cardiac complications & contraindications

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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

Last reviewed 01/2018