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Drug-Induced QTc Prolongation

Mohamed Salih

Background

  • QT is measured from the start of the Q-wave to the end of the T-wave (use the lead with the longest measurement) and is the time of ventricular depolarization + repolarization
  • QTc is the corrected estimate of QT assuming a rate of 60 bpm since QT decreases with tachycardia and increases with bradycardia
  • QTcF (Fridericia) + QTcB (Bazett) on ECG are different equations and either may be used
  • QTcB tends to overestimate QT
  • A 2016 Meta-analysis (PMID 27317349) recommended QTcF over QTcB
  • Prolongation is defined as a QT > 440 ms in males or >460 in females
  • Usually not very high concern until QTcF > 500 ms.
  • Life threatening risk of prolonged QTcF >500ms = Torsades de pointes (TdP) (see “Wide Complex Tachycardias” in the Cardiology section for evaluation and management of TdP).
  • Two main causes of prolonged QT:
    • Congenital/Hereditary (long QT syndrome/channelopathies)
    • Acquired (Drug induced, anorexia, bradycardia, MI/BBB, hypothermia, hypothyroidism, hypokalemia, hypomagnesemia, hypocalcemia, increased ICP)
    • Think of ABCDE for common offenders:
      • A: Anti”A”rrythmics (think class IA (procainamide, disopyramide) and class III (amiodarone, sotalol, dofetilide))
      • B: Anti”B”iotics: (azoles, macrolides, quinolones)
      • C: Anti”C”ychotics: 1st gen>2nd gen (chlorpromazine, haloperidol, risperidone)
      • D: Anti “D”epressants: SSRIs, TCAs
      • E: Anti”E”metics: (5-HT3 antagonists) ondansetron, also droperidol
    • Other high risk medications: methadone, Arsenic (chemo), quinines (antimalarials), hydroxychloroquine
    • Special note on ondansetron (Zofran): Risk IV > PO (ODT). Risk is greater if using IV dose > 16mg, concomitant QT prolonging meds, concomitant other congenital and/or acquired QT prolongation condition
    • Conclusion: Single dosing of IV/PO Zofran without additional risk factors is safe, but if the patient is on other QTc prolonged medications or has risk factors for QTc prolongation, would get ECG prior to giving or before giving another dose of Zofran within 2 hours

Presentation

  • Most commonly asymptomatic
  • Other symptoms include palpitations, seizure, syncope, SCD

Evaluation

  • Always evaluate with a recent EKG
  • If pt is at high risk; ie: receiving antibiotics ± antiemetics while inpatient with QT >500, can monitor with EKG q 2-3 days
  • It important to add QTc for crossover resident in your handoff

Management

  • If stable:
    • Stop the offending medication (see ABCDE’s above)
    • Aggressive electrolyte repletion (K and Mg especially)
    • Serial EKG monitoring ± monitor on telemetry
    • If offending medication is needed, think of alternatives regimens (zofran scopolamine patch/alcohol wipe sniff; neutropenic ppx LVQ/fluconazole cefdinir/micafungin, etc.)
  • If unstable:
    • ABCs
    • Defibrillation if pulseless
    • Empiric IV magnesium
    • If drug induced, contact cardiology for overdrive pacing as IV magnesium does not work
    • Refer to ACLS protocol