QTc (Corrected QT Interval)

Bazett is the most used and the least reliable. It over-corrects above roughly 90/min and under-corrects below 60/min, so it manufactures a prolonged QTc in tachycardia and conceals one in bradycardia. The 2009 AHA/ACCF/HRS statement recommends against the power-function corrections, reaffirmed in 2017; Fridericia and Framingham track the rate far better.

All four are shown rather than one because their disagreement is itself the finding. At 60/min they are identical by construction. The further the rate is from 60, the more the answer depends on which correction you picked — and at the extremes that difference can move a patient across a decision threshold.

Correct the potassium, magnesium and calcium, and review the drug chart before treating a number. QTc prolongation is usually acquired and usually multifactorial.

Measure the QT in the lead where it is longest, usually II or V5–V6, and beware counting a U wave as part of the T. Rate correction is also unreliable in atrial fibrillation and in bundle branch block, where the QRS itself is widened.