1QTc Normal Range & Interpretation
Fridericia: QTc = QT ÷ RR^(1/3) [preferred at HR >60 or <100]
Framingham: QTc = QT + 0.154×(1−RR)
Hodges: QTc = QT + 1.75×(HR−60)
RR interval = 60 ÷ Heart Rate
Normal QTc Values
- Normal males: QTc ≤440 ms
- Normal females: QTc ≤460 ms (women have naturally longer QTc due to hormonal effects)
- Borderline prolonged (males): 441–460 ms — monitor, identify and remove risk factors
- Borderline prolonged (females): 461–480 ms
- Prolonged: >460 ms males / >480 ms females — avoid QT-prolonging drugs, check electrolytes
- Critical / High TdP risk: >500 ms in either sex — drug withdrawal, electrolyte correction, cardiac monitoring, consider admission
Why QTc Varies by Formula
All QTc formulas attempt to remove the effect of heart rate on QT duration. Bazett's formula (QT ÷ √RR) is most widely used but overcorrects at high heart rates (>100 bpm) — making QTc falsely high in tachycardia. Fridericia (cube root correction) performs better at high and low heart rates and is recommended by the FDA and most regulatory bodies for drug-induced QT assessment. For clinical bedside use, Bazett remains the standard.
Common QT-Prolonging Drugs
- Amiodarone High
- Sotalol High
- Quinidine High
- Procainamide High
- Disopyramide High
- Dofetilide High
- Azithromycin High
- Clarithromycin High
- Ciprofloxacin Moderate
- Moxifloxacin High
- Chloroquine High
- Fluconazole Moderate
- Haloperidol High
- Quetiapine Moderate
- Olanzapine Moderate
- Citalopram ≥40mg High
- Tricyclics (TCA) High
- Lithium Moderate
- Ondansetron ≥32mg IV High
- Methadone High
- Domperidone High
- Tamoxifen Moderate
- Arsenic trioxide High
Risk Factors for Drug-Induced QT Prolongation
- Female sex (naturally longer baseline QTc)
- Hypokalaemia (most common electrolyte cause — repolarisation reserve reduced)
- Hypomagnesaemia
- Hypocalcaemia
- Bradycardia — longer RR interval gives more time for delayed repolarisation
- Congenital Long QT syndrome
- Heart failure, left ventricular hypertrophy
- Combining multiple QT-prolonging drugs
2Frequently asked questions
What is QTc and why is it important?
QTc (corrected QT interval) adjusts the measured QT interval for heart rate, since QT naturally shortens at faster rates. A prolonged QTc indicates delayed ventricular repolarisation, which increases the risk of Torsades de Pointes (TdP) — a potentially fatal polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation.
What is a normal QTc interval?
Normal QTc: <440 ms in men, <450 ms in women. Borderline prolonged: 440–470 ms (men), 450–480 ms (women). Prolonged: >470 ms (men), >480 ms (women). Critical/high TdP risk: >500 ms in either sex. The female sex has an inherently longer QTc and higher TdP risk than males at equivalent QTc values.
Which drugs commonly prolong QTc?
Common culprits in Indian clinical practice: antipsychotics (haloperidol, quetiapine, risperidone), antiemetics (metoclopramide, ondansetron, domperidone), antibiotics (azithromycin, ciprofloxacin, levofloxacin, clarithromycin), antifungals (fluconazole), antiarrhythmics (amiodarone, sotalol), antimalarials (chloroquine, hydroxychloroquine), antidepressants (TCAs). Check CredibleMeds (AZsert.com) for complete up-to-date list.
What electrolyte abnormalities prolong QTc?
Hypokalaemia (most important — potassium stabilises cardiac membrane; low K prolongs repolarisation), hypomagnesaemia (often co-exists with hypokalaemia), and hypocalcaemia all prolong QTc. Always correct electrolytes before administering QT-prolonging drugs. Target K >4.0 mEq/L and Mg >0.8 mmol/L in patients at risk of TdP (known long QT, receiving QT-prolonging drugs).
What is Torsades de Pointes and how is it treated?
TdP is a polymorphic VT with characteristic twisting of QRS complexes around the isoelectric line, occurring in the setting of prolonged QTc. It may self-terminate or degenerate to VF. Treatment: IV magnesium sulphate 2 g bolus (first-line regardless of Mg level), temporary cardiac pacing or isoproterenol to increase heart rate (shortens QT), withdraw offending drug, correct electrolytes, defibrillation if degenerates to VF. Avoid class IA/IC antiarrhythmics.
What formulas are used to calculate QTc?
Bazett formula: QTc = QT / √RR (most widely used, tends to overcorrect at fast rates). Fridericia formula: QTc = QT / RR^0.333 (more accurate at extremes of HR). Hodges formula: QTc = QT + 1.75 × (HR − 60). Bazett is standard on ECG machines but overestimates QTc at HR >100. Fridericia is preferred in tachycardia. Measure QT in lead II or V5 from Q wave onset to T wave end.
Medical disclaimer: This calculator is for educational and clinical decision-support purposes only. It does not replace clinical judgment or specialist consultation. RxMedCalc is not liable for clinical decisions made solely on this tool.