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QTc Calculator — Drug-Induced QT Prolongation

Bazett, Fridericia, Framingham and Hodges correction with sex-specific thresholds and a quick reference for QT-prolonging drugs used in Indian outpatient, psychiatry and inpatient practice. Calculations run locally in your browser.

QT interval

Measure from the start of the QRS to the end of the T wave on lead II or V5 (use the tangent method). Average across 3–5 beats; exclude U waves where possible.

ms

Heart rate / RR interval

Enter the heart rate in beats per minute, or the RR interval in milliseconds.

Bazett over-corrects at HR > 100 and under-corrects at HR < 60 — prefer Fridericia or Framingham at the extremes.

Sex

Cut-offs follow AHA/ACCF/HRS 2009: normal < 450 ms (male) / < 460 ms (female); ≥ 500 ms confers high risk of torsades.

QTc result

QT-prolonging drugs commonly prescribed in India

CredibleMeds “Known Risk of TdP” entries that appear frequently on Indian prescriptions. Check every new prescription, especially with hypokalaemia, hypomagnesaemia, bradycardia, or a baseline QTc ≥ 450/460 ms.

  • Macrolides: azithromycin, clarithromycin, erythromycin
  • Fluoroquinolones: ciprofloxacin, levofloxacin, moxifloxacin, ofloxacin
  • Antiemetics: ondansetron (especially IV), domperidone
  • Antipsychotics: haloperidol (IV especially), chlorpromazine, quetiapine, ziprasidone, risperidone, amisulpride, pimozide
  • Antidepressants: citalopram (> 40 mg/day; > 20 mg/day if > 60 years), escitalopram (> 20 mg/day)
  • Antiarrhythmics: amiodarone, sotalol, dofetilide, ibutilide, procainamide, quinidine
  • Antimalarials / antiprotozoals: chloroquine, hydroxychloroquine, quinine, artemether–lumefantrine, pentamidine
  • Antifungals: fluconazole (high dose), voriconazole, itraconazole
  • Other: methadone, oxaliplatin, arsenic trioxide, donepezil

Reference: CredibleMeds QTdrugs list (crediblemeds.org), AHA/ACCF/HRS 2009 scientific statement, and NICE BNF cautions. List is not exhaustive — always cross-check before prescribing.

Disclaimer: This calculator is for clinical decision support only and is not a substitute for clinical judgment. QTc interpretation depends on lead quality, U-wave artefact, QRS width (use JTc or alternative correction if QRS > 120 ms or paced), electrolytes, comorbidities and the full drug list. Always integrate with the patient's individual ECG and risk profile.

References

  1. Bazett HC. An analysis of the time-relations of electrocardiograms. Heart 1920;7:353–370.
  2. Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Med Scand 1920;53:469–486.
  3. Sagie A, Larson MG, Goldberg RJ, et al. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). Am J Cardiol 1992;70(7):797–801.
  4. 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.
  5. Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: AHA/ACCF/HRS scientific statement. Circulation 2010;121:1047–1060.
  6. CredibleMeds. QTdrugs list. Available at crediblemeds.org (accessed 2026).

Severity bands follow AHA/ACCF/HRS 2009: normal < 450 ms (male) / < 460 ms (female); borderline 450–469 ms (male) / 460–479 ms (female); prolonged 470–499 (male) / 480–499 (female); high TdP risk ≥ 500 ms or any rise > 60 ms above baseline.

Record ECG findings, QTc values and drug-monitoring notes directly in your patient chart with the Dr. Notes app — works offline, keeps records secure.

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