368 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



DISCUSSION 



Dr. George Clowes: It is apparent that the recordable electropotentials of the 

 brain disappear below certain temperatures. This "silence of the brain" is also seen 

 after the carotid arteries are occluded at somewhat higher temperatures. It seems to 

 me that the situation is different in each instance. In the first, the brain metabolic 

 and electrical activity probably is almost totally obliterated by cold. In the latter, at 

 a degree of cold normally not obliterating the brain waves, removal of the blood 

 supply causes an event to occur within the brain which stops organized electrical 

 discharge. 



It has been shown at normal temperatures that anesthetic agents in sufficient 

 concentrations can depress and ultimately obliterate the brain waves. This is re- 

 versible. We have demonstrated that severe hypercapnia uncomplicated by hypoxia 

 has this same effect. However, if an animal's brain waves are obliterated for more 

 than 20 minutes by excess carbon dioxide or anesthetics, it will die in a state 

 resembling normovolemic shock within 12 to 18 hours. Pure hypoxia, on the other 

 hand, produces little effect on the electroencephalogram until the arterial oxygen 

 content has fallen below a critical level of three volumes per cent, after which death 

 rapidly follows. It seems to me that the neurosurgeon who clamps the carotid 

 arteries is dealing with a situation similar to that which I have described at normal 

 temperatures. I should like to ask, how long is it possible to obliterate the cerebral 

 circulation under hypothermia with disappearance of brain waves and still have the 

 animal recover fully? 



Dr. Lougheed: I don't think that I can answer the first part of Dr. Clowes' 

 question very well, because when one occludes the cervical vessels to clip the 

 aneurysm, it is a rather tense moment and it is difficult to coordinate the electro- 

 encephalographer with the surgeon, so there are gross artefacts in the EEG. We 

 have not noticed that the EEG right after this period of occlusion has been dis- 

 turbed. The portions of EEG that are not disturbed by artefact during the occlusion 

 were normal. 



Animals breathing 100 per cent nitrogen at a temperature of 25° C. have not 

 shown any change in their brain waves for a period of 15 to 22 minutes. I have 

 never taken an animal or a patient to the stage of electro-silence, so I can't answer 

 that part of it. 



Dr. B otter ell: This work was done primarily as a clinical effort to improve our 

 success, or lack of it, in dealing with these ruptured aneurysms. 



One can expand a bit on what Dr. Lougheed said about the EEG, for there are 

 surprisingly few changes in ten minutes of bilateral carotid and vertebral artery 

 occlusion, There is some lowering of the amplitude and disappearance of fast 

 activity, but nothing in the way of spikes or high voltage slow waves. Dr John Scott 

 has been studying these records. We have the needle electrodes in the scalp and a 

 constant recording throughout the operation and cooling. 



It might interest you to know that the pH of these patients has ranged from 7 A 

 to 7.45 in the last dozen cases. We have been able only to make these physiological 

 observations in the course of our clinical treatment, so they are perhaps not as full 

 as some of the experimental work. 



