Corrected QT Interval Calculator (QTc)
The QT interval on an ECG reflects ventricular repolarization time. It varies with heart rate, so it's corrected (QTc) for accurate risk assessment. Enter QT interval and heart rate to calculate QTc.
Understanding the QT Interval
The QT interval represents the time it takes for the heart's ventricles to depolarize (contract) and repolarize (recover). It's measured from the beginning of the Q wave to the end of the T wave on an ECG.
Normal QT duration depends on heart rate. At 60 beats per minute, normal QT is about 390-450 ms. As heart rate increases, QT shortens. Without correction, a QT that's normal at rest might appear abnormal during exercise.
QT prolongation delays repolarization, creating electrical instability. This can trigger torsades de pointes, a life-threatening arrhythmia that degenerates into ventricular fibrillation and sudden death. Drug-induced QT prolongation causes thousands of deaths annually.
Medical disclaimer: This calculator is for educational purposes only. QT interval measurement and interpretation require expertise in ECG reading. QT prolongation can be life-threatening and requires immediate medical evaluation by a cardiologist.
Causes of QT Prolongation
Medications are the most common cause of QT prolongation. Antiarrhythmics (sotalol, dofetilide), antibiotics (azithromycin, fluoroquinolones), antipsychotics (haloperidol, ziprasidone), and antidepressants (citalopram, tricyclics) all prolong QT. The FDA maintains a list of QT-prolonging drugs at CredibleMeds.org.
Electrolyte abnormalities—especially hypokalemia, hypomagnesemia, and hypocalcemia—prolong QT and increase arrhythmia risk. Diuretics causing potassium loss are a common culprit. Correcting electrolytes before starting QT-prolonging drugs is standard practice.
Congenital long QT syndrome (LQTS) is a genetic disorder affecting cardiac ion channels. Patients present with syncope, seizures, or sudden death triggered by exercise, loud noises, or swimming. QTc is typically >480 ms. Treatment includes beta-blockers, lifestyle modifications, and sometimes an implantable defibrillator.
Acquired LQTS also occurs in bradycardia (slow heart rate), hypothyroidism, intracranial hemorrhage, and malnutrition (refeeding syndrome). Women have longer baseline QT than men, making them more susceptible to drug-induced prolongation.
Bazett vs. Fridericia: Which Formula to Use?
The Bazett formula (QTc = QT / √RR) is simple and widely used but has limitations. It overcorrects at heart rates above 90 bpm and undercorrects below 60 bpm. This can falsely classify patients as high or low risk.
The Fridericia formula (QTc = QT / ∛RR) is more accurate across a range of heart rates. It's preferred in clinical trials, pacemaker settings, and atrial fibrillation where heart rate varies widely. Many cardiologists now favor Fridericia over Bazett.
Both formulas assume regular rhythm. In atrial fibrillation with variable RR intervals, QT measurement becomes unreliable. Averaging QT over multiple beats or using Fridericia improves accuracy.
Regardless of formula, QTc >500 ms is universally recognized as high risk. At this level, torsades de pointes risk is 2-5% per year without intervention. Stopping causative drugs, correcting electrolytes, and close monitoring are mandatory. Refractory cases may need pacing, mexiletine, or left cardiac sympathetic denervation.
Frequently Asked Questions
What is the QT interval?
The QT interval on an electrocardiogram (ECG) measures the time from the start of the Q wave to the end of the T wave. It represents the duration of ventricular depolarization and repolarization.
Why does QT need to be corrected?
QT interval shortens at faster heart rates and lengthens at slower rates. QTc corrects for heart rate, allowing consistent risk assessment. A QT of 400 ms is normal at 60 bpm but prolonged at 100 bpm.
What is the Bazett formula?
The Bazett formula is QTc = QT / √RR, where RR is the interval between heartbeats in seconds. It's the most widely used but overcorrects at high heart rates and undercorrects at low rates.
What is the Fridericia formula?
The Fridericia formula is QTc = QT / ∛RR. It's more accurate than Bazett at extreme heart rates and is preferred in clinical trials and patients with atrial fibrillation.
What is a dangerous QTc?
QTc >470 ms in men or >480 ms in women is prolonged and increases torsades de pointes risk. QTc >500 ms carries high risk of sudden cardiac death and requires urgent intervention.