logo
AHA / ACCF / HRS & CredibleMeds Guidelines

QT Interval & TdP Risk Calculator

Multi-formula rate correction (Fridericia, Bazett, Framingham, Hodges) with sex-specific thresholds, bundle branch block adjustments, Tisdale risk stratification, and CredibleMeds drug precautions.

AHA Preferred: Fridericia (∛RR)
Critical Danger: QTc ≥ 500 ms
Clinical Advisory on Formula Selection:Bazett's formula significantly overcorrects at heart rates > 80 bpm (generating false positives for prolonged QT) and undercorrects at heart rates < 60 bpm. The AHA/ACC and FDA strongly recommend Fridericia (QT / ∛RR) for primary clinical decision making.

1. ECG Parameters

Bogossian Adj

Tisdale QT Risk Score

Validated TdP Risk Stratification
Score: 0 (Low Risk (≤6))
Rate-Corrected Electrocardiographic Analysis

QTc Forecast & Arrhythmia Risk

QTc (FRIDERICIA)
453ms
Normal: < 450 ms
Fridericia (∛RR)
453ms
AHA Gold Standard
Bazett (√RR)
464ms
Standard
Borderline QTc (450–470 ms)BORDERLINE

Moderate risk. Repolarization reserve is slightly impaired.

Protocol Directive: Maintain serum K⁺ ≥ 4.0 mEq/L and Mg²⁺ ≥ 2.0 mg/dL. Re-evaluate baseline before starting second QT-prolonging agent.

Simultaneous Formula Comparison

FormulaEquationCalculated QTcClinical Evaluation
Fridericia (Gold Standard)QT / ∛RR453 msPreferred by AHA/FDA across all heart rates
BazettQT / √RR464 msHistorical; overestimates in tachycardia
Framingham (Linear)QT + 154(1−RR)452 msRobust linear population regression
HodgesQT + 1.75(HR−60)448 msAlternative linear model

QTc Trajectory Across Heart Rates (40–130 bpm)

Red Line: 500 ms Hazard

CredibleMeds QT Drug Precautions

Known & Possible Torsades de Pointes (TdP) Risk
Amiodarone(Cordarone / Pacerone)
Known Risk of TdP

Dosing Guidance: Reduce dose or discontinue if QTc > 500 ms or increases by > 60 ms over baseline.

Directly blocks IKr channels; lower incidence of TdP than other class III agents due to concurrent calcium/beta blockade, but requires baseline and serial ECGs.

Sotalol(Betapace)
Known Risk of TdP

Dosing Guidance: Strict dose reduction in renal impairment. Discontinue if QTc exceeds 500 ms.

Marked reverse use-dependence (highest TdP risk at slower heart rates / bradycardia). 100% renally eliminated.

Haloperidol(Haldol)
Known Risk of TdP

Dosing Guidance: IV administration carries substantially higher risk than PO. Limit IV doses or switch to atypical agent if QTc > 480 ms.

FDA boxed warning for IV administration. Continuous telemetry recommended during IV therapy.

Citalopram(Celexa)
Known Risk of TdP

Dosing Guidance: Max 40 mg/day (Max 20 mg/day in age > 60 yr, hepatic impairment, or CYP2C19 poor metabolizers).

FDA safety communication warns against doses > 40 mg/day due to dose-dependent QTc prolongation.

Azithromycin(Zithromax)
Known Risk of TdP

Dosing Guidance: Avoid in patients with baseline prolonged QTc, hypokalemia, hypomagnesemia, or bradycardia.

FDA warning regarding fatal cardiac arrhythmias; consider Doxycycline or beta-lactams as safer alternatives in high-risk patients.

Levofloxacin / Ciprofloxacin(Levaquin / Cipro)
Known Risk of TdP

Dosing Guidance: Dose-adjust for renal clearance. Avoid co-administration with other QT-prolonging agents.

Moxifloxacin carries highest risk among quinolones; Levofloxacin is intermediate; Ciprofloxacin is lowest.

Ondansetron(Zofran)
Possible Risk

Dosing Guidance: Max single IV dose is 16 mg. Avoid repeat doses in patients with baseline QTc > 480 ms.

ECG monitoring recommended in patients with electrolyte abnormalities or heart failure.

Methadone(Dolophine / Methadose)
Known Risk of TdP

Dosing Guidance: Perform baseline ECG. Obtain follow-up ECG if daily dose exceeds 100 mg/day or if risk factors present.

Potent hERG channel blocker. Clinically significant QTc prolongation commonly observed at doses > 100-120 mg/day.

Fluconazole(Diflucan)
Possible Risk

Dosing Guidance: Renally adjust. Monitor closely when co-administered with CYP3A4/CYP2C9 substrates.

Dual mechanism: direct IKr channel block and CYP inhibition increasing systemic levels of other QT drugs.