QTc Calculator
Calculates corrected QT interval (QTc) using multiple formulas to assess risk of torsades de pointes and guide medication management.
ECG Measurements
Measured QT interval from ECG in milliseconds
Heart rate in beats per minute
Normal QTc ranges differ by sex
Correction Formulas
QTc (Bazett)
Enter values to calculate
About This Calculator
The QTc Calculator corrects the QT interval for heart rate, which is essential for identifying patients at risk for dangerous arrhythmias like torsades de pointes (TdP).
The QT interval represents ventricular depolarization and repolarization. Because it naturally varies with heart rate (longer at slower rates), correction formulas are needed to standardize measurements.
Correction Formulas: • Bazett (QTcB): QTc = QT / √RR - Most widely used, but overcorrects at high HR and undercorrects at low HR • Fridericia (QTcF): QTc = QT / ³√RR - More accurate at extreme heart rates, preferred by FDA for drug studies • Framingham (QTcFra): QTc = QT + 0.154 × (1 - RR) - Linear correction • Hodges (QTcH): QTc = QT + 1.75 × (HR - 60) - Heart rate linear correction
Clinical Significance: • Prolonged QTc increases risk of torsades de pointes, a potentially fatal arrhythmia • Many medications can prolong QT (antiarrhythmics, antipsychotics, antibiotics, etc.) • Electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia) can prolong QT • Congenital long QT syndrome requires specialized management
When to Measure: • Before starting QT-prolonging medications • When combining multiple QT-prolonging drugs • In patients with syncope or seizures of unclear etiology • During electrolyte disturbances
Formula
QTcB = QT / √RR, QTcF = QT / ³√RR (RR in seconds)Multiple correction formulas exist: **Bazett Formula (most common):** QTcB = QT / √(RR interval) Where RR = 60/HR (in seconds) **Fridericia Formula (preferred for drug studies):** QTcF = QT / ³√(RR interval) **Framingham Formula:** QTcFra = QT + 0.154 × (1 - RR) × 1000 **Hodges Formula:** QTcH = QT + 1.75 × (HR - 60) Normal QTc values: • Men: <450 ms • Women: <460 ms Borderline: 450-470 ms (men), 460-480 ms (women) Prolonged: >470 ms (men), >480 ms (women)
Clinical Considerations
- •Bazett formula is less accurate at extreme heart rates (<50 or >90 bpm)
- •Always verify QT measurement technique (use lead II or V5-V6)
- •U waves should not be included in QT measurement
- •Consider congenital long QT syndrome in young patients
- •Multiple QT-prolonging drugs have additive effects
- •Always check potassium, magnesium, and calcium levels
Limitations
- •No single correction formula is perfect for all heart rates
- •Inter-observer variability in QT measurement can be significant
- •Bundle branch blocks and ventricular pacing complicate interpretation
- •Does not account for T wave morphology abnormalities
- •Risk calculators (e.g., Tisdale score) may be more predictive for drug-induced TdP
Interpretation Guide
| Range | Classification | Recommendation |
|---|---|---|
| <-440 | Normal | Normal corrected QT interval. QT-prolonging medications can generally be used with standard monitoring. |
| 440-470 | Borderline | Borderline QTc prolongation. Use caution with QT-prolonging medications. Check and correct electrolytes. Consider repeat ECG. |
| 470-500 | Prolonged | QTc prolonged. Avoid QT-prolonging drugs if possible. Correct electrolyte abnormalities. Consider cardiology consultation. |
| 500-1000 | Markedly Prolonged | Markedly prolonged QTc - high risk for torsades de pointes. Discontinue QT-prolonging medications. Urgent cardiology consultation. Consider telemetry monitoring. |
Frequently Asked Questions
Which QTc formula should I use?
Bazett is most commonly used clinically, but Fridericia is preferred by the FDA for drug studies and is more accurate at heart rates outside 60-100 bpm. When heart rate is very fast or slow, report both values.
What medications prolong the QT interval?
Common QT-prolonging drugs include: antiarrhythmics (sotalol, amiodarone, dofetilide), antipsychotics (haloperidol, ziprasidone), antibiotics (fluoroquinolones, azithromycin, erythromycin), antidepressants (citalopram, escitalopram), antiemetics (ondansetron, droperidol), and methadone. See CredibleMeds.org for a comprehensive list.
What is torsades de pointes?
Torsades de pointes ("twisting of the points") is a polymorphic ventricular tachycardia associated with QT prolongation. It can degenerate into ventricular fibrillation and cardiac arrest. Treatment includes IV magnesium, stopping offending drugs, and correction of electrolytes.
How do I measure the QT interval correctly?
Measure from the beginning of the Q wave (or R wave if no Q) to the end of the T wave. Use leads II or V5-V6 where T waves are usually most distinct. Do not include U waves. Average multiple beats if rhythm is irregular.
What electrolyte abnormalities affect QT?
Hypokalemia, hypomagnesemia, and hypocalcemia all prolong the QT interval. Always check and correct these before starting QT-prolonging medications. Hypokalemia is the most common and dangerous in combination with QT-prolonging drugs.
References
1. Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920. doi: 10.1177/000331972002200313
2. Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen. Acta Medica Scandinavica. 1920
3. Rautaharju PM, Surawicz B, Gettes LS, et al.. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009. doi: 10.1161/CIRCULATIONAHA.108.191096
View Source →4. Drew BJ, Ackerman MJ, Funk M, et al.. Drug-Induced Prolongation of the QT Interval. Circulation. 2010. doi: 10.1161/CIRCULATIONAHA.109.192704
View Source →Last updated: 2025-01-15
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