436 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



anestlietized clogs, and maintaining it quite steadily at rectal temperatures down 

 to 25° C. at least. The maximal cooling efficiency is conditioned by the dimensions 

 of the catheter in relation to the large vessel diameters to a greater extent than by 

 the rate of blood flow, at least within the range studied so far. 



The advantages of this technique are the relatively atraumatic procedure resulting 

 in minimal derangement of normal circulatory conditions, the possibility of main- 

 taining a steady predetermined degree of hypothermia while maintaining any desired 

 room temperature, and the possibility of continuously measuring and recording the 

 number of calories withdrawn or added in relation to the animal's temperature 

 pattern. Limitations of the procedure are that the maximal rate of heat withdrawal 

 cannot w^ell exceed a value of 1200 cal./min. X m-, and the obvious dependence of 

 cooling upon a functioning circulation. 



In all, this technique of intravascular cooling would seem to have a great deal to 

 offer in the study of temperature regulation as well as of the physiology of the rela- 

 tively undisturbed hypothermic animal ; although no clinical applications have been 

 attempted so far, one can envision a number of situations where the close tempera- 

 ture control effected by intravascular cooling could be of very real value. 



Dr. L. I. Goldberg: In a study of the effects of total, preganglionic, sympathetic 

 block (produced by epidural injection of 0.45 per cent procaine solution) on cardiac 

 arrhythmias developing during hypothermia in the dog, it was found that ventricular 

 fibrillation occurred at higher temperatures in blocked animals than in dogs with- 

 out such sympathetic block. Hypothermia was rapidly produced, in the initial series, 

 by a veno-venous, extracorporeal method, in which blood was circulated through 8 

 feet of plastic tubing immersed in w-ater at 0° C. In a later, smaller series with 

 blocked dogs, experimental conditions were unchanged, except that hypothermia 

 was produced more gradually by application of ice packs. The animals, cooled in 

 this manner, developed ventricular fibrillation at significantly lower temperatures 

 than the animals cooled rapidly by the extracorporeal shunt. In a third series of 

 blocked dogs, hypothermia was produced by the extracorporeal shunt, but the rate 

 of temperature decrement was controlled in order to approximate the more gradual 

 rate of cooling produced by application of ice packs. Ventricular tachycardia or 

 ventricular fibrillation developed at higher temperatures in three or four of these 

 latter dogs, also, than in those made hypothermic by application of ice packs. Pre- 

 liminary results are tabulated below. 



Adult, mongrel dogs, weighing between 10 and 20 kg., were used in this study. 

 Anesthesia was induced with intravenous thiopental sodium and maintained in the 

 third stage of surgical anesthesia. Epidural, preganglionic, sympathetic block was 

 produced according to the technique described by Brewster, Isaacs and \\'aino- 

 Andersen (Am. J. Physiol. 175: 399, 1953). In this technique, laminectomy is 

 performed and polyethylene catheters are inserted into the epidural space at varying 

 levels. Procaine solution (0.45 per cent) is injected every 15 minutes through the 

 catheters until sympathetic block is achieved. After a total block is established, 

 additional procaine is injected every 30 minutes for the duration of the experiment. 

 Non-blocked dogs were prepared in the same way, l)ut normal saline solution was 

 substituted for the procaine solution. 



All dogs were mechanically ventilated by an lunerson Resuscitator with 100 per 



