410 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



of heparin to each Hter of blood. The blood was perfused from an ordinary 2 liter 

 graduated cylinder with the use of a sigma pump. 



The technique employed consists of cooling these animals to a rectal temperature 

 of 32 to 34° C. The right brachial (subclavian) artery is temporarily occluded 

 distal to the internal mammary artery. A catheter is inserted into the internal mam- 

 mary artery in order to take pressures during the period of bypass. A systolic pres- 

 sure of 80 to 120 millimeters of mercury is maintained during the period of bypass. 



The left brachial (subclavian) artery is occluded temporarily. The superior cava 

 is occluded around a catheter that has been inserted through the azygos vein. Blood 

 is drained from the superior vena cava by gravity. The inferior vena cava is oc- 

 cluded and the aorta cross-clamped distal to the origin of the left brachial artery. 

 The sigma pump is turned on and blood is perfused in a proximal direction into the 

 right common carotid artery. The blood, therefore, is pumped into the arch of the 

 aorta. It can only perfuse the heart through the coronary arteries and the brain and 

 head through the left common carotid artery. 



We have used the right subclavian artery for the site of perfusion instead of the 

 carotid artery in some dogs, and the results have been the same. 



The last 20 dogs were cooled to a rectal temperature of 32 to 34° C. and the 

 right ventricle was open for 15 to 30 minutes. The aorta was cross-clamped for 

 30 to Z7 minutes. 



In these dogs none of the 20 hearts fibrillated. Seventeen of twenty dogs are 

 long-term survivors. The other three dogs died within the first 48 hours postopera- 

 tively. Gross and microscopic studies revealed pulmonary congestion. 



The method described is safe and allows open heart surgery in the dog for 

 periods of time up to 30 minutes. 



During the past year Dr. Robert Gaertner and I have used hypothermia in more 

 than 250 dogs in order to perform intracardiac procedures. All of these animals had 

 inflow occlusion. Early in our experiments we cooled the dogs to a final rectal tem- 

 perature of 20 to 25° C. and maintained inflow occlusion for 10 to 12 minutes. It 

 soon became evident, however, that with inflow occlusion for longer than 7 or 8 

 minutes, the mortality rate was extremely high. We also noted that the incidence of 

 ventricular fibrillation was very high at temperatures lower than 30° C. Tempera- 

 tures of 30 to 32° C. safely protected the brain against damage for periods of in- 

 flow occlusion of 8 minutes. The incidence of fibrillation with inflow occlusion for 

 8 minutes at 30 to 32° C. was low and the recovery rate high. We therefore rec- 

 ommend that procedures requiring inflow occlusion l)e performed at 30 to 2)2° C. 

 and that these procedures recjuire less than 8 minutes. 



Dr. I . K. R. McMillan: Two or three years ago we noticed something which at 

 the time was regarded as ridiculous, namely, that we lost a lot more dogs from 

 ventricular fibrillation in the summer than in the winter. This was in England, but 

 Dr. Swan has had the same experience, and it was also reported by Cookson about 

 five years ago. It has occurred sv:fficiently often in our experience that 1 think it is 

 a matter that needs investigating. It is one of the interesting side issues of hyi:)0- 

 thermia which is little discussed, and perhaps ni}' comment may stimulate others 

 to add their observations. 



Dr. IViUiam P. Loiujiiiire. Jr.: We have recently reviewed 100 cases in which 



