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A previous report from this laboratory (1) presented the results of hematologic studies in nine dogs cooled below 200 C. During induction of hypothermia the animals showed a marked rise in red cell count, hemoglobin, and hematocrit, together with a nearly complete disappearance of white cells and platelets. On rewarming, the values all returned to normal. A number of studies have been carried out to learn more about the mechanism of these changes. The present paper describes some of the studies dealing especially with the changes in platelets and white cells. The experiments were performed on a group of 22 adult mongrel dogs. Hypothermia was induced by a method of surface cooling previously reported (2). Each dog was anesthetized with intravenous thiopental. An endotracheal catheter was inserted and connected to an automatic respirator utilizing room air. Thermo-couples in the rectum and esophagus permitted continuous temperature recordings by a Brown potentiometer. A continuous three limb lead electrocardiogram and aortic blood pressure were also recorded. The animal was placed in an ice bath at 40 C. and cooled until ven-tricular fibrillation occurred or the rectal temperature reached 170 C. The heart was defibrillated electrically. Each experiment, from the onset of hypothermia until rewarming had raised the body temperature to approximately 30° C., occupied approximately five hours. Platelet counts were carried out by the method of Brecher and Cronkite (3) using phase microscopy. Two pipettes were filled and two chambers for each pipette were counted and the average was taken as the result for each determination. By this technique, the normal range for platelet counts in dogs is 200,000 to 350,000 per cu. mm. Leukocyte counts were carried out by the standard hemocytometer technique using 1: 20 dilution. Both sides of two hemocytometer chambers were counted. Hemoglobin concentration was measured by the cyanmethemoglobin method (4). Hematocrit was measured in capillary tubes after high-speed centrifuga-tion (5). Protocol 1: Bone marrow aspirations and biopsies were carried out from the ribs and tibias of three dogs before, during, and after hypothermia. Protocol 2: Studies with radioactive platelets were carried out in dogs using a donor (Dog A) and a recipient (Dog B). Two mc. of Pa (NaHP"0O, Abbott) was injected into Dog A. One week later, Dog A was bled by cardiac puncture, and 500 ml. of blood was drawn into a plastic bag containing disodium versinate solution. Simultaneously 500 ml. of blood was taken from …

A CAUSE OF THE THROMBOCYTOPENIA AND LEUKOPENIA THAT OCCUR IN DOGS DURING DEEP HYPOTHERMIA