MYOCARDIAL IRRITABILITY IN EXPERIMENTAL 

 IMMERSION HYPOTHERMIA 



A. H. HEGNAUER and B. G. COVINO 



lly])(itlienni:i may he induced experiiiK-nlally by a mniiher of iiieans and at vari- 

 ous rates of cooling. Maintenance of the hypothermic state may be l)riel or \n-o- 

 longed, and at any level which is nut immediately lethal. The condition has been 

 studied in a wide variety of species either for comparative ])uri)Oses or for specific 

 measurements, each of which was more readily or economically accomplished in 

 one than in another. The dog has been the principal species employed for the study 

 of hypothermic myocardial irritability and the hypothermic state has generally been 

 induced by immersion in cold water. This section is therefore concerned primarily 

 with immersion hypothermia in the dog. 



Terminus in acute experimental hypothermia in anesthetized dogs is either (aj 

 ventricular fibrillation between 26° and 19° C, or (b) asystole between 18° and 

 14° C. The proportion of deaths in each category is related to the nature of the 

 anesthetic administered preliminary to experimentation. Under light ether or thio- 

 pental, and in the absence of extensive pre-immersion surgery, ventricular fibrilla- 

 tion terminates about 20 to 30 per cent of the experiments, whereas under pento- 

 barbital anesthesia the rate is 60 to 70 per cent.^- - Seconal, in amounts required to 

 l)lock shiveriiig, resembles pentobarbital in this regard.'^ No attempt has yet been 

 made to determine the basis for these differences, most studies having been made 

 under pentobarbital anesthesia. 



Several approaches to the problem of hypothermic ventricular fibrillation have 

 been explored, primarily to discover practical means for its control. From the 

 strictly physiological point of view the direct measurement of ventricular thresholds 

 as modified by hypothermia would a])pear to be a fruitful starting point, and such 

 an approach has recently been made and reported in a series of papers by Covino 

 and collaborators, including the wa-iter. Measurements of diastolic thresholds* as 

 well as of points earlier in the cardiac cycle were made.''- ''• ' In the latter papers are 

 given thresholds for early systole in some hypothermic dogs which are of a startling 

 nature, and appeared to form a basis for the fre(iuently observed ventricular fibril- 

 lation. These data have just recenth- been shown to be artefacts resulting from the 

 particular arrangement of the stimulating circuits, the nature of which is at the 

 moment only incompletely clear. Suf^ce it to say that the character of change in the 

 refractory period of the ventricle in hypothermia is not correctly portrayed in those 

 papers, and in fact there may be little if any change. 



Diastolic thresholds in relation to character of death in hypothermia. 1 or 

 these measurements* two stimulating electrodes were attached to the heart, one to 

 the auricle to serve as pacemaker and the other to the left ventricle. A Grass Model 

 3C square wave stimulator drove the heart via the auricular electrode. The rate 

 -was maintained just sufficiently above spontaneous to assure control, and was ap- 

 propriately reduced as body temperature decreased. The testing electrode on the 

 ventricle led from a second stimulator which in turn was triggered by the first. 



327 



