QTc Calculator

What is the QT Interval?

The QT interval on an electrocardiogram (ECG) represents the total time for ventricular depolarization and repolarization — from the beginning of the QRS complex to the end of the T wave. It reflects the duration of the ventricular action potential and is measured in milliseconds (ms).

Because the QT interval is inversely related to heart rate (it shortens as heart rate increases), a rate-corrected value (QTc) is necessary for clinical interpretation. Without correction, a naturally shorter QT at a fast heart rate might mask pathological prolongation, and vice versa.

QTc Correction Formulas

Bazett Formula (1920)

The most widely used formula. However, it overcorrects at high heart rates and undercorrects at low heart rates. Best at normal resting heart rates (60-100 bpm).

Fridericia Formula (1920)

Preferred over Bazett at heart rates outside the normal range. The FDA now recommends Fridericia for clinical drug trials evaluating QT prolongation risk.

Framingham Formula (1992)

A linear correction derived from the Framingham Heart Study cohort. Performs well across a range of heart rates.

Hodges Formula (1983)

Uses heart rate directly rather than RR interval. Simple linear correction that avoids the overcorrection issues of Bazett at high heart rates.

In all formulas, RR = 60 / Heart Rate (in seconds).

Normal Values & Interpretation

Category	Males (ms)	Females (ms)	Risk
Normal	< 430	< 450	Low
Borderline	430 – 450	450 – 470	Moderate — monitor
Prolonged	> 450	> 470	High — evaluate cause
Dangerously Prolonged	> 500 (either sex)		Very high — risk of TdP

Note: Women have physiologically longer QT intervals than men, primarily due to hormonal effects on cardiac ion channels. This sex difference is reflected in the different normal thresholds.

ECG QT Interval Diagram

Long QT Syndrome (LQTS)

Long QT Syndrome is a disorder of cardiac repolarization characterized by prolongation of the QT interval on ECG and a predisposition to ventricular tachyarrhythmias, particularly torsades de pointes (TdP), which can degenerate into ventricular fibrillation and sudden cardiac death.

LQTS can be:

• Congenital (inherited): Caused by mutations in cardiac ion channel genes. At least 17 genetic subtypes have been identified (LQT1-LQT17). The most common are LQT1 (KCNQ1), LQT2 (KCNH2/hERG), and LQT3 (SCN5A).
• Acquired: Usually drug-induced, but also caused by electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia), structural heart disease, or hypothermia.

Clinical features include syncope (often triggered by exercise, emotional stress, or auditory stimuli depending on subtype), seizures (often misdiagnosed as epilepsy), and sudden cardiac death. Diagnosis relies on ECG findings, clinical history, family history, and genetic testing.

Drug-Induced QT Prolongation

Many commonly used medications can prolong the QT interval. The most common mechanism is blockade of the hERG potassium channel (IKr), which is critical for cardiac repolarization.

Drug Class	Examples	Risk Level
Antiarrhythmics (Class III)	Amiodarone, Sotalol, Dofetilide	High
Antipsychotics	Haloperidol, Ziprasidone, Thioridazine	High
Antibiotics	Erythromycin, Moxifloxacin, Azithromycin	Moderate
Antidepressants	Citalopram, Escitalopram (high-dose)	Moderate
Antiemetics	Ondansetron, Domperidone	Moderate
Antimalarials	Chloroquine, Hydroxychloroquine	Moderate
Opioids	Methadone	Moderate-High

Risk factors for drug-induced QT prolongation include female sex, advanced age, electrolyte disturbances, bradycardia, congenital LQTS, heart failure, and concomitant use of multiple QT-prolonging drugs. The website CredibleMeds.org maintains an updated list of drugs associated with QT prolongation.

Torsades de Pointes (TdP)

Torsades de pointes ("twisting of the points") is a specific form of polymorphic ventricular tachycardia that occurs in the setting of QT prolongation. It is characterized by a gradual change in the amplitude and direction of the QRS complexes, creating a distinctive "twisting" pattern on the ECG.

TdP may be self-terminating (causing syncope) or may degenerate into ventricular fibrillation and cardiac arrest. The risk of TdP increases significantly when the QTc exceeds 500 ms.

Acute management of TdP:

• IV magnesium sulfate 2g bolus (first-line treatment even if Mg levels are normal)
• Overdrive pacing (temporary transvenous pacing at 90-110 bpm)
• Isoproterenol infusion (to increase heart rate if pacing unavailable)
• Discontinue all QT-prolonging drugs
• Correct electrolyte abnormalities (K+, Mg2+, Ca2+)
• Defibrillation if TdP degenerates to ventricular fibrillation

Worked Example

A 55-year-old female patient on sotalol has an ECG showing QT interval = 420 ms and heart rate = 80 bpm.

By the Bazett formula, QTc = 485 ms, which is prolonged for a female (threshold: 470 ms). This warrants clinical attention — consider dose adjustment or alternative antiarrhythmic. The Fridericia result (462 ms) is borderline, highlighting why the Bazett formula can overestimate QTc at elevated heart rates.

Frequently Asked Questions

Which QTc formula should I use?

Bazett is the most widely used and recognized, making it the default in most clinical settings. However, Fridericia is increasingly preferred (especially by the FDA for drug studies) because it is more accurate at heart rates above 60 or below 100 bpm. For research and drug-safety evaluations, report Fridericia.

Why is the QT interval longer in women?

Testosterone shortens the QT interval by enhancing repolarizing potassium currents. After puberty, males develop shorter QT intervals than females. This sex difference means women are at approximately 2-3 times higher risk for drug-induced TdP compared to men.

Can exercise affect QTc measurement?

Yes. QTc should ideally be measured from a resting 12-lead ECG. During exercise, heart rate increases significantly, and the Bazett formula may overestimate QTc. If an exercise ECG is the only available source, use the Fridericia or Hodges correction.

What should I do if QTc is above 500 ms?

A QTc above 500 ms significantly increases the risk of torsades de pointes. Actions include: immediately review and discontinue any QT-prolonging medications, check and correct electrolytes (K+, Mg2+), place on continuous telemetry monitoring, and obtain urgent cardiology consultation.