Corrected QT Interval (QTc)
Heart rate-corrected QT interval using Bazett, Fridericia, and Framingham formulas
Calculator
Why use it
Identifies patients at risk for torsades de pointes and sudden death. Guides drug safety monitoring (many drugs prolong QT). Essential for evaluating inherited long QT syndromes.
About
The corrected QT interval (QTc) adjusts the measured QT interval for heart rate, enabling comparison across different heart rates. Prolonged QTc is associated with increased risk of torsades de pointes (TdP) and sudden cardiac death. Multiple correction formulas exist; Bazett is most commonly used but overestimates at high HR and underestimates at low HR. Fridericia is preferred at extreme heart rates.
When to use
- ECG interpretation when QT prolongation is suspected
- Monitoring QT-prolonging medications (antiarrhythmics, antipsychotics, antibiotics)
- Evaluation of syncope or unexplained cardiac arrest
- Screening for inherited long QT syndrome
Formula
Pearls & pitfalls
- Normal QTc: < 450 ms (males), < 460 ms (females).
- Bazett formula overestimates QTc at HR > 100 and underestimates at HR < 60.
- Fridericia is recommended for HR < 60 or > 100 bpm.
- Always correct K⁺ and Mg²⁺ before attributing QT prolongation to medications.
- QTc > 500 ms or increase > 60 ms from baseline is considered high risk for TdP.
- Common QT-prolonging drugs: haloperidol, ondansetron, fluoroquinolones, methadone, amiodarone.
Next steps
QTc > 500 ms: High risk for TdP — review medications, correct electrolytes (K⁺, Mg²⁺), cardiology consult. QTc 450-500 ms (male) or 460-500 ms (female): Monitor closely, avoid additional QT-prolonging agents.
Evidence
Bazett formula published 1920, remains the most commonly used despite known limitations. Fridericia (1920) shown to be more accurate at extreme heart rates. AHA/ACC/HRS 2017 guidelines recommend QTc monitoring with QT-prolonging drugs.