1QTc Correction Formulas & Normal Ranges
• Bazett Formula: QTc = QT / √RR — Most widely used; overcorrects at fast heart rates (>100 bpm).
• Fridericia Formula: QTc = QT / ∛RR — Preferred at extremes of heart rate (<60 or >100 bpm).
• Framingham Formula: QTc = QT + 0.154 × (1 − RR)
• Hodges Formula: QTc = QT + 1.75 × (HR − 60)
• RR Interval (s): RR = 60 / Heart Rate (bpm)
Clinical QTc Cutoffs & Torsades de Pointes (TdP) Risk
- Normal Males: ≤ 440 ms
- Normal Females: ≤ 460 ms (naturally longer repolarization due to estrogen effects).
- Borderline Prolonged: 441–470 ms (males) / 461–480 ms (females).
- Abnormally Prolonged: > 470 ms (males) / > 480 ms (females).
- Critical / High TdP Risk: > 500 ms in either sex (carries a dramatically elevated risk of malignant ventricular arrhythmias).
2Common QT-Prolonging Drugs & Electrolyte Risk Factors
High-Risk QT Prolonging Drugs
- Antiarrhythmics: Amiodarone, Sotalol, Quinidine, Dofetilide.
- Antipsychotics: Haloperidol, Quetiapine, Ziprasidone, Thioridazine.
- Antiemetics / GI: Ondansetron (high IV doses), Domperidone.
- Antimicrobials: Macrolides (Azithromycin, Clarithromycin), Fluoroquinolones (Ciprofloxacin, Moxifloxacin), Chloroquine.
Crucial Electrolyte & Metabolic Deficits
- Hypokalemia (Serum K < 4.0 mEq/L): Destabilizes cardiac membrane repolarization reserve. Always replete K ≥ 4.0.
- Hypomagnesemia (Serum Mg < 0.8 mmol/L): Co-exists frequently with potassium depletion and triggers TdP.
- Hypocalcemia: Prolongs the ST segment portion of the QT interval.
- Bradycardia: Lengthens the diastolic pause and promotes early afterdepolarizations (EADs).
3Frequently Asked Questions
What is QTc and why is it important?+
QTc (corrected QT interval) measures ventricular depolarization and repolarization normalized to heart rate. A prolonged QTc reflects delayed repolarization, predisposing the heart to Torsades de Pointes (TdP)—a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation and sudden cardiac arrest.
What is Torsades de Pointes and how is it treated?+
Torsades de Pointes (TdP) is a characteristic twisting-around-the-isoelectric-line polymorphic VT associated with prolonged QTc. Acute management includes Intravenous Magnesium Sulfate (2 g IV bolus) regardless of serum magnesium levels, withdrawing offending QT-prolonging drugs, correcting potassium to ≥4.5 mEq/L, and overdrive pacing or isoprenaline if recurrent.
Which correction formula should be used?+
Bazett's formula is the universal standard programmed into most automated ECG machines, but it overcorrects during tachycardia (HR >100 bpm) and undercorrects during bradycardia (<60 bpm). Fridericia's formula is preferred when heart rates deviate significantly from normal.
Clinical Takeaway: A QTc exceeding 500 ms represents a medical alert. Immediately check serum potassium and magnesium, discontinue culprit medications, and place the patient on continuous telemetry.
Medical Disclaimer: This QTc calculator provides decision-support calculations for clinicians. Manual ECG interval measurements can be subject to lead selection and T-wave offset determination artifacts. Always correlate with clinical findings.