Qtc Calculator
Calculate qtc quickly with our cardiovascular system tool. Get results based on evidence-based formulas with clear explanations.
Reviewed for accuracy by Rahul Singh, Health & Wellness Specialist
Medical disclaimer: This calculator is provided for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. Results are general estimates and may not reflect your individual circumstances. Always consult a qualified healthcare professional before making decisions about your health.
Qtc Calculator
Calculator
Adjust values & calculateEnter your values below. Every result is computed in your browser — no data is sent to any server.
Formula: Bazett: QTc = QT / sqrt(RR); Fridericia: QTc = QT / RR^(1/3)
Worked example — QTc Bazett: 425 ms | QTc Fridericia: 409 ms | Normal range for male (< 450 ms)
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 Bazett: QTc = 380 / sqrt(0.800) = 380 / 0.894 = 425 ms Fridericia: QTc = 380 / (0.800)^(1/3) = 380 / 0.928 = 409 ms Hodges: QTc = 380 + 1.75 x (75-60) = 380 + 26.3 = 406 ms Framingham: 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 Bazett: QTc = 520 / sqrt(1.034) = 520 / 1.017 = 511 ms Fridericia: QTc = 520 / (1.034)^(1/3) = 520 / 1.011 = 514 ms Both formulas show QTc > 500 ms in a female patient (upper limit 460 ms). QTc > 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.
How does the Bazett formula work and what are its limitations?
The Bazett formula, published by Henry Cuthbert Bazett in 1920, calculates QTc by dividing the measured QT interval by the square root of the preceding RR interval in seconds: QTc = QT / sqrt(RR). It is the most widely used QT correction formula and remains the standard in most clinical guidelines and drug development regulatory submissions. However, the Bazett formula has significant limitations. It overcorrects at high heart rates (above 100 bpm), yielding falsely prolonged QTc values, and undercorrects at low heart rates (below 60 bpm), potentially missing true prolongation. These inaccuracies become clinically relevant in patients with tachycardia from any cause and in athletes with physiological bradycardia. For this reason, the Fridericia formula is increasingly preferred, particularly in clinical pharmacology studies and for patients with heart rates outside the 60-100 range.
What is the Fridericia formula and when should it be used instead of Bazett?
The Fridericia formula calculates QTc by dividing the QT interval by the cube root of the RR interval: QTc = QT / RR^(1/3). It was proposed by Louis Sigurd Fridericia in 1920, the same year as the Bazett formula, but has gained increasing recognition for its superior rate correction. The Fridericia formula provides more consistent QTc values across a wider range of heart rates, particularly avoiding the overcorrection seen with Bazett at higher heart rates. The ICH E14 guidance for thorough QT studies in drug development now recommends Fridericia correction as the primary method. Clinical situations where Fridericia is preferred include evaluation of QT-prolonging drug effects, patients with tachycardia or bradycardia, and serial QTc monitoring where heart rate varies significantly between measurements. Many electrophysiologists now routinely report both Bazett and Fridericia values.
What medications commonly cause QT prolongation?
Numerous medications from diverse drug classes can prolong the QT interval by blocking the hERG potassium channel (IKr), which is critical for cardiac repolarization. Class III antiarrhythmics (amiodarone, sotalol, dofetilide) are the most potent QT-prolonging agents and are used specifically for their repolarization-prolonging effects. Antipsychotics (haloperidol, ziprasidone, thioridazine), certain antibiotics (fluoroquinolones, macrolides, azole antifungals), methadone, and ondansetron are commonly implicated in acquired QT prolongation. The risk of TdP increases synergistically when multiple QT-prolonging drugs are combined, when electrolyte abnormalities (hypokalemia, hypomagnesemia) are present, or in patients with baseline QTc prolongation or structural heart disease. The CredibleMeds database (www.crediblemeds.org) maintains a regularly updated list of drugs categorized by their QT-prolonging risk.
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.
What is Long QT Syndrome and how is it diagnosed?
Long QT Syndrome (LQTS) is a group of inherited cardiac channelopathies characterized by prolonged ventricular repolarization and increased risk of syncope, seizures, and sudden cardiac death from TdP. The most common genetic subtypes are LQT1 (KCNQ1 mutation affecting IKs current), LQT2 (KCNH2/hERG mutation affecting IKr current), and LQT3 (SCN5A mutation affecting sodium current). Diagnosis is based on the Schwartz diagnostic criteria, which incorporate QTc duration (greater than 480 ms scores 3 points), clinical history (syncope, family history of sudden death), and T-wave morphology. Genetic testing confirms the diagnosis in approximately 75% of clinically affected individuals. The prevalence is estimated at 1 in 2,000 to 1 in 2,500 individuals. Treatment includes beta-blockers (especially for LQT1 and LQT2), avoidance of QT-prolonging drugs, and implantable cardioverter-defibrillator placement for high-risk patients.
How do electrolyte abnormalities affect the QT interval?
Electrolyte imbalances significantly impact cardiac repolarization and QT duration through their effects on ion channel function. Hypokalemia is the most common electrolyte cause of QT prolongation because reduced extracellular potassium decreases IKr current (the same channel blocked by QT-prolonging drugs), impairs repolarization, and creates early afterdepolarizations. The effect is synergistic with QT-prolonging medications, making hypokalemia a major risk factor for drug-induced TdP. Hypomagnesemia independently prolongs the QT interval and also promotes potassium wasting, creating a dual mechanism for QT prolongation. Hypocalcemia prolongs the ST segment (phase 2 of the action potential) but has less arrhythmogenic potential than hypokalemia. Treatment of QT prolongation from electrolyte abnormalities requires aggressive replacement, targeting potassium levels above 4.0 mEq/L and magnesium above 2.0 mg/dL in at-risk patients.
What is the clinical significance of Short QT Syndrome?
Short QT Syndrome (SQTS) is a rare inherited channelopathy characterized by QTc intervals below 340 ms (some criteria use 360 ms) and an increased risk of atrial fibrillation, syncope, and sudden cardiac death from ventricular fibrillation. Unlike LQTS, shortened repolarization in SQTS is caused by gain-of-function mutations in potassium channels (KCNH2, KCNQ1, KCNJ2) or loss-of-function mutations in calcium channels (CACNA1C, CACNB2), leading to accelerated repolarization and shortened refractory periods. The abbreviated refractory period facilitates reentrant arrhythmias. SQTS was first described in 2000 and remains exceptionally rare, with fewer than 200 cases reported worldwide. Diagnosis requires QTc shortening plus either symptoms (syncope, cardiac arrest), family history of sudden death, or confirmed pathogenic genetic mutation. Treatment is primarily ICD implantation, with quinidine as an adjunctive pharmacological therapy that can lengthen the QT interval.
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.
References
Reviewed for accuracy by Rahul Singh, Health & Wellness Specialist · Editorial policy
Related Calculators
🧮Addiction Calculator
Calculate addiction with inputs, formulas, and instant results.
🧮Alcohol Unit Calculator
Calculate alcohol unit with inputs, formulas, and instant results.
🧮AUDIT-C Calculator
Calculate audit ccalculator with inputs, formulas, and instant results.
🧮AUDIT Full Questionnaire Calculator
Calculate auditfull questionnaire with inputs, formulas, and instant results.
🧮AUDIT Score Calculator
Calculate audit score with inputs, formulas, and instant results.
🧮CAGE Questionnaire Calculator
Calculate cagequestionnaire with inputs, formulas, and instant results.
🧮Cigarette Calculator
Calculate cigarette with inputs, formulas, and instant results.
🧮DAST-10 Drug Abuse Screening Calculator
Calculate dast10drug abuse with inputs, formulas, and instant results.