QTc Fridericia Calculator

Calculate the corrected QT interval using Fridericia's formula to assess cardiac repolarization time adjusted for heart rate.

How to Use This Calculator

Enter the QT interval measurement in milliseconds (ms)
Enter the patient's heart rate in beats per minute (bpm)
Select the patient's gender for appropriate reference ranges
Click Calculate to see the corrected QT interval and its interpretation

Formula Used

QTc = QT / (RR interval)^(1/3)

Where:

QTc = Corrected QT interval
QT = Measured QT interval in milliseconds
RR interval = Time between heart beats in milliseconds (60000/Heart Rate)

Example Calculation

Real-World Scenario:

A patient with a heart rate of 70 bpm has a measured QT interval of 400 ms. Calculate the corrected QT interval using Fridericia's formula.

Given:

QT interval = 400 ms
Heart rate = 70 bpm

Calculation:

RR interval = 60000 / 70 = 857.14 ms

QTc = 400 / (857.14)^(1/3) = 400 / 9.51 = 42.06 × 10 = 420.6 ms

Result: The corrected QT interval is 420.6 ms, which is at the upper limit of normal for a male patient and borderline prolonged for a female patient.

Why This Calculation Matters

Practical Applications

Assessing risk of arrhythmias and sudden cardiac death
Monitoring patients on QT-prolonging medications
Evaluating electrolyte disturbances affecting cardiac repolarization
Screening for congenital long QT syndrome

Key Benefits

More accurate than Bazett's formula at high heart rates
Standardizes QT interval across different heart rates
Helps identify patients at risk for torsades de pointes
Guides medication dosing and monitoring decisions

Common Mistakes & Tips

The QT interval should be measured from the beginning of the Q wave to the end of the T wave. Use lead II or V5 when possible, and average measurements over 3-5 consecutive beats. Avoid including U waves in the measurement unless they merge with the T wave.

While Bazett's formula is commonly used, it tends to overcorrect at high heart rates and undercorrect at low heart rates. Fridericia's formula (QTc = QT/RR^(1/3)) provides more accurate correction across a wider range of heart rates, making it preferable in many clinical situations.

Frequently Asked Questions

A QTc interval greater than 450 ms in males and greater than 470 ms in females is generally considered prolonged. Values between 430-450 ms in males and 450-470 ms in females are considered borderline. A QTc greater than 500 ms is associated with a significantly increased risk of torsades de pointes.

QTc should be monitored when starting medications known to prolong QT interval (such as certain antibiotics, antipsychotics, and antiarrhythmics), in patients with electrolyte abnormalities (especially hypokalemia and hypomagnesemia), in patients with known congenital long QT syndrome, and when evaluating syncope of unknown cause.

Bazett's formula (QTc = QT/RR^(1/2)) uses a square root correction, while Fridericia's formula (QTc = QT/RR^(1/3)) uses a cube root correction. Research has shown that Fridericia's formula provides more accurate QT correction across a wider range of heart rates, particularly at higher heart rates where Bazett's formula tends to overcorrect.

References & Disclaimer

Medical Disclaimer

This calculator is for educational and informational purposes only. It 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: Comparison of Formulas - National Center for Biotechnology Information
AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram - American Heart Association
2022 ESC Guidelines for the Management of Patients with Ventricular Arrhythmias - European Society of Cardiology

Accuracy Notice

The QTc calculation provided by this tool is based on Fridericia's formula and is intended as a reference only. Clinical interpretation should consider the complete clinical context, including patient history, medications, electrolyte status, and other relevant factors. This calculator has not been validated for use in pediatric patients.

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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