What are normal QTc values?

Typical reference ranges are: men: normal QTc 450 ms; women: normal QTc 470 ms. Many clinicians consider QTc ≥ 500 ms clearly high risk for torsades de pointes, regardless of sex. Always interpret QTc in the clinical context and according to local guidelines.

Which QTc formula should I use?

Bazett is the most widely used but overcorrects at high heart rates and undercorrects at low heart rates. Fridericia and Framingham perform better across a wider heart-rate range and are often preferred in drug safety and research. Hodges is another linear correction. Many centers report Bazett plus one alternative method; follow your local protocol.

How do I measure the QT interval correctly?

Use a 12‑lead ECG at standard speed (25 mm/s). Choose a lead with a clear T wave (often II or V5). Measure from the start of the QRS complex to the end of the T wave (where it returns to the baseline). Avoid including U waves. Average 3–5 consecutive beats in sinus rhythm. In irregular rhythms, manual measurement and clinical judgment are essential.

When should I worry about a prolonged QTc?

QTc ≥ 500 ms or an increase of ≥ 60 ms from baseline is commonly used as a high‑risk threshold, especially in patients on QT‑prolonging drugs. Marked prolongation, syncope, family history of sudden death, electrolyte disturbances, or bradycardia should prompt urgent clinical evaluation. This calculator cannot diagnose or rule out disease.

QTc Calculator – Corrected QT Interval (Bazett, Fridericia, Framingham)

What is QTc?

The QT interval on the ECG represents the time from the start of ventricular depolarization to the end of repolarization. Because the QT interval shortens at higher heart rates and lengthens at lower heart rates, the corrected QT interval (QTc) adjusts QT to a standardized heart rate of 60 bpm.

Prolonged QTc is associated with an increased risk of torsades de pointes and sudden cardiac death, especially in congenital long QT syndrome and in patients taking QT‑prolonging drugs (e.g. certain antiarrhythmics, antipsychotics, macrolide antibiotics, methadone).

QTc formulas used in this calculator

Let \(QT\) be the measured QT interval in seconds and \(RR\) the RR interval in seconds (RR = 60 / heart rate).

Bazett formula

\[ QTc_{Bazett} = \frac{QT}{\sqrt{RR}} \]

Most widely used; tends to overcorrect at high heart rates and undercorrect at low heart rates.

Fridericia formula

\[ QTc_{Fridericia} = \frac{QT}{\sqrt[3]{RR}} \]

Often more accurate than Bazett across a wide heart‑rate range; commonly used in drug safety studies.

Framingham formula

\[ QTc_{Framingham} = QT + 0.154 \times (1 - RR) \]

Linear correction derived from the Framingham Heart Study.

Hodges formula

\[ QTc_{Hodges} = QT + 0.00175 \times (HR - 60) \]

Linear correction based directly on heart rate (HR in bpm).

Normal QTc values and risk thresholds

Reference ranges vary slightly between sources. This calculator uses commonly cited cut‑offs:

Men: normal < 430 ms; borderline 430–450 ms; prolonged > 450 ms.
Women: normal < 450 ms; borderline 450–470 ms; prolonged > 470 ms.
High‑risk threshold: QTc ≥ 500 ms is generally considered clearly prolonged and associated with higher torsades risk.

Always interpret QTc in the context of the clinical situation, electrolyte status, concomitant medications, and family history. Local or specialty‑society guidelines may define different thresholds.

How to measure QT and use this QTc calculator

Obtain a standard 12‑lead ECG at 25 mm/s and 10 mm/mV.
Choose a lead with a clearly visible T wave and minimal noise (often II, V5, or V6).
Measure QT from the beginning of the QRS complex to the point where the T wave returns to the baseline. Do not include U waves; if present, use the nadir between T and U as the end of T.
Average multiple beats (3–5 consecutive beats in sinus rhythm) to reduce beat‑to‑beat variability.
Determine heart rate or RR interval from the ECG:
- Heart rate (bpm) can be read from the ECG machine or calculated (e.g. 300 / number of large squares between R waves).
- RR interval (s) is the time between two consecutive R peaks.
Enter QT and rate/RR into the calculator. It will convert units, compute RR, and display QTc using all four formulas.
Review the interpretation and compare Bazett with alternative formulas, especially at very high or low heart rates.

Clinical caveats and limitations

Automated ECG QT/QTc values may be inaccurate; manual verification is recommended, especially when results will affect therapy.
In atrial fibrillation or markedly irregular rhythms, QT measurement and correction are less reliable.
Bundle branch block, ventricular pacing, and wide QRS complexes can artificially prolong QT; specialized methods (e.g. JT interval) may be more appropriate.
Electrolyte disturbances (hypokalemia, hypomagnesemia, hypocalcemia), bradycardia, and structural heart disease all modify torsades risk.
This calculator does not diagnose long QT syndrome or determine fitness for specific drugs or procedures.

Disclaimer

This QTc calculator is intended for educational and decision‑support use by healthcare professionals. It does not provide medical advice, diagnosis, or treatment and must not be used as a substitute for clinical judgment, ECG over‑reading by a qualified clinician, or local protocols and guidelines.

QTc Calculator

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