The effect of histamine-receptor blockade on the hemodynamic responses to protamine

A prospective randomized study was undertaken to evaluate the use of histamine-receptor blockade to prevent protamine-induced hypotension in patients undergoing elective aortocoronary artery bypass graft surgery. Normotensive patients with good left ventricular function who did not receive vasoactive or inotropic agents after cardiopulmonary bypass (CPB) were allocated to either a study or a control group. After CPB, patients in the study group received 10 mg of chlorpheniramine, and 400 mg of cimetidine, intravenously, five minutes before infusion of protamine. In both groups, protamine was injected via a central venous cannula at 2.5 mg/sec. Hypotension in the control group was significantly more severe and prolonged than in the study group. Systolic and mean arterial blood pressures in the control group fell 35% and 34%, differing significantly (P less than 0.01) from the study group in whom the reduction was 24% for both systolic and mean pressures. In addition, the control group showed significant falls in central venous pressure and increases in heart rate following protamine. In conclusion, it appears that the hemodynamic changes are only partially mediated through histamine action, and the normal response to protamine may be modified, but not abolished, by H1 and H2 receptor blockade. This blockade may, however, prevent the reduction in right heart preload and increase in heart rate following protamine.