How to Calculate Q Tc
What is Q Tc?
Corrected QT interval accounts for heart rate, predicting arrhythmia risk. QTc >460 ms females, >450 ms males increases torsades risk.
Formula
Apply Bazett formula: QTc = QT / √RR (RR in seconds)
- QT
- QT value — Variable used in the calculation
- RR
- RR value — Variable used in the calculation
Step-by-Step Guide
- 1Measure QT interval from ECG
- 2Apply Bazett formula: QTc = QT / √RR (RR in seconds)
- 3Or use other formulas for extreme heart rates
Worked Examples
Input
QT 420 ms, HR 80 (RR 0.75 s)
Result
QTc ≈ 420/√0.75 ≈ 485 ms (prolonged)
Elevated arrhythmia risk
Common Mistakes to Avoid
- ✕Using Bazett formula at extreme heart rates (use Fridericia for HR >100)
- ✕Forgetting electrolyte abnormalities (low K, Mg, Ca) prolong QT
Frequently Asked Questions
Which drugs prolong QT?
Antiarrhythmics, antipsychotics, antiemetics, fluoroquinolones, and many others. Check before prescribing.
What QTc values are safe?
<450 ms males, <460 ms females normal; 450-480 borderline; >500 ms high torsades risk.
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