QTc Interpretation Thresholds
| Category | Males | Females |
|---|
| Normal | <440 ms | <450 ms |
| Borderline | 440–470 ms | 450–480 ms |
| Prolonged | >470 ms | >480 ms |
| Critical (TdP risk) | >500 ms (both sexes) |
RR = 60 / HR (seconds)
Bazett: QTc = QT / √RR
Fridericia: QTc = QT / RR^(1/3)
Framingham: QTc = QT + 0.154 × (1 - RR)
Hodges: QTc = QT + 1.75 × (HR - 60)
Clinical Notes:
• Bazett over-corrects at high HR and under-corrects at low HR
• Fridericia preferred at extremes of heart rate
• QTc >500 ms: discontinue offending drugs, correct electrolytes, consider telemetry
• Common causes: antipsychotics, antibiotics, antiarrhythmics, hypokalemia, hypomagnesemia
