Toolkit / Psychopharmacology
QTc calculator (Bazett, Fridericia, Framingham, Hodges)
Correct a measured QT interval for heart rate with four standard formulas, side by side. Useful before starting or raising antipsychotics, citalopram/escitalopram, tricyclics, methadone and other QT-prolonging drugs.
Thresholds used
| Men < 450 | Within normal limits |
| Women < 460 | Within normal limits |
| 450/460–500 | Prolonged |
| > 500 | Markedly prolonged: high risk |
| Rise ≥ 60 | From pre-drug baseline: high risk |
Notes
- Measure QT in lead II or V5 from the start of the QRS to the end of the T wave, ideally averaged over 3–5 beats. Exclude U waves. Machine-read QTc is usually Bazett and should be checked by hand when it is borderline.
- Bazett over-corrects at fast heart rates and under-corrects at slow ones. At heart rates outside roughly 60–90 bpm, prefer Fridericia or Framingham. This page leads with Fridericia for that reason.
- Thresholds here follow the AHA/ACCF/HRS 2009 recommendations: QTc 450 ms or more in men, 460 ms or more in women is prolonged. A QTc over 500 ms, or a rise of 60 ms or more from the pre-drug baseline, is high risk for torsades de pointes and needs prompt action.
- The Maudsley Prescribing Guidelines use slightly different bands (normal under 440 ms in men and under 470 ms in women; above 500 ms associated with increased arrhythmia risk). Check which your unit follows.
- QTc is only one risk factor. Also check potassium, magnesium and calcium, other QT-prolonging drugs, bradycardia, structural heart disease, family history of sudden death, and drug interactions. Drug-specific QT risk: CredibleMeds (crediblemeds.org).
References
- Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353–370.
- Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Med Scand. 1920;53:469–486.
- Sagie A, Larson MG, Goldberg RJ, Bengtson JR, Levy D. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). Am J Cardiol. 1992;70(7):797–801.
- Hodges M, Salerno D, Erlien D. Bazett’s QT correction reviewed: evidence that a linear QT correction for heart rate is better. J Am Coll Cardiol. 1983;1:694.
- Rautaharju PM, Surawicz B, Gettes LS, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram. Part IV: the ST segment, T and U waves, and the QT interval. Circulation. 2009;119(10):e241–e250.
- Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010;121(8):1047–1060.
- Taylor DM, Barnes TRE, Young AH. The Maudsley Prescribing Guidelines in Psychiatry. 14th ed. Wiley-Blackwell; 2021.
An educational aid for clinicians, not a substitute for clinical judgement. Check every result against the ECG, the drug chart and your local guidelines. Nothing you enter on this page is stored or sent.
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Thresholds used
| Men | Prolonged at 450 ms or more |
| Women | Prolonged at 460 ms or more |
| All | Over 500 ms: markedly prolonged, high risk |
| Change | Rise of 60 ms or more from pre-drug baseline: high risk |
Notes
- Measure QT in lead II or V5 from the start of the QRS to the end of the T wave, ideally averaged over 3–5 beats. Exclude U waves. Machine-read QTc is usually Bazett and should be checked by hand when it is borderline.
- Bazett over-corrects at fast heart rates and under-corrects at slow ones. At heart rates outside roughly 60–90 bpm, prefer Fridericia or Framingham. This page leads with Fridericia for that reason.
- Thresholds here follow the AHA/ACCF/HRS 2009 recommendations: QTc 450 ms or more in men, 460 ms or more in women is prolonged. A QTc over 500 ms, or a rise of 60 ms or more from the pre-drug baseline, is high risk for torsades de pointes and needs prompt action.
- The Maudsley Prescribing Guidelines use slightly different bands (normal under 440 ms in men and under 470 ms in women; above 500 ms associated with increased arrhythmia risk). Check which your unit follows.
- QTc is only one risk factor. Also check potassium, magnesium and calcium, other QT-prolonging drugs, bradycardia, structural heart disease, family history of sudden death, and drug interactions. Drug-specific QT risk: CredibleMeds (crediblemeds.org).
References
- Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353–370.
- Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Med Scand. 1920;53:469–486.
- Sagie A, Larson MG, Goldberg RJ, Bengtson JR, Levy D. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). Am J Cardiol. 1992;70(7):797–801.
- Hodges M, Salerno D, Erlien D. Bazett’s QT correction reviewed: evidence that a linear QT correction for heart rate is better. J Am Coll Cardiol. 1983;1:694.
- Rautaharju PM, Surawicz B, Gettes LS, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram. Part IV: the ST segment, T and U waves, and the QT interval. Circulation. 2009;119(10):e241–e250.
- Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010;121(8):1047–1060.
- Taylor DM, Barnes TRE, Young AH. The Maudsley Prescribing Guidelines in Psychiatry. 14th ed. Wiley-Blackwell; 2021.
This is an educational aid for clinicians, not a substitute for clinical judgement. Scores and calculations support, but never replace, a full assessment. All entries were made by the user; the website does not verify them and accepts no responsibility for decisions based on them. Nothing entered on the website is stored or sent.