Hemodynamic effects of H2-receptor antagonists. Histamine H2-receptor antagonists (H2RAs) often are administered to intensive care unit patients in an attempt to reduce gastric acidity and to prevent stress-related mucosal damage. These agents have an extremely low overall incidence and severity of adverse reactions; however, hemodynamically significant hypotension has been noted. Clinical studies with rapidly administered intravenous cimetidine in critically ill patients have demonstrated a depression in blood pressure in up to 75 percent of patients. Ranitidine, also studied in this setting, does not appear to induce similar hemodynamic changes. The newer H2RAs, famotidine and nizatidine, have not been evaluated in critically ill patients. 