QTc Prolongation Calculator

The raw QT interval on an ECG shortens as the heart rate climbs and lengthens as it slows, because ventricular repolarization (the T wave) is tied to the cardiac cycle length. A QT of 400 ms means very different things at HR 50 (long, RR = 1.2 s) versus HR 110 (borderline-short, RR = 0.55 s). Correcting for rate (QTc) lets you compare values across patients and across the same patient at different rates, and it is what every published QT prolongation threshold (450 ms male, 460 ms female, 500 ms TdP risk) refers to. Reporting an uncorrected QT and applying those thresholds is the most common error in QT interpretation.

Both formulas date to 1920 and use the same RR interval, but with different exponents: Bazett uses a square root (QTc = QT / sqrt(RR)), while Fridericia uses a cube root (QTc = QT / cbrt(RR)). Bazett systematically overestimates QTc at heart rates above 100 bpm (it flags healthy tachycardia as prolonged) and underestimates at heart rates below 60 (it misses real prolongation in bradycardia). Fridericia and Framingham (a linear correction, also from the same era and refined by Sagie 1992) track the underlying rate adjustment more closely at the extremes and are preferred in drug-safety research and ICU work. The catch: Bazett is still the most widely reported formula on automated ECG carts, so most published thresholds were derived against Bazett. Use Fridericia outside 60 to 100 bpm and always check both.

Women have a baseline QTc that runs roughly 10 ms longer than men starting at puberty, driven by sex-hormone effects on the cardiac potassium channels (IKr) that control repolarization. The AHA/ACCF/HRS 2009 scientific statement codified this into separate cutoffs: upper limit of normal is 450 ms in males and 460 ms in females. Borderline runs to 470 ms in both sexes, and above 500 ms is the high-risk band for either sex. Forgetting the sex-specific cut-off in a female patient leads to false-positive prolongation alerts at the 451 to 460 ms range, which can trigger unnecessary medication holds.

Torsades de Pointes (TdP, French for 'twisting of the points') is a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation and sudden cardiac death. It is driven by early afterdepolarizations from prolonged repolarization, exactly the mechanism that a long QTc reflects. Risk rises sharply above QTc 500 ms in observational data: in drug-induced QT prolongation, the relative risk of TdP roughly doubles for every additional 10 ms above 500 ms. At QTc above 500 ms, current practice is to hold any non-essential QT-prolonging medication, check potassium and magnesium (and replace to high-normal: K above 4.0, Mg above 2.0), repeat the ECG, and arrange cardiology review. Symptoms (palpitations, presyncope, syncope) at any QTc warrant the same workup.

The class with the highest known TdP risk is class IA and class III antiarrhythmics: quinidine, procainamide, disopyramide, sotalol, dofetilide, ibutilide, and amiodarone (lower TdP risk than its prolongation suggests). Outside cardiology, common culprits include macrolide antibiotics (azithromycin, erythromycin, clarithromycin), fluoroquinolones (levofloxacin, moxifloxacin, ciprofloxacin), azole antifungals (fluconazole, ketoconazole, voriconazole), antipsychotics (haloperidol IV, ziprasidone, thioridazine), SSRIs (citalopram and escitalopram in particular, with dose-related risk), tricyclic antidepressants, methadone (cumulative, dose-dependent), and the 5HT3 antiemetic ondansetron. CredibleMeds.org maintains the authoritative public list (Known, Possible, Conditional Risk of TdP) and is the reference cited by US and European guidelines. Check it before starting any new drug in a patient with QTc above 470 ms or with prior TdP.