ICU pharmacology and drug infusions

QT prolongation, torsades and key ICU drug interactions

Reviewed Nov 2026·1 references
New· views rounded down, updated weekly

Stack-check QT drugs and risk factors (low K/Mg, bradycardia, female sex, structural heart disease). Stop offending drugs when QTc exceeds 500 ms or rises more than 60 ms, give IV magnesium for torsades, and watch the handful of interactions that cause real ICU harm.

At the bedside

Timed steps
  1. 1

    Baseline ECG

    Screen

    QTc before and after starting QT drugs; correct K to ≥ 4.0 mmol/L and Mg ≥ 2 mg/dL.

  2. 2

    Act

    Threshold

    QTc > 500 ms or increase > 60 ms: stop or switch QT-prolonging drugs, continuous monitoring.

  3. 3

    Treat

    Torsades

    Magnesium 2 g IV; unstable or pulseless: defibrillate; recurrent: overdrive pacing or isoprenaline to raise heart rate (if not congenital LQT).

  4. 4

    Check

    Interactions

    Carbapenem + valproate (valproate levels collapse – seizures; avoid); linezolid + SSRIs/fentanyl/methylene blue (serotonin syndrome); fluconazole/voriconazole + midazolam, fentanyl, tacrolimus (CYP3A4 inhibition); rifampicin (induction); amiodarone + warfarin/digoxin (halve digoxin).

Torsades de pointes

Magnesium sulphate

IV
Dose
2 g over 1-2 min if pulseless, otherwise over 10-15 min; repeat once or infuse 0.5-1 g/h
Range
1–2 g
Worked example
2 g = 4 mL of 50% solution diluted to 20 mL.

Common QT-prolonging ICU drugs

ClassExamplesNotes
AntipsychoticsHaloperidol IV, quetiapine, olanzapineIV haloperidol highest risk; ECG monitoring
AntiemeticsOndansetron, domperidone, metoclopramideDose-related
AntimicrobialsAzithromycin, clarithromycin, levofloxacin, moxifloxacin, fluconazole, voriconazole, hydroxychloroquine, bedaquilineCombined use common in India
AntiarrhythmicsAmiodarone, sotalol, procainamideAmiodarone TdP uncommon
OthersMethadone, tricyclics, citalopram/escitalopram, terlipressin (bradycardia)Check before combining

Use CredibleMeds (crediblemeds.org) for a current list.

Red flag · call your senior now
QTc > 500 ms with ventricular ectopy or pauses (short-long-short sequences).
Pitfall
Treating torsades with amiodarone.
Pitfall
Bazett over-corrects at high heart rates; use Fridericia when HR > 100.
Pearl
Hypokalaemia and hypomagnesaemia are the most fixable QT risks.

Calculators for this topic

Corrected QT (Bazett)

QT ÷ √(RR in seconds). Over-corrects at fast heart rates.

References

  1. 1

    Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010;121(8):1047-1060

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