cardiac complications & contraindications

Last reviewed 01/2018

High plasma concentrations of the non-sedative antihistamines may be achieved by several means:

• simple overdose

• slowing the rate of hepatic excretion:
• by competing for the specific cytochrome P450 isoenzyme in the liver e.g. erythromycin and the antifungal imidazoles
• as a result of hepatic failure

High doses of non-sedative antihistamines prolong the cardiac QT interval and predispose to arrythmias such as ventricular fibrillation and torsade de pointes.

Thus, the following precautions should be observed when prescribing non-sedative antihistamines:

• do not exceed the recommended dose

• avoid simultaneous administration of:
• antifungal imidazoles e.g. ketoconazole
• macrolide antibiotics e.g. erythromycin

• avoid simultaneous administration of drugs which prolong the QT interval, including amiodarone, bretylium, disopyramide, procainamide, quinidine, sotalol, neuroleptic drugs and tricyclic antidepressants

• potassium or magnesium depletion is a contraindication

• symptomatic liver disease is a contraindication

• pre-existing QT prolongation (> 450 ms) is a contraindication