'Conventional' antihistamines slow cardiac repolarization in isolated perfused (Langendorff) feline hearts

We examined the effects of 'conventional' antihistamines on cardiac repolarization by using the isolated perfused feline heart model. Representative drugs from the major classes of antihistamines were tested. Each of the antihistamines evaluated in this study elicited a dose-dependent slowing of cardiac repolarization, as indicated by the QT prolongations observed from electrocardiogram (ECG) tracings recorded during these experiments. The concentrations of drugs tested ranged from I to 30 μM. Of the drugs analyzed, clemastine and hydroxyzine appeared to be the most potent (relative EC⁵⁰ values, 5.2 and 6.6 μM, respectively), causing the QT to lengthen by as much as 40-50% at a concentration of 10 μM. Brompheniramine, chlorpheniramine, and diphenhydramine displayed intermediate potencies with respect to QT prolongation (relative EC⁵⁰ values, 11-13 μM), whereas cyproheptadine, chlorcyclizine, and promethazine were the least potent of the antihistamines tested (relative EC₅₀ values, 16-20 μM). It is concluded that the antihistamines evaluated in this study act directly on the heart to slow cardiac repolarization. These findings could have important clinical relevance for patients taking excessive dosages of conventional antihistamines and those at risk of developing cardiac arrhythmias.