title: Studies on the metabolic response to thermal injury in the rat
Creators: name: Richards, John Rooney
Publisher: name: University of Glasgow
Year: 1979

description: A gradient layer calorimetry system to hold one rat was designed and constructed by the author in collaboration with the Department of Clinical Physics and Bioengineering of the Western Regional Hospital Board. The calorimeter is capable of operating over a range of ambient temperature and humidity conditions between 20°C to 30°C and at 0.01 to 15g H2O/m3/dry air background humidity. This range is below that of thermally neutral conditions of 30-33°C for the laboratory rat. By controlling the temperature of the gradient layer box water jacket, its surrounding insulated enclosure and the laboratory in which the calorimetry system was housed, a "no-load" thermopile output from the gradient layer box was achieved within ± 0.15mV of zero at operating temperatures of 20°C and 30°C. These conditions remained stable to within ± 0.02mV over 3 hours giving a ± 0.5% level of accuracy for thermopile measurement of heat loss for a 250g rat during this period. Because of the close agreement between direct and indirect measurements of heat loss and production, with a difference of less than 2.4% at 20°C, and the stability of rat body heat content during calorimetry, restraint of the rat and body thermometry proved unnecessary. This was an advance compared with previous rat calorimeter design and operation. Special animal facilities were designed and built by the author in the Institute of Physiology to house the experimental rats under study. These allowed close control of ambient temperature, noise, dust levels and spread of bacteria. Semi-automated microkjeldahl nitrogen assay methods were adapted for use in performing rat metabolic studies. In thermoneutral environments (30°C), calorimetry showed that normal male Wistar rats obeyed Voit's Surface Law with measured total heat loss between 1097-1039 2 kcal/m[2]/day or 5.42-5.13 W/kg. In a colder environment of 20°C, resting metabolic expenditure (RME) in normal rats was 16% greater than that found in thermoneutral conditions. An ambient temperature of 20°C therefore represents a mild cold stress for the laboratory rat. A reproducible burn injury model was developed by the author for use in the rat, in which the depth and extent of the burn could be precisely controlled. Calorimetry and metabolic studies were carried out on rats with 20% and 25% of body surface area full skin depth burns. This study constitutes the first gradient layer calorimeter measurement of partitioned heat losses after burn injury in rats to be made at the typical hospital ward temperature and humidity conditions of 20°C with high relative humidity. The 25% BSA burn proved to be the most satisfactory injury "model" which, when food intake was slightly restricted, resulted in severe weight loss in the rat comparable to that seen in extensively burned man. After a 25% BSA burn, RME increased steadily to a maximum of 15.7 W/kg by the 55th post burn day, 137% greater than control values. At the end of the study, on the 6 3rd post burn day, RME remained elevated at 15.2 W/kg, 121% greater than control values. Evaporative heat loss increased after injury to a maximum value of 6.4 W/kg on the 55th post burn day, 806% greater than controls. Dry or sensible heat loss also increased after a 25% BSA burn to a maximum of 10.2 W/kg on the 35th post burn day, 66% greater than controls. On the 63rd post burn day the rate of dry heat loss was still 54% greater than controls, at 9.5 W/kg. Thermometry carried out in the 20% BSA burn study showed that there was a high rate of heat transfer from core to skin at 20°C, comparable to that measured in uninjured control rats at 30°C ambient temperature. This suggested that the burned rats' hypothalamic "set-point" for thermoregulation had been altered after injury as has been noted in man. In the burned rat at 20°C ambient temperature, and with a loss of appropriate surface insulation, increased evaporative heat loss presented a major thermoregulatory challenge.