Clinical & Hospital Pharmacy

QT Interval & TdP Risk Calculator

Corrects the ECG QT interval for heart rate (QTc) with four formulas, applies sex-specific limits and scores the risk of torsades de pointes.

QTc · Fridericia

453ms

Borderline QTc (450–470 ms)

Moderate risk. Repolarization reserve is slightly impaired.

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

ECG measurements

Sex (sets the normal limit)

QT interval

Start of QRS to end of T wave. Uncorrected QT is usually 350–450 ms.

Heart rate

bpm

Updates the RR interval. Resting adult 60–100 bpm.

RR interval

Or enter the RR interval — the heart rate updates to match (60000 ÷ RR).

Baseline QTc (optional)

A pre-treatment QTc, to see the change. A rise of 60 ms or more is critical.

QRS duration

Normal is under 120 ms. Above 120 ms the wide-QRS adjustment is applied automatically.

Bundle branch block / wide QRSApplies the Bogossian adjustment: QT − 0.5 × (QRS − 100), with QRS taken as at least 120 ms. It removes the conduction delay so repolarisation is judged on its own.

Formula shown in the result

Working

Every formula, from the same effective QT.

RR interval used (60 ÷ HR)0.857s

Effective QT430ms

Normal limit for this sex< 450ms

Formula	Equation	QTc	Notes
Fridericia	QT / ∛RR	453 ms	Preferred by AHA/FDA across all heart rates
Bazett	QT / √RR	464 ms	Historical; overestimates in tachycardia
Framingham	QT + 154(1−RR)	452 ms	Robust linear population regression
Hodges	QT + 1.75(HR−60)	448 ms	Alternative linear model

QTc across heart rates (40–130 bpm)

The same QT corrected at other rates. Dashed red: 500 ms danger line; dashed green: the normal limit.

Tisdale QT risk score

For hospitalised patients: predicts QTc prolongation from clinical risk factors.

Score 0 — Low Risk (≤6)

0–6 low, 7–10 moderate, 11 or more high risk. Female sex (+1) is counted from the Sex choice above.

Age ≥ 68 years+1Loop diuretic therapy (furosemide, etc.)+1Serum potassium ≤ 3.5 mEq/L+2Admission (baseline) QTc ≥ 450 ms+2Acute myocardial infarction+2Heart failure with reduced ejection fraction+3Severe sepsis / septic shock+3Taking ≥ 2 QT-prolonging medicines+3

QT-prolonging drug precautions

High-yield drugs with a known or possible risk of TdP (CredibleMeds categories).

Filter by drug, brand or class

Amiodarone (Cordarone / Pacerone)

Known Risk of TdP

Class III Antiarrhythmic

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

Class III Antiarrhythmic / Beta-Blocker

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

Typical Antipsychotic

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

SSRI Antidepressant

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

Macrolide Antibiotic

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

Fluoroquinolone Antibacterial

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

5-HT3 Receptor Antagonist Antiemetic

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

Opioid Agonist

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

Triazole Antifungal

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.

Rate corrections (RR in seconds = 60 ÷ heart rate)

Fridericia: QTc = QT ÷ ∛RR
Bazett: QTc = QT ÷ √RR
Framingham: QTc = QT + 154 × (1 − RR)
Hodges: QTc = QT + 1.75 × (HR − 60)

Wide QRS (Bogossian)

QT adjusted = QT − 0.5 × (QRS − 100), QRS taken as ≥ 120 ms, result not below 200 ms

Applied when bundle branch block is ticked or QRS is over 120 ms. Every QTc is rounded to the nearest millisecond.

Limits (AHA/ACCF/HRS)

Men: normal < 450 ms, borderline 450–470, prolonged above 470. Women: normal < 460 ms, borderline 460–480, prolonged above 480. Critical at 500 ms or more for both; short below 340 ms. A rise of 60 ms or more from baseline is also critical.

References

1. Drew BJ et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the AHA and ACCF. Circulation. 2010;121(8):1047-1060.

2. Tisdale JE et al. Development and validation of a risk score for QT interval prolongation in hospitalized patients. Circ Cardiovasc Qual Outcomes. 2013;6(4):479-487.

3. Bogossian H et al. QTc evaluation in patients with bundle branch block. Ann Noninvasive Electrocardiol. 2014;19(6):568-575.

CredibleMeds drug database

Common questions

Bazett divides by the square root of RR, which over-corrects at fast heart rates and under-corrects at slow ones — so a tachycardic patient can look falsely prolonged. Fridericia's cube root stays much closer to the true value across the usual heart-rate range, which is why AHA/ACC and FDA recommend it.

A QTc of 500 ms or more, or a rise of 60 ms or more from baseline, carries a markedly higher risk of torsades de pointes. At that point, non-essential QT-prolonging drugs are usually stopped, potassium and magnesium replaced, and the patient monitored.

A wide QRS lengthens the QT without any change in repolarisation, so the raw QTc overstates risk. Tick the bundle branch block box and enter the QRS duration to subtract half of the excess QRS width before correcting. Any QRS above 120 ms triggers the same adjustment.

It is a validated score for hospitalised patients that adds points for risk factors such as age ≥ 68, female sex, loop diuretics, low potassium, sepsis, heart failure and multiple QT-prolonging drugs. A high score flags patients who need ECG monitoring before and after starting a QT-prolonging drug.

Either. Changing one updates the other (RR in ms = 60000 ÷ heart rate), and the calculation uses the heart rate.

About QTc

The QT interval shortens as heart rate rises, so it is corrected to a rate of 60 bpm (QTc) before it is compared with a limit. A long QTc raises the risk of torsades de pointes (TdP), a polymorphic ventricular tachycardia that can cause sudden death — and many common drugs prolong it.

Use it when

Before starting, or while monitoring, a QT-prolonging drug

Combining two or more drugs that prolong the QT interval

A hospitalised patient has risk factors (low potassium, sepsis, heart failure)

Comparing a new ECG with a baseline QTc

Keep in mind

Bazett's formula overcorrects above 80 bpm (false prolonged QTc) and undercorrects below 60 bpm. AHA/ACC and FDA recommend Fridericia for clinical decisions.

Verify automated QT measurements by hand with the tangent method in lead II or V5.

Check serum potassium and magnesium before escalating therapy in a patient with a borderline or prolonged QTc.

Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy