144 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



Dr. Audrey U. Smith: Freezing usually started about 10 minutes after immersion 

 in fluid at -5° C. One hour of freezing at -5° C. was just about the limit for 

 reviving hamsters by the simple method shown. When we froze them for 75 minutes 

 they recovered apparently completely, but they usually died within a few hours. The 

 commonest cause of death was gastric hemorrhage, so far as we could tell from 

 autopsies. Animals frozen for as long as 96 minutes have recovered heartbeat and 

 breathing but they usually died without recovering posture and consciousness. Ani- 

 mals frozen for 180 minutes have recovered heartbeats only. 



Dr. And jus: As to the rat, the longest period below zero in the rat was 40 minutes 

 with subsequent recovery. The lowest temperature reached was -3.3° C. m the 

 rectum and —5.7° C. under the skin. 



Dr. A. L. Hopkins: I would like to ask if you had the formation of ice in the 

 tissues of these animals or if you had vitrification. 



Dr. Smith: There is no question about it. The animals either froze or became 

 supercooled. In the frozen animals masses of ice were present in the tissues. Some of 

 the animals were cut in half instead of being resuscitated. Ice crystals could be picked 

 out of the body wall, the peritoneal cavity, the hollow of viscera and solid organs. 

 There was no doubt about the presence of ice. We were cooling the animals slowly ; 

 the physical conditions were not those under which vitrification could have been 

 expected. 



Dr. Jacob Fine: How do you explain the effect of the time factor in that case? 

 Dr. Smith: During immersion of a body in a sub-zero bath, freezing occurs 

 gradually. The ice front progresses inwards from the surface to the interior of the 

 animal. The longer the duration of freezing, the higher the proportion of water 

 frozen in any situation. At the end of an hour of freezing, 90 per cent of the water 

 in the skin may be converted into ice, and as much as 50 per cent of the total body 

 water may be frozen. In some animals as much as 62 per cent of the water in the 

 brain has become frozen. Meanwhile the concentration of electrolytes in the tissue 

 fluid surrounding the individual cells is increasing and will eventually reach a level 

 which damages the various cells and tissues. Unless the whole animal can be per- 

 meated with at least 10 per cent of glycerol we do not think we will be able to freeze 

 100 per cent of the body water and revive the hamster afterwards. 



Dr. R. IV. Brauer: Dr. Smith was kind enough to give us the operating instruc- 

 tions for their cooling procedures. Being curious about the oxygen in those, we have 

 repeated some of the experiments since, using 100 per cent oxygen instead of air 

 in the jar. Those animals go out precisely as rapidly as the ones in the air-filled jars 

 and, furthermore, if you control doing the same animal repeatedly, on alternate days 

 with oxygen and with air, the temperature lowering proceeds equally well imder 

 both conditions. 



Dr. F. D. Moore: What happens if you do not build up the CO,.? 

 Dr. Andjiis: If an animal is enclosed in the jar and the COo is absorbed, then in 

 the same period of time the animal will not cool sufficiently and will die. Conversely, 

 if the CO. is allowed to accumulate, the animal will cool belcnv 20° and will survive 

 that confinement. The simplest and most effective way is to combine the two : oxygen 

 lack and CO2 excess. 



Dr. Dripps: Most effective in what way — rapidity, survival? 



