QTc & drug risk
QT corrected four ways with the right formula flagged for the heart rate, sex-specific and >500 ms thresholds, a bundle-branch-block adjustment, the Tisdale risk score, and what to do next, with a prompt to check every drug at CredibleMeds.
1ECG
Use the same formula and lead for the pre-drug and current ECG. In atrial fibrillation, average QT and RR over several beats.
2Electrolytes
3QT-prolonging drugs
4Tisdale risk score
Age ≥68 (+1), female sex (+1), K ≤3.5 (+2) and QTc ≥450 ms by Bazett (+2, from this ECG) come from the fields above; the drug count from section 3.
How it works
Formulas (QT and RR in ms; RR = 60 000 ÷ HR)
Fridericia: QT ÷ ∛(RR in s)
Framingham (Sagie 1992): QT + 154 × (1 − RR in s)
Hodges (1983): QT + 1.75 × (HR − 60)
Which one: the AHA/ACCF/HRS 2009 ECG standards recommend linear regression functions (Framingham, Hodges) rather than Bazett. The 2010 AHA/ACCF in-hospital statement notes Bazett gives overlong QTc at faster rates, particularly above 85 bpm, and expects Fridericia to take a growing role. In a 6609-patient hospital cohort, Fridericia and Framingham corrected best; Bazett over-corrected at high rates and under-corrected at low rates (Vandenberk 2016). Between 60 and 85 bpm the formulas differ little. Below 50 or above 100 bpm every formula is extrapolating beyond the rates where they were compared (Vandenberk studied rates under 90 bpm), so near-threshold values deserve caution.
Caveat: the >500 ms rule and the Tisdale derivation used Bazett. Switching formula changes how many patients cross 500 ms, so a Bazett value over 500 deserves a look even when Fridericia is lower.
Thresholds
| Finding | Meaning |
|---|---|
| QTc >470 ms (men), >480 ms (women) | Above the 99th percentile — abnormally prolonged (Drew 2010) |
| QTc >500 ms (either sex) | Highly abnormal; TdP risk 2–3-fold higher |
| Rise ≥60 ms from the pre-drug baseline | ECG sign of risk after starting a QT drug; prompt action |
Each 10 ms of QTc adds roughly 5–7% to TdP risk in congenital long QT; there is no threshold at which TdP becomes certain. Other warning signs: T–U distortion after a pause, macroscopic T-wave alternans, new ectopy or non-sustained polymorphic VT after a pause.
Wide QRS
A wide QRS lengthens QT without adding repolarization risk, so narrow-QRS limits do not apply directly. Drew 2010 offers two consistent approaches: subtract the change in QRS width after a new block, or follow the JT interval (end of QRS to end of T). The Bogossian formula, modified QT = QT − 50% × QRS, then rate-corrected as usual, matched native QTc in intermittent left bundle-branch block and after TAVI-induced block (Bogossian 2014, 2017). In right-ventricular pacing it overestimated QTc by about 25–30 ms; in right bundle-branch block with fascicular block, Bogossian plus Hodges matched best, while Bogossian plus Bazett or Fridericia deviated. Whatever the method, use it consistently over time.
Tisdale score (2013)
| Risk factor | Points |
|---|---|
| Age ≥68 y · female sex · loop diuretic | 1 each |
| Serum K ≤3.5 mEq/L · admission QTc ≥450 ms · acute MI | 2 each |
| Sepsis · heart failure · one QTc-prolonging drug | 3 each |
| ≥2 QTc-prolonging drugs | 3 more (6 total for drugs) |
Low ≤6, moderate 7–10, high ≥11 (maximum 21). In validation, QTc >500 ms or a rise >60 ms occurred in 15%, 37% and 73%. Derived in 900 cardiac-care-unit patients at one centre; performance outside that setting is less certain.
Which drugs count
The AHA 2020 statement lists antiarrhythmic agents, antimicrobial drugs, psychotropic medications and methadone, plus a growing list of neurological, anticancer and other drugs, as able to prolong QT and provoke torsades. Risk categories change several times a year, so check each drug at CredibleMeds (free registration), and check for interactions that raise drug levels.
Sources
- Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the AHA and ACCF. Circulation 2010;121:1047–60. doi:10.1161/CIRCULATIONAHA.109.192704 — 99th-percentile QTc 470 ms (men) / 480 ms (women); >500 ms highly abnormal; rise ≥60 ms; Bazett overcorrects above ~85 bpm; bundle-branch-block methods; actions; magnesium 2 g IV, pacing >70 bpm, potassium 4.5–5 mmol/L for TdP.
- Rautaharju PM, Surawicz B, Gettes LS, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram, part IV. J Am Coll Cardiol 2009;53:982–91. doi:10.1016/j.jacc.2008.12.014 — recommends linear regression functions rather than Bazett for rate correction (as quoted in Majeed et al., Ann Noninvasive Electrocardiol 2020;25:e12756).
- Tisdale JE, Chung MK, Campbell KB, et al. Drug-induced arrhythmias: a scientific statement from the AHA. Circulation 2020;142:e214–33. doi:10.1161/CIR.0000000000000905 — drug classes that cause TdP (abstract); maintain potassium >4 mEq/L and magnesium >2 mg/dL (ACC “Ten points to remember”, 21 Sep 2020).
- Vandenberk B, Vandael E, Robyns T, et al. Which QT correction formulae to use for QT monitoring? J Am Heart Assoc 2016;5:e003264. doi:10.1161/JAHA.116.003264 — formula definitions (Bazett, Fridericia, Framingham [Sagie 1992], Hodges 1983); Fridericia and Framingham best rate correction; Bazett over- and under-corrects at high and low rates.
- Tisdale JE, Jaynes HA, Kingery JR, et al. Development and validation of a risk score to predict QT interval prolongation in hospitalized patients. Circ Cardiovasc Qual Outcomes 2013;6:479–87. doi:10.1161/CIRCOUTCOMES.113.000152 — Table 5 points, maximum 21; low ≤6, moderate 7–10, high ≥11. ≥2 drugs scored 3 + 3 (as in MDCalc).
- Bogossian H, Frommeyer G, Ninios I, et al. New formula for evaluation of the QT interval in patients with left bundle branch block. Heart Rhythm 2014;11:2273–7. doi:10.1016/j.hrthm.2014.08.026; clinical validation Ann Noninvasive Electrocardiol 2017;22:e12393 (doi:10.1111/anec.12393); RV pacing: Frommeyer, PACE 2017;40:409–16 and Weipert, Clin Res Cardiol 2018;107:1033–9 (overestimates by ~25–30 ms); RBBB/bifascicular: Erkapic, Clin Cardiol 2020;43:957–62.
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