
QT 400 ms at 75 beats/min → RR = 60 / 75 = 0.8 s
Bazett: 400 / √0.8 = 447 ms
Fridericia: 400 / ∛0.8 = 431 ms
Framingham: 400 + 154 × (1 − 0.8) = 431 ms
Hodges: 400 + 1.75 × (75 − 60) = 426 ms
QT 320 ms at 110 beats/min: Bazett 433 ms, Fridericia 392 ms