Drug-Induced QTc Prolongation

Mohamed Salih, MD and Matthew Kern, MD

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)
  • The time of ventricular depolarization + repolarization
  • Prolongation is defined as QT > 440ms in males or >460ms in females
  • 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) are commonly included on ECG report (other formulas are available on MDCalc). QTcB often overestimates QT but is most accurate for bradycardia; QTcF is more accurate for tachycardia. “B for brady, F for fast”.
  • Ventricular pacing and/or bundle branch block artificially prolongs QT.
    • One quick correction is QT - (QRS - 120), then plug in the result to the correction formula.
    • Can also use Mayo Clinic QTc Calculator to correct in cases of wide QRS or Afib (simply google this)
  • QTc > 500 ms increases risk of Torsades de pointes (TdP), a life-threatening wide complex VT (see “Wide Complex Tachycardias” in the Cardiology section for evaluation and management of TdP).
  • Two main causes of prolonged QT:
    • Congenital/Hereditary (long QT syndromes/channelopathies)
    • Acquired (Drug induced, anorexia, bradycardia, MI/BBB, hypothermia, hypothyroidism, hypokalemia, hypomagnesemia, hypocalcemia, increased ICP)
  • Think of ABCDE for common medication offenders:
    • A: Anti”A”rrythmics (class IA (procainamide, disopyramide) and class III (amiodarone, sotalol, dofetilide)).
      • Less clinical concern with amiodarone since its multiple mechanisms reduce risk of TdP despite QT prolongation.
    • B: Anti”B”iotics: (azoles, macrolides, quinolones)
    • C: Anti”C”ychotics: 1st gen>2nd gen (chlorpromazine, haloperidol, risperidone)
    • D: Anti “D”epressants: SSRIs, TCAs o E: Anti”E”metics: Ondansetron, prochlorperazine, droperidol
  • Other high risk medications: methadone, Arsenic (chemo), quinines (antimalarials), hydroxychloroquine
  • Crediblemeds.org is a great resource to look up specific meds. It also shows risk of QT prolongation vs risk of inducing TdP (not always equal)
  • Special note on ondansetron (Zofran): Risk IV > PO/ODT. Risk is greater if using IV dose > 16mg, other concomitant QT prolonging meds, concomitant congenital and/or acquired QT prolongation condition.
  • Consider ECG if patient has these risk factors or if re-dosing within 2 hours.

Presentation

  • Most commonly asymptomatic
  • Possible palpitations, seizure, syncope, SCD

Evaluation

  • Check recent EKG
  • If pt is at high risk e.g. receiving antibiotics ± antiemetics while inpatient with QT >500, can monitor with EKG q 2-3 days
  • Include the most recent QTc in your handoff for crossover resident

Management

  • If stable:
    • Stop the offending medication (see ABCDE’s above) if possible. Many meds have non-QT prolonging alternatives, e.g. scopolamine patch/alcohol wipe sniff for Zofran, doxycycline for azithromycin.
    • Aggressive electrolyte repletion (K and Mg especially)
    • Serial EKG monitoring ± monitor on telemetry
  • If progression to TdP:
    • Address ABCs
    • ACLS, defibrillation if pulseless
    • Empiric IV magnesium o STAT page cardiology for overdrive pacing and likely transfer to CCU
    • FYI: Most episodes of TdP self-terminate, but patients are likely to have multiple episodes. Up to 25% of TdP cases convert to VF.
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