Calculate corrected QT interval using Bazett, Fridericia, and Framingham formulas simultaneously. Drug safety screening, Torsades de Pointes risk assessment, electrolyte guidance, and EMR-ready cardiac documentation.
Amiodarone (prolongs QT but low TdP risk due to homogeneous repolarization effect)
Special case
Reference: CredibleMeds.org (AZCERT) for complete updated drug risk classification. Combinations of two or more QT-prolonging drugs significantly increase TdP risk even when each individual drug produces only modest prolongation.
QTc Correction Formulas — Comparison
Formula
Equation
Best Used For
Bazett
QTc = QT ÷ √(RR)
Most widely used clinically; standard for drug trials; over-corrects at HR >100 bpm
Fridericia
QTc = QT ÷ ∛(RR)
More accurate at heart rate extremes; preferred in ICU, tachycardia, and bradycardia
Framingham
QTc = QT + 0.154×(1−RR)
Linear correction; best for ambulatory/Holter monitoring; reliable at all heart rates
QTc Interval — Clinical Reference for Cardiac Safety
The QT interval on a standard 12-lead ECG represents the duration of ventricular electrical depolarization and repolarization — from the beginning of the QRS complex to the end of the T-wave.
Because the QT interval shortens at faster heart rates and lengthens at slower rates, clinicians use the corrected QT interval (QTc) to normalize measurements to a heart rate of 60 bpm, allowing comparison across different patients and conditions.
Prolonged QTc is the most common cause of drug withdrawal from clinical use and is a major concern in ICU patients receiving multiple medications.
Why Bazett, Fridericia, and Framingham Give Different Values
All three formulas aim to remove the effect of heart rate from the QT interval, but use different mathematical models.
Bazett uses a square root correction (QT ÷ √RR) — the simplest and most widely used, but it systematically over-corrects at high heart rates (>100 bpm), producing falsely elevated QTc values that may lead to unnecessary drug discontinuation in tachycardic patients.
Fridericia uses a cube root (QT ÷ ∛RR) — more accurate at heart rate extremes and the preferred formula in ICU settings and pharmacological research.
Framingham (Sagie) uses a linear correction — best suited for ambulatory/Holter monitoring across a wide physiological heart rate range.
This calculator computes all three simultaneously so clinicians can compare.
Torsades de Pointes — Prevention and Management
Torsades de Pointes (TdP) is a polymorphic ventricular tachycardia associated with QT prolongation.
Risk factors beyond QTc >500 ms include: female sex (2× higher TdP risk), hypokalemia (K⁺ <3.5 mEq/L), hypomagnesemia (Mg²⁺ <0.8 mmol/L), bradycardia, structural heart disease, and concurrent use of multiple QT-prolonging drugs.
Prevention: maintain K⁺ >4.0 mEq/L and Mg²⁺ >1.0 mmol/L in high-risk patients, avoid combining QT-prolonging drugs, and perform baseline + serial ECG monitoring.
Management of TdP: IV magnesium sulfate 2g bolus over 1–2 minutes (even if Mg²⁺ is normal); correct hypokalemia; discontinue causative drugs; temporary overdrive pacing at 90–110 bpm if bradycardia-dependent TdP.
QTc Monitoring in the ICU — Practical Protocol
Baseline ECG: Before starting any QT-prolonging drug; document QT, HR, and QTc formula used
Repeat ECG: 4–8 hours after drug initiation or dose change, then daily if QTc >450 ms
QTc 450–500 ms: Review all QT-prolonging medications; correct electrolytes (K⁺ target >4.0, Mg²⁺ target >1.0); increase ECG frequency
QTc >600 ms or symptomatic TdP: ICU-level monitoring; ACLS; consider overdrive pacing; AVOID class IA/III antiarrhythmics (they will further prolong QT)
Does amiodarone cause Torsades de Pointes despite prolonging QTc?
Amiodarone is unique — it significantly prolongs the QTc interval (often by 60–100 ms) but rarely causes Torsades de Pointes. This apparent paradox occurs because amiodarone prolongs repolarization uniformly across all cardiac layers (epicardium, mid-myocardium, endocardium), reducing transmural dispersion of repolarization — the key substrate for TdP. Most other QT-prolonging drugs preferentially affect the mid-myocardial M-cells, increasing dispersion and TdP risk. Therefore, amiodarone-induced QTc prolongation above 500 ms does not require automatic drug discontinuation, unlike other QT-prolonging agents — but should still be monitored closely.
Why do women have a higher QTc than men?
Women have longer intrinsic QTc intervals than men by approximately 10–20 ms. This sex difference is driven by the effects of sex hormones on cardiac ion channels — specifically, testosterone in men shortens the QT interval by enhancing certain repolarizing potassium currents (IKr, IKs), while estrogen has a smaller effect on prolongation. This accounts for why women have a higher normal QTc threshold (460 ms vs 440 ms for men) and a 2–3 times higher risk of drug-induced Torsades de Pointes for the same degree of QTc prolongation. Post-menopausal women lose some of this risk difference as estrogen levels fall.
How do I measure the QT interval on an ECG?
Measure the QT interval in lead II or V5 (where T-wave amplitude is typically largest and endpoint clearest): (1) Identify the start of the QRS complex — the first deflection from the isoelectric baseline; (2) Identify the end of the T-wave — where the T-wave returns to the isoelectric baseline (use the tangent method: draw a line along the steepest downslope of the T-wave and mark where it crosses the baseline); (3) Measure the distance between these points in milliseconds. Average 3–5 consecutive beats. If prominent U-waves are present (hypokalemia), measure to the end of the T-wave only, not the U-wave, unless the T and U are fused. Always note the lead and heart rate used for measurement.
Which QTc formula should I use — Bazett or Fridericia?
Use Fridericia in the ICU, especially for patients with tachycardia (HR >100 bpm) or bradycardia (HR <50 bpm) — Bazett over-corrects at high rates, producing falsely high QTc values that may lead to unnecessary drug discontinuation. Use Bazett if comparing to historical ECG values calculated with Bazett (for consistency), or if your institution uses Bazett as the standard (most automated ECG systems use Bazett). Use Framingham for ambulatory Holter monitoring. This calculator computes all three simultaneously — when in doubt, report all three and note any clinically significant discrepancy.