Major
Amiodarone × Metronidazole
Class III antiarrhythmics (Vaughan Williams)×Imidazole derivatives (antiprotozoal / antibacterial)
Mechanism
Metronidazole prolongs the QT interval. Combined with amiodarone, torsades de pointes has been reported, especially with hypokalaemia.
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. Alternative antibiotics without QT effect: a cephalosporin, amoxicillin-clavulanate, or fosfomycin. For anaerobic infection requiring metronidazole: ECG before start and at day 2.