Pulmonary hypertension after heparin-protamine: roles of left-sided infusion, histamine, and platelet-activating factor. Severe pulmonary hypertension after protamine neutralization of heparin is an infrequent but life-threatening event following cardiopulmonary bypass. The effect of left ventricular infusion of protamine on pulmonary hypertension as well as a possible role of platelet-activating factor (PAF) or histamine in the heparin-protamine reaction was investigated in 30 pigs in four different groups during general anesthesia. Group 1 animals received 250 IU/kg heparin, followed by 100 mg protamine intravenously after 15 min. In group 2 protamine was infused into the left ventricle. Group 3 animals received the histamine H1- and H2-antagonists clemastine and ranitidine 5 min before protamine infusion. In group 4 the PAF receptor blocker WEB 2086 was given 5 min before protamine. Platelet-activating factor was measured by a bioassay in serum samples of group 1 and group 4 animals. In all four groups protamine caused severe pulmonary hypertension, thromboxane A2 release, and a transient decrease in leukocyte counts. No PAF release was detected after protamine infusion. Neither left ventricular infusion of protamine nor histamine or PAF antagonists prevented or attenuated the reactions after protamine infusion. The authors conclude that left ventricular infusion of protamine provides no protection from pulmonary hypertension, and that histamine and PAF are not involved in the acute pulmonary vasoconstriction after protamine neutralization of heparin. 