Qtc Bazett
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The QT interval on an ECG reflects the time it takes the heart's ventricles to contract and then reset electrically, but this raw interval naturally shortens at faster heart rates and lengthens at slower ones. The corrected QT (QTc) adjusts for heart rate so that values can be compared meaningfully regardless of how fast the heart is beating.

Bazett's formula divides the measured QT interval by the square root of the RR interval (the time between heartbeats in seconds), which normalizes it as if the heart rate were 60 beats per minute. It's the most widely used correction formula in clinical practice, despite known limitations.

Bazett's formula tends to overcorrect at high heart rates and undercorrect at low heart rates, meaning it can flag a falsely prolonged QTc in someone with tachycardia or miss a genuinely prolonged QTc in someone with bradycardia. Alternative formulas like Fridericia's are sometimes preferred for this reason, especially at heart rate extremes.

General clinical thresholds flag a QTc above about 450 ms in men or 460 ms in women as prolonged, with values above 500 ms associated with a notably higher risk of dangerous heart rhythm disturbances. These thresholds can vary somewhat by clinical guideline and context.

A prolonged QTc interval increases the risk of a specific dangerous arrhythmia called torsades de pointes, which can be triggered by certain medications, electrolyte imbalances, or inherited heart conditions. This is why many drugs are screened for their effect on QT interval before and during use.