REVERSIBLE INHIBITION OF HUMAN PLATELET ACTIVATION BY HYPOTHERMIA IN-VIVO AND IN-VITRO

A hypothermia-induced hemorrhagic diathesis is associated with cardiop ulmonary bypass, major surgery, and multiple trauma, but its pathophys iological basis is not well understood. We examined the by hypothesis that hypothermia reversibly inhibits human platelet activation in vitr o and in vivo. Platelet activation was studied in normal volunteers by whole blood flow cytometric analysis of modulation of platelet surfac e GMP-140 and the glycoprotein (GP) Ib-IX complex in: a) shed blood em erging from a standardized in vivo bleeding time wound; b) peripheral blood activated in vitro with either thrombin (in the presence of gly- pro-arg-pro, an inhibitor of fibrin polymerization) or the stable thro mboxane (TX) A(2) analogue U46619. Platelets in peripheral whole blood were activated at temperatures between 22 degrees C and 37 degrees C. the forearm skin temperature was maintained at temperatures between 2 2 degrees C and 37 degrees C prior to and during the bleeding time inc ision. Platelet aggregation was studied in shed blood by flow cytometr y and in peripheral blood by aggregometry. Generation of TXB(2) (the s table metabolite of TXA(2)) was determined by radioimmunoassay. In vit ro, hypothermia inhibited both thrombin- and U46619-induced upregulati on of GMP-140, downregulation of the GPIb-IX complex, platelet aggrega tion, and TXB(2) generation. These inhibitory effects of hypothermia w ere all completely reversed by rewarming the blood to 37 degrees C. In vivo, platelet activation was inhibited by hypothermia as shown by 5 independent assays of shed blood: upregulation of GMP-140, downregulat ion of the GPIb-IX complex, platelet aggregate formation, TXB(2) gener ation, and the bleeding time. In summary, by a combination of immunolo gic, biochemical, and functional assays, we demonstrate that hypotherm ia inhibits human platelet activation in whole blood in vitro and in v ivo. Rewarming hypothermic blood completely reverses the activation de fect. These results suggest that maintaining normothermia or rewarming a hypothermic bleeding patient may reduce the need for platelet trans fusions.