The effect of environmental temperature on energy metabolism following thermal injury in the rat and surgical trauma in man

Animal Studies Energy metabolism following a standard thermal injury was studied using a purpose built small animal calorimeter. The effects of two environmental temperatures and two levels of dietary intake were investigated in three groups of rats over a period of approximately 50 days following injury. (1) Injured animals kept at 20°C with a daily food intake of 15g showed a marked weight loss (30%) while pair fed uninjured animals grew 41%. The burn animals had no increase in sensible heat losses, though their insensible losses were 2.6 times those of the controls, giving them a hypermetabolism of 21%. (2) Keeping the food intake at 15g per day but increasing the environmental temperature to 30°C, increased the insensible losses of the injured rats by 25%, but decreased their sensible losses by 55%. This meant that their total heat losses were reduced by 26%, but as the control animals also decreased their heat losses at the warmer temperature, the burn animals were still 21% hypermetabolic. The burn rats put on 25% in weight, almost as much as was lost by the similarly fed rats kept at 20°C. (3) A final group of animals was kept at 20°C but the rats were allowed an increased food intake of 18g per day. The heat losses were very similar to those of the other group of rats kept at 20°C, though because of the higher dietary intake, the injured animals showed a small weight gain over the experiment (4%). Measurement of the thermogenic effect of the diet showed it to be significantly different in burn and control animals (2 & 7% of food energy respectively). Using this factor, plus measurements of the metabolisable food energy, allowed calculation of energy balance from the post-absorptive calorimetry results. This method gave a more positive energy balance for all groups of rats than the energy balance calculated from chemically measured body tissue changes. This was probably because the calorimetered heat losses, measured under quiet conditions, were less than the average daily energy expenditures which the body composition changes reflected. However, the difference in energy balance between burn and control animals was similar using either method of calculation. Increasing the environmental temperature from 20 to 30°C has the same beneficial effect on energy balance as increasing the food intake by 5.1g (35%). Human Studies - A continuous open circuit indirect calorimeter using a ventilated hood design was constructed to investigate the effect of environmental temperature on heat production in patients following surgery. The indirect calorimeter was calibrated and tested using nine volunteers, who showed a mean fasting metabolic rate of 99% of the value predicted from tables. After taking a mixed diet via a nasogastric tube for 4.5 hours, their mean resting metabolic rate increased to 113% of the fasting value and their respiratory quotient increased to 0.88, in agreement with the values expected for this diet. The heat production of 20 patients was measured pre- operatively and for the first 4 days following moderate severity elective surgery. Ten patients were nursed at 20°C and ten at 28°C. There were no significant differences between the heat productions and respiratory quotients of the groups at the two environmental temperatures. The heat production increased to 113% of the preoperative value on the first post-operative day, reducing to 103% by the fourth day. Respiratory quotient decreased significantly over the post-operative period, indicating an increased utilisation of fat reserves. In contrast to the results of other studies, the mean increase of 8% in heat production over the four days following elective surgery was significant