Major
Amiodarone × Quetiapine
Class III antiarrhythmics (Vaughan Williams)×Atypical antipsychotics
Mechanism
Amiodarone prolongs QT and weakly blocks CYP3A4 (the main quetiapine metabolic route). Quetiapine is a CYP3A4 substrate with QT effect at high doses. Additive QT prolongation plus moderate quetiapine level rise.
Symptoms
QT prolongation on ECG. Dizziness, syncope, palpitations. Severe cases: polymorphic ventricular tachycardia (torsades de pointes). Risk is higher with hypokalaemia and hypomagnesaemia.
Management
Avoid the combination at high quetiapine doses. At low doses (≤100 mg) and normal potassium and magnesium, the combination is acceptable with ECG monitoring at 2 weeks. Alternative antipsychotics with minimal QT effect: olanzapine or aripiprazole.