A patient's ECG shows QT interval of 480 ms with a heart rate of 64 bpm (RR interval 938 ms). Calculate the corrected QT interval (QTc) using Bazett's formula, and determine if it is prolonged.
- A QTc = 496 ms; prolonged (normal upper limit males < 440 ms, females < 460 ms) ✓
- B QTc = 480 ms; normal for this heart rate
- C QTc = 460 ms; borderline, no action required
- D QTc = 520 ms; severely prolonged requiring urgent treatment
Correct answer: A. QTc = 496 ms; prolonged (normal upper limit males < 440 ms, females < 460 ms)
Explanation
Bazett's formula: QTc = QT / √RR (in seconds). RR = 60/64 = 0.938 s; √0.938 = 0.969. QTc = 0.480 / 0.969 = 0.4953 s = 495.3 ms ≈ 496 ms. This is significantly prolonged (normal QTc: males < 440 ms, females < 460 ms; >500 ms carries high risk of torsades de pointes). Common causes include hypokalemia, hypomagnesemia, drugs (antiarrhythmics, antipsychotics, macrolides), and congenital long QT syndrome. This warrants investigation and elimination of causative agents.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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