EXTERNAL COOLING METHODS— MULLER and DAMMANN 419 



ture has been reduced only moderately because it is slower than most of the other 

 methods. 



Mechanics of hypothermia. In order to answer certain questions regarding the 

 mechanics of hypothermia, fifteen groups using hypothermia were polled. 



1. Raic of cooling. All but one of the 15 favored rapid over slow cooling. Most 

 gave as the reason convenience for the surgeon and the operating teams. A more 

 concrete advantage of rapid cooling is that the period of anesthesia is reduced. It 

 was also felt that the incidence of frostbite and possible fat necrosis is reduced by 

 rapid cooling. Another important reason for cooling rapidly involves alterations in 

 coronary blood flow. Berne-" has found in experimental animals that from 39.5° to 

 28° C. the aortic pressure decreased slowly, whereas below 28° C. it dropped 

 sharply. In contrast to this, the coronary blood flow decreased precipitously in the 

 early stages of hypothermia, declined more gradually as the blood pressure fell 

 between 33.5° and 21.5° C, and remained fairly constant at 20° C. This constancy 

 remained in spite of a further decrease in aortic pressure. It would seem advan- 

 tageous, therefore, to lower the temperature rapidly, especially during the early 

 phase of hypothermia when the coronary blood flow is relatively reduced. Lewis^^ 

 stated that the only incidence of irreversible ventricular fibrillation which he has 

 encountered was in a patient in whom rapid cooling was used. He, therefore, pre- 

 fers relatively slow cooling. 



2. Rate of rc-cvarming. Lewis^^ found a higher survival rate in rats which were 

 rewarmed rapidly. Lind and Senning^'' noted small necrotic foci in the myocardium 

 of dogs in all of their hypothermic experiments and felt that time might be a fac- 

 tor; that is, the longer the duration of the experiment, the more extensive the 

 lesions. Drift occurs with re warming just as it does with cooling and one should 

 discontinue active warming when the body temperature is within 2-4° C. of normal. 

 This minimizes the development of hyperpyrexia. 



3. Optimal teniperatnre. This temperature depends on w^hether or not one is 

 using hypothermia as an adjunct or whether one plans to interrupt the cardiac 

 venous return for intracardiac surgery. When uSed as an adjunct, one desires to re- 

 duce the oxygen consumption significantly but, at the same time, not reduce the tem- 

 peratures to levels which carry a greater risk of serious cardiac arrhythmias. At 

 30° C. the oxygen consumption is roughly one-half of normal and the general level 

 of 29° to 32° C. is being used by most investigators when the entire circulation is 

 not interrupted. When inflow occlusion is used, a lower temperature is desired. A 

 range of 25° to 28° C. is generally advocated and Swan-^ states that for periods of 

 inflow- occlusion as long as 12 minutes, the temperature should be lowered to at 

 least 25° C. Below 30° to 32° C. the risk of hypothermia becomes greater because 

 it is generally agreed that ventricular arrhythmias begin at approximately this level 

 and become more frequent as the temperature is reduced. In achieving the optimum 

 temperature, one must take into consideration the drift which occurs after surface 

 cooling has been discontinued. The drift is usually one-half to two-thirds the num- 

 ber of degrees of lowering at the time the cooling agent is discontinued, but de- 

 pends on a number of factors including the rate of cooling and the size of the pa- 

 tient. If cooling has been rapid, it is likely that the drift will be greater. Drift is 

 likely to be greater in large patients and adults and less in the smaller patient w'here 



