Engineering aspects of controlled hypothermi

The investigation presented concerned applications of experimental and clinical hypothermia. The techniques and apparatus commonly utilized for its production have been reviewed and are described, both as regards to surface and extra-corporeal cooling. The heat transfer phenomenon has been examined both from the physiological and engineering standpoint. To enable the transient state of heat transfer to be studied and to provide another means of predicting the after-fall invitable with surface cooling, the concept of an isothermic body, having little or no temperature gradient, is considered. A study has been made on a large number of dogs in collaboration with the Department of Anaesthetics, Royal Infirmary, Glasgow, to investigate the effects of hypothermia on the circulatory system. Thirty-one cases are reported here for the purposes of hypothermic cabinet calibration; for pre-determination of the total heat extraction of the animal and for the study of temperature gradients across the muscles of the hind legs during the cooling process. In the experiments the Forrester-Brown (1961) technique of air cooling was employed, the cooling cabinet being part of the operating table for the animals. In all cases the desired temperature level of the oesophagus was 30°C. The average rate of cooling employed was 4.2°C/hr which fulfilled the experimental requirements. The analytical techniques of Forrester and Brown were extended to evaluate the specific heat of the perfused and intact animal and to assess the heat extraction pattern with and without metabolic heat generation of the experimental animal. The predictive results of the after-fall of the oesophageal temperature in all the experiments carried out showed an average error of less than 1% with dogs of body weights ranging from 8.7-37 kg and of various breed. Muscle temperature gradients have been studied in eleven dogs subjected to hypothermia. After the active cooling process, during the period of stabilization, negative temperature gradients were experienced in six animals. The random variations of the heat transfer coefficient from skin to air suggested no consistency in the pattern. However, the very consistently gradual decrease of the thermal conductivity of the muscle with oesophageal temperature indicated the necessity for less and less heat extraction from the dog to maintain heat balance as hypothermia progressed. The above work is presented in the thesis in Sections I-VI, treating respectively a review of hypothermic techniques; the heat transfer phenomenon in vivo; the experimental programme, the description of apparatus; pilot tests and the experimental procedures. The theoretical and experimental findings are discussed in Section VII followed by a summary re-statement of the conclusion in Section VIII. The thesis terminates with a bibliography and appendices, the latter presenting certain ancillary and calculations in detail.