QTc Bazett Calculator

Calculate the corrected QT interval (QTc) using Bazett's formula to assess cardiac repolarization time and identify potential arrhythmia risks.

How to Use This Calculator

Select your input method (intervals in milliseconds or heart rate in bpm)
Enter the QT interval in milliseconds (from start of Q wave to end of T wave)
Enter either the RR interval in milliseconds or the heart rate in bpm
Click Calculate QTc to see the corrected QT interval using Bazett's formula
Review the interpretation to understand if the QTc is normal, borderline, or prolonged

Formula Used

QTc = QT / √RR

Where:

QTc = Corrected QT interval in milliseconds
QT = Measured QT interval in milliseconds
RR = Time between consecutive QRS complexes in seconds (RR interval in ms ÷ 1000)

Heart Rate Conversion: RR (ms) = 60,000 ÷ Heart Rate (bpm)

Example Calculation

Real-World Scenario:

A 45-year-old patient presents for an ECG evaluation. The cardiologist measures a QT interval of 400ms with a heart rate of 75 bpm.

Given:

QT Interval = 400ms
Heart Rate = 75 bpm

Calculation:

First, convert heart rate to RR interval: RR = 60,000 ÷ 75 = 800ms = 0.8s

Then apply Bazett's formula: QTc = 400 ÷ √0.8 = 400 ÷ 0.894 = 447ms

Result: The corrected QT interval is 447ms, which is borderline prolonged and may warrant further evaluation.

Why This Calculation Matters

Practical Applications

Assessing risk of torsades de pointes and other ventricular arrhythmias
Monitoring patients on QT-prolonging medications
Evaluating electrolyte disturbances and their cardiac effects
Screening for congenital long QT syndrome

Key Benefits

Standardizes QT interval across different heart rates
Helps identify patients at risk for serious arrhythmias
Guides medication management and dosing decisions
Supports early detection of cardiac conduction abnormalities

Common Mistakes & Tips

The QT interval should be measured from the beginning of the Q wave to the end of the T wave in the lead where it is longest. Common errors include stopping at the peak of the T wave or not accounting for U waves when they merge with the T wave. Use multiple leads and average the measurements when possible.

Bazett's formula tends to overcorrect at high heart rates (>100 bpm) and undercorrect at low heart rates (<50 bpm). For patients with extreme heart rates, consider using alternative correction formulas like Fridericia (QTc = QT / RR^(1/3)) or Framingham (QTc = QT + 0.154×(1-RR)).

Frequently Asked Questions

For adult males, a normal QTc is typically less than 430ms. For adult females, a normal QTc is typically less than 450ms. Values between 430-450ms in males and 450-470ms in females are considered borderline. Values greater than 450ms in males and 470ms in females are considered prolonged and may increase the risk of arrhythmias.

Women naturally have longer QTc intervals due to hormonal influences on cardiac ion channels. Testosterone tends to shorten the QT interval, while estrogen and progesterone can prolong it. This is why women are at higher risk for drug-induced torsades de pointes and why the normal reference ranges differ between sexes.

Many medications can prolong the QT interval, including certain antibiotics (macrolides, fluoroquinolones), antiarrhythmics (amiodarone, sotalol), antipsychotics (haloperidol, ziprasidone), antiemetics (ondansetron), and methadone. Patients taking these medications should have their QTc monitored, especially when multiple QT-prolonging drugs are prescribed.

References & Disclaimer

Medical Disclaimer

This QTc calculator is for educational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of a qualified healthcare provider with any questions you may have regarding a medical condition.

References

QTc Calculator: Bazett's Formula - National Center for Biotechnology Information
AHA/ACC/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram - American Heart Association
Bazett's Formula: History and Limitations - Journal of Electrocardiology

Accuracy Notice

The accuracy of QTc calculation depends on precise measurement of the QT interval and RR interval. This calculator uses Bazett's formula, which has known limitations at extreme heart rates. Clinical correlation and professional judgment are essential when interpreting QTc values.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

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