Qtc Calculator
Calculate qtc quickly with our cardiovascular system tool. Get results based on evidence-based formulas with clear explanations.
Formula
Bazett: QTc = QT / sqrt(RR); Fridericia: QTc = QT / RR^(1/3)
Where QT = measured QT interval in milliseconds, RR = RR interval in seconds (calculated as 60/heart rate). The Bazett formula uses the square root of RR, while Fridericia uses the cube root. Both formulas normalize the QT interval to a heart rate of 60 bpm for standardized comparison.
Worked Examples
Example 1: Normal QTc Assessment
Problem: A 45-year-old male has a QT interval of 380 ms on ECG with heart rate 75 bpm. Calculate QTc using multiple correction formulas.
Solution: RR interval = 60/75 = 0.800 seconds\n\nBazett: QTc = 380 / sqrt(0.800) = 380 / 0.894 = 425 ms\nFridericia: QTc = 380 / (0.800)^(1/3) = 380 / 0.928 = 409 ms\nHodges: QTc = 380 + 1.75 x (75-60) = 380 + 26.3 = 406 ms\nFramingham: QTc = 380 + 0.154 x (1000-800) = 380 + 30.8 = 411 ms
Result: QTc Bazett: 425 ms | QTc Fridericia: 409 ms | Normal range for male (< 450 ms)
Example 2: Drug-Induced QT Prolongation
Problem: A 60-year-old female on sotalol has QT 520 ms with HR 58 bpm. Evaluate QTc and assess TdP risk.
Solution: RR interval = 60/58 = 1.034 seconds\n\nBazett: QTc = 520 / sqrt(1.034) = 520 / 1.017 = 511 ms\nFridericia: QTc = 520 / (1.034)^(1/3) = 520 / 1.011 = 514 ms\n\nBoth formulas show QTc > 500 ms in a female patient (upper limit 460 ms).\nQTc > 500 ms indicates markedly prolonged with high TdP risk.
Result: QTc Bazett: 511 ms | Markedly Prolonged | TdP Risk: Significantly Elevated | Consider drug adjustment
Frequently Asked Questions
What is the QTc interval and why is it important in clinical medicine?
The QTc (corrected QT) interval is the QT interval on an electrocardiogram adjusted for heart rate, representing the total duration of ventricular depolarization and repolarization. The QT interval naturally shortens at faster heart rates and lengthens at slower rates, so correction is necessary for meaningful interpretation. QTc prolongation is clinically significant because it indicates delayed ventricular repolarization, which creates a substrate for potentially fatal polymorphic ventricular tachycardia known as Torsades de Pointes (TdP). Normal QTc values are generally below 450 ms in males and 460 ms in females. QTc monitoring is essential when prescribing QT-prolonging medications, evaluating syncope, screening for inherited channelopathies (Long QT Syndrome), and managing electrolyte abnormalities that affect cardiac repolarization.
What is Torsades de Pointes and how does QTc prolongation cause it?
Torsades de Pointes (TdP) is a specific form of polymorphic ventricular tachycardia that occurs in the setting of QT prolongation. The name means twisting of the points in French, describing the characteristic ECG appearance where the QRS axis appears to rotate around the baseline. TdP occurs when prolonged repolarization creates heterogeneous recovery of excitability across the ventricular myocardium, allowing early afterdepolarizations (EADs) to trigger reentrant circuits. The risk of TdP increases roughly exponentially with QTc duration, with significant risk emerging above 500 ms. However, TdP can also occur at lower QTc values, particularly with rapid QTc increase (delta QTc greater than 60 ms from baseline). TdP can self-terminate causing syncope, or degenerate into ventricular fibrillation causing sudden cardiac death. Acute treatment includes intravenous magnesium, overdrive pacing, and isoproterenol to increase heart rate.
How should QTc be monitored in hospitalized patients on QT-prolonging drugs?
QTc monitoring protocols for hospitalized patients receiving QT-prolonging medications should include a baseline 12-lead ECG before drug initiation with calculation of QTc using a consistent formula. Follow-up ECGs should be obtained after achieving steady-state drug levels (typically 3-5 half-lives), after dose increases, and when other QT-prolonging drugs or conditions are added. Electrolytes (potassium, magnesium, calcium) should be checked and corrected before and during therapy. Drug discontinuation or dose reduction should be strongly considered if QTc exceeds 500 ms or if delta QTc exceeds 60 ms from baseline, as these thresholds substantially increase TdP risk. Continuous telemetry monitoring should be employed for patients with QTc above 500 ms or those receiving intravenous QT-prolonging medications. Automated QTc monitoring algorithms are increasingly available in electronic health records and can provide real-time alerts.
What is the JTc interval and when is it more useful than QTc?
The JTc (corrected JT) interval measures the time from the end of the QRS complex to the end of the T wave, corrected for heart rate, and specifically reflects ventricular repolarization duration without the confounding effect of depolarization time. The JT interval equals the QT interval minus the QRS duration. JTc becomes clinically important in patients with bundle branch block or intraventricular conduction delays, where QRS prolongation artificially increases the QT interval without actually affecting repolarization. In patients with left bundle branch block or ventricular pacing, the QTc can be misleadingly prolonged, while the JTc provides a more accurate assessment of repolarization. Some experts recommend using JTc when QRS duration exceeds 120 ms. Additionally, JTc may be superior to QTc for risk stratification in patients with heart failure and conduction disease, and for assessing drug-induced repolarization changes in patients with pre-existing bundle branch block.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.
Can I use the results for professional or academic purposes?
You may use the results for reference and educational purposes. For professional reports, academic papers, or critical decisions, we recommend verifying outputs against peer-reviewed sources or consulting a qualified expert in the relevant field.