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Diphenhydramine prevents the haemo-dynamic changes of cimetidine in ICU patients


Cimetidine, a histomine 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 < 0.01) two minutes after cimetidine. Also, systemic vascular resistance (SVR) decreased during the eight-minute observation period (P < 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.


La cimétidine, un bloqueur des récepteurs histaminiques de type 2 (H2), induit une vasodilatation entraînant de l’ hypotension surtout chez les grands malades. Une stimulation des récepteurs H1 pourrait en être la cause. Nous avons done étudié l’ influence de la diphenhydramine, un bloqueur H1, sur les effets hémody-namiques de la cimétidine aux soins intensifs. Chaque patient participait à deux jours de séance expérimental soit une injection de 200 mg de cimétidine iv précédée en une occasion randomisee de 40 mg de diphenhydramine iv. Deux minutes après l’ injection de cimétidine seule, la tension artérielle moyenne passait de 107,4 ± 8,4 à 86,7 ± 11, 4 mmHg (P < 0, 01) el la résistance vasculaire systémique diminuait pendant au moins huit minutes (P < 0,01). Lorsque que la cimétidine était précédée de diphenhydramine, les variables hémodynamiques restaient stables. Les bloqueurs des récepteurs histaminiques H1 peuvent done prévenir l’ hypotension associée à la cimétidine puisque que cette dernière stimule les récepteurs H1 entraînant une vasodilatation.


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Correspondence to K. Omote.

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Omote, K., Namiki, A., Iwasaki, H. et al. Diphenhydramine prevents the haemo-dynamic changes of cimetidine in ICU patients. Can J Anaesth 38, 210–212 (1991).

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Key words

  • Blood Pressure: hypotension
  • Histamine: cimetidine, diphenhydramine