QTc Calculator — Corrected QT Interval using Bazett and Fridericia

About

The QT interval represents the total duration of ventricular depolarization and repolarization on the surface electrocardiogram, measured from the beginning of the QRS complex to the end of the T wave. Because the QT interval varies inversely with heart rate — shortening at faster rates and lengthening at slower rates — a corrected QT interval must be calculated to allow meaningful clinical interpretation independent of rate. The Bazett formula, published in 1920 by Henry Cuthbert Bazett, uses a square root correction: QTc = QT / √RR, where RR is the interval between successive R waves in seconds. The Fridericia formula, published the same year by Louis Sigurd Fridericia, uses a cube root correction: QTc = QT / ∛RR. At heart rates between 60 and 90 beats per minute, both formulas produce similar results. However, at heart rates above 90 bpm, Bazett tends to overcorrect, producing falsely prolonged QTc values, while Fridericia performs more reliably. At heart rates below 60 bpm, both formulas may undercorrect. Other correction formulas exist, including the Framingham linear regression formula, Hodges formula, and individual-specific correction methods, but Bazett remains the most widely used in clinical practice due to its simplicity and historical precedence. The clinical significance of QTc prolongation lies in its association with torsade de pointes, a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation and cause sudden cardiac death. Both congenital and acquired forms of long QT syndrome are recognized, with over 17 genes implicated in the congenital forms (most commonly KCNQ1, KCNH2, and SCN5A). Acquired QT prolongation is far more common in clinical practice and results from medications (antiarrhythmics, antipsychotics, antibiotics, antihistamines, antidepressants), electrolyte disturbances (hypokalemia, hypomagnesemia, hypocalcemia), bradycardia, myocardial ischemia, intracranial hemorrhage, and hypothyroidism. The incidence of drug-induced torsade de pointes is estimated at 1-10 per 100,000 patient-years, with higher rates in women, older adults, and patients with pre-existing cardiac disease. Gender-specific QTc thresholds are essential because women have inherently longer QTc intervals by approximately 10-20 ms, attributed to the modulating effects of sex hormones on cardiac ion channels, particularly the slower repolarization reserve in women.

Formula

Bazett: QTc = QT / √(RR). Fridericia: QTc = QT / ∛(RR). Where RR = 60 / Heart Rate (seconds).

The QTc calculator implements two correction formulas to provide a comprehensive rate-corrected QT assessment. The Bazett formula (QTc = QT / √RR) calculates the square root of the RR interval in seconds, where RR = 60 / heart rate in beats per minute. For example, at a heart rate of 60 bpm, RR = 1.0 second and √RR = 1.0, so the correction factor is 1. At a heart rate of 100 bpm, RR = 0.6 seconds and √RR = 0.775, resulting in a larger correction. The Fridericia formula uses the cube root of RR (QTc = QT / ∛RR), which produces a smaller correction factor at elevated heart rates, explaining its greater accuracy in tachycardia. The calculator provides both results simultaneously, allowing the clinician to compare them. If the Bazett QTc is substantially higher than the Fridericia QTc, particularly at heart rates above 90 bpm, the Fridericia value is likely more reliable. Gender-specific interpretation thresholds are applied automatically based on the patient's sex. For adult males, the normal QTc is ≤430 ms (corrected Bazett), borderline is 431-450 ms, and prolonged is >450 ms. For adult females, the normal QTc is ≤450 ms (corrected Bazett), borderline is 451-470 ms, and prolonged is >470 ms. For pediatric patients (1-15 years), normal is ≤440 ms and prolonged is >460 ms regardless of sex. In the presence of wide QRS complex (bundle branch block, ventricular pacing), the QT interval is artifactually prolonged because it includes ventricular activation time. In this situation, the JT interval (JT = QT - QRS) can be used as an alternative, with a corrected JTc of ≤360 ms considered normal. The calculator can also be used to monitor serial QTc changes in patients started on QT-prolonging medications, with a change of >60 ms from baseline or an absolute QTc >500 ms generally considered a threshold for dose adjustment or discontinuation.

Score Interpretation

QTc assessment is a cornerstone of arrhythmia risk stratification in both inpatient and outpatient settings. Clinicians routinely encounter QT-prolonging medications, and awareness of drug-induced QT prolongation is critical for patient safety. Over 100 medications across multiple therapeutic classes are known to prolong the QT interval, including class IA and III antiarrhythmics (quinidine, sotalol, amiodarone, dofetilide), macrolide antibiotics (azithromycin, erythromycin), fluoroquinolones (levofloxacin, moxifloxacin), antipsychotics (haloperidol, ziprasidone, quetiapine), antidepressants (citalopram, escitalopram), antiemetics (ondansetron, domperidone), antimalarials (chloroquine, hydroxychloroquine), opiate agonists (methadone), and antifungal agents. The risk of drug-induced torsade de pointes increases when multiple QT-prolonging drugs are combined, when drugs are given in high doses or by rapid intravenous infusion, or when concomitant electrolyte disturbances exist. Hypokalemia is the most common electrolyte abnormality predisposing to QT prolongation, as low extracellular potassium reduces the repolarization reserve by decreasing the rapid component of the delayed rectifier potassium current (IKr). Hypomagnesemia potentiates the proarrhythmic effects of hypokalemia, making the synergistic effects of diuretic therapy particularly dangerous. The QTc interval is also used in the diagnosis and monitoring of congenital long QT syndrome (LQTS). Genetic testing is recommended for patients with QTc >500 ms after excluding acquired causes, those with a Schwartz score ≥3.5, and family members of known LQTS probands. LQTS genotype-phenotype correlations are well-established: LQT1 (KCNQ1 mutation) typically presents with exercise-induced events, LQT2 (KCNH2 mutation) is triggered by auditory stimuli or emotional stress, and LQT3 (SCN5A mutation) occurs during sleep or rest. The European Medicines Agency and FDA recommend ECG monitoring for all drugs with known QT-prolonging potential during clinical development, and regulatory decisions have been influenced by QT prolongation signals. In addition, QTc prolongation is an independent predictor of mortality in the general population, with a meta-analysis of over 100,000 individuals showing a hazard ratio of 1.3 for cardiovascular mortality for each 50 ms increase in QTc beyond the normal range.

Reference Ranges