Hemodynamics

QTc (Fridericia) Calculator

Heart-rate-corrected QT by the cube root of RR, which corrects better than Bazett at faster rates.

Reviewed by Alex G, BSN, RN, CEN, CCRN, CMC, NREMT-P · From the Reference tab of the app

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Formula

QTc = QT ÷ ∛RR
RR (seconds) = 60 ÷ heart rate
Example: QT 400 ms at 90/min: RR 0.667 s, ∛0.667 = 0.874, QTc = 458 ms (Bazett gives 490)

How it is used

• About 470 ms in men and 480 ms in women is the 99th percentile in healthy adults; over 500 ms is highly abnormal in either sex and carries a 2- to 3-fold higher torsades risk in congenital long QT (Drew 2010).
• A rise of 60 ms or more from the pre-drug baseline after a QT-prolonging drug calls for prompt action, especially with other warning signs on the ECG (Drew 2010).
• Bazett gives overlong values at faster rates, especially above about 85/min; in 6,609 patients Fridericia and Framingham corrected for rate best and Bazett worst (Drew 2010; Vandenberk 2016).
• That comparison was in sinus rhythm under 90/min; correction in atrial fibrillation or sinus tachycardia is unproven, and the hospital protocol should name one formula so serial values compare (Vandenberk 2016; Drew 2010).

Key points

• A QTc over 500 ms or a rise of 60 ms on a QT-prolonging drug needs prompt action: tell the prescriber, look for interacting drugs, bradycardia and low potassium or magnesium, and keep a defibrillator at hand (Drew 2010).

Same formula as the app. Open it in the web app.

Related one-pagers

• Rhythm GalleryBazett's QTc (QT ÷ √RR, in seconds) overcorrects at fast rates, where Fridericia (QT ÷ cube root of RR) is more accurate. A QTc over 450 ms in men or 460 ms in women is long (AHA/ACCF/HRS 2009; Drew 2010 uses 470 and 480); over 500 ms, or a rise of more than 60 ms, carries high torsades risk (Drew 2010). A wide QRS inflates the QT: trend one lead and method. V1 best separates VT from aberrancy and right from left bundle patterns, and II shows P waves best (Sandau 2017).
• Toxidromes and AntidotesQTc over 500 ms, torsades · Methadone, antipsychotics, citalopram, sotalol · Magnesium, potassium, overdrive pacing
• CEN Mental Health: Crisis, Suicide Risk, Eating Disorders and AbuseAdmission supported by any of (SAHM 2022): HR under 50 awake or 45 asleep; BP under 90/45; T under 35.6 °C (96 °F); 75% of median BMI or less; orthostatic pulse rise over 40 (under 19 y) or over 30, or BP fall over 20 systolic or 10 diastolic; low K, Na or phosphate; long QTc; syncope; acute food refusal; suicidal thoughts.
• Blood Products and TransfusionCitrate binds calcium (long QTc, tetany, paresthesias): check ionized calcium

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For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your institution’s protocols. Not a substitute for clinical judgment.