Diphenhydramine prevents the haemodynamic changes of cimetidine in ICU patients.

Cimetidine, a histamine 2 (H2) antagonist, produces a decrease in arterial pressure due to vasodilatation, especially in critically ill patients. This may be because cimetidine acts as a histamine agonist. We, therefore, investigated the effects of the histamine 1(H1) receptor antagonist, diphenhydramine, on the haemodynamic changes observed after cimetidine in ICU patients. Each patient was studied on two separate days. In a random fashion, they received cimetidine 200 mg iv on one day, and on the other, a pretreatment of diphenhydramine 40 mg iv with cimetidine 200 mg iv. In the non-pretreatment group, mean arterial pressure (MAP) decreased from 107.4 +/- 8.4 mmHg to 86.7 +/- 11.4 mmHg (P less than 0.01) two minutes after cimetidine. Also, systemic vascular resistance (SVR) decreased during the eight-minute observation period (P less than 0.01). In contrast, in the pretreatment group, little haemodynamic change was seen. We conclude that an H1 antagonist may be useful in preventing hypotension caused by iv cimetidine, since the vasodilating activity of cimetidine is mediated, in part, through the H1 receptor.