Drug-Induced QTc Prolongation

Mohamed Salih, Matthew Kern, & Reece Frechette

Background

  • QT interval: Q-wave onset to T-wave end; use lead with longest measurement 
  • QTc interval: adjusts QT for patient’s HR 
  • Prolonged QTc: >450 ms (males), >460 ms (females) 
  • High-risk of TdP: QTc >500 ms or Δ≥60 ms from baseline 
  • TdP Risk Factors: QTc >500 ms, female, age >65, bradycardia, electrolyte abnormalities, HFrEF, acute MI, ≥2 QT-prolonging drugs, hepatic/renal impairment, prior drug-induced TdP, congenital LQTS 
  • QTc formulas: Fridericia (QTcF) preferred in tachycardia; Bazett (QTcB) preferred in bradycardia as overestimates at fast HR ("F for Fast, B for Brady”) 
  • Wide QRS (BBB/pacing): correct with QT – (QRS – 120), then apply formula 
  • Wide QRS + AFib: Mayo Clinic QTc calculator

Causes of prolonged QT

  • Congenital/Hereditary (long QT syndromes/channelopathies) 
  • Acquired (Drugs, hypoK/hypoMg/hypoCa, bradycardia, MI/BBB, hypothermia, hypothyroidism, ↑ICP) 
  • Think of ABCDE for common medication offenders: 
  • Anti”A”rrhythmics: Class IA (procainamide, quinidine), Class III (sotalol, dofetilide). Amiodarone: lower TdP risk despite QT prolongation 
  • Anti”B”iotics: Macrolides, fluoroquinolones (moxi > levo > cipro), azoles 
  • Anti”Cy”chotics: 1st gen > 2nd gen (haloperidol, chlorpromazine, ziprasidone) 
  • Anti”D”epressants: TCAs, SSRIs (citalopram/escitalopram highest) 
  • Anti”E”metics: Ondansetron (IV > PO; avoid IV >16 mg), droperidol 
  • Other: Methadone, arsenic, antimalarials, loperamide (abuse doses) 
  • Resource: CredibleMeds.org

Monitoring: Baseline ECG before TdP-risk drugs, consider ECG q2-3 days; daily if on antiarrhythmics, +/- repeat ECG if re-dosing within 2 hours. Include QTc in nighlt handoff.

Management

  • If stable: discontinue offending med and implement alternatives (eg. scopolamine for ondansetron, doxycycline for azithromycin) 
  • Replete K >4.0, Mg >2.0 - Serial ECG ± telemetry 
    • If TdP: Pulseless: Defibrillation, ACLS. Stable: IV MgSO4 1-2 g (effective regardless of serum Mg). Refractory: STAT cardiology → CCU for advanced management considerations (Overdrive pacing or isoproterenol (↑HR shortens QT); lidocaine) 
    • Most TdP self-terminates but if recurs; 25% → VF
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