On a 12-lead ECG, the QT interval measures 520 ms at a heart rate of 60 bpm (RR interval = 1000 ms). Using Bazett's formula, what is the corrected QT (QTc) and how should this be interpreted?
- A QTc = 368 ms; normal
- B QTc = 400 ms; normal
- C QTc = 520 ms; prolonged and carries risk of Torsades de Pointes ✓
- D QTc = 480 ms; borderline but clinically insignificant
Correct answer: C. QTc = 520 ms; prolonged and carries risk of Torsades de Pointes
Explanation
Bazett's formula: QTc = QT / √(RR in seconds) = 520 / √1 = 520 ms. At a rate of 60 bpm, the RR is exactly 1.0 s, so QTc equals the raw QT. A QTc > 500 ms is significantly prolonged and predisposes to early afterdepolarizations triggering Torsades de Pointes, a polymorphic ventricular tachycardia.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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