The corrected QT interval (QTc) using Bazett's formula for a patient with heart rate 100 bpm and QT interval 380 ms is approximately:
- A 380 ms — unchanged as HR is fast
- B 600 ms — markedly prolonged
- C 480 ms — prolonged ✓
- D 380 ms — normal since tachycardia shortens QT
Correct answer: C. 480 ms — prolonged
Explanation
Bazett's formula: QTc = QT / √RR (in seconds). At HR 100 bpm, RR interval = 60/100 = 0.6 s. QTc = 380 ms / √0.6 = 380 / 0.775 ≈ 490 ms. The closest option is 480 ms. Normal QTc is <440 ms (men) / <460 ms (women). This value of ~490 ms is prolonged, indicating drug toxicity or electrolyte abnormality risk. Bazett's formula overcorrects at high heart rates (tends to overestimate QTc during tachycardia).
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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