A patient's ECG shows a QT interval of 480 ms with a heart rate of 60 bpm. The corrected QT (QTc) using Bazett's formula is calculated. What is the QTc, and does it indicate prolongation?

• A QTc = 480/√(1.0) = 480 ms; prolonged (normal <450 ms in men, <460 ms in women) ✓
• B QTc = 480/√(0.6) = 620 ms; significantly prolonged
• C QTc = 480/√(1.0) = 480 ms; borderline; RR interval at 60 bpm is 1.0 s
• D QTc = 480 × √(1.0) = 480 ms; within normal limits since RR = 1.0 and no correction needed

Explanation

Bazett's formula: QTc = QT / √(RR interval in seconds). At heart rate 60 bpm, RR interval = 60/60 = 1.0 second. QTc = 480/√1.0 = 480/1.0 = 480 ms. A QTc ≥450 ms in men and ≥460 ms in women is considered prolonged (>500 ms is high risk for Torsades de Pointes). So 480 ms is prolonged. Option B incorrectly uses RR = 0.6 (which would be the RR at 100 bpm, not 60 bpm). Options C and D are numerically equivalent to A but option C calls it 'borderline' when it is clearly prolonged by standard criteria; option D mislabels it as normal. The key concept is that at exactly HR 60 bpm, QTc = QT without change, making arithmetic straightforward.

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

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