A patient's corrected QT interval (QTc) is calculated using Bazett's formula. The measured QT is 480 ms at a heart rate of 60 bpm (RR interval = 1000 ms). What is the QTc, and is it prolonged?

• A QTc = 480 ms; definitely prolonged (threshold 450 ms in men, 470 ms in women)
• B QTc = 440 ms; normal
• C QTc = 520 ms; markedly prolonged
• D QTc = 480 ms; borderline prolonged (normal < 440 ms in men, < 460 ms in women) ✓

Explanation

Bazett's formula: QTc = QT / √RR (in seconds) = 480 / √1.0 = 480 / 1.0 = 480 ms. At heart rate 60 bpm, RR = 1.0 s and √1.0 = 1, so QTc equals the measured QT. The normal upper limit is < 440 ms in men and < 460 ms in women; > 500 ms carries risk of torsades de pointes. A QTc of 480 ms is prolonged in men but borderline in women — clinically significant and warrants investigation for channelopathies, electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia), or offending drugs.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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