Augmented Thermic Effect of Amino Acids under General Anaesthesia: A Mechanism Useful for Prevention of Anaesthesia-Induced Hypothermia

Abstract

1. Intravenous infusion of amino acids stimulates energy expenditure and heat accumulation in normal man. To find out whether such stimulation also occurs during general anaesthesia, thermogenesis was measured in 21 patients before, during and after anaesthesia and surgery. 2. Ten patients received a mixture of 19 amino acids (240 kJ/h) infused intravenously throughout the anaesthesia. The other 11 patients, serving as controls, received saline. Using catheters previously inserted into the pulmonary and a systemic artery, cardiac output, arteriovenous oxygen difference, pulmonary oxygen uptake and mixed blood temperature were measured. 3. During anaesthesia and surgery, the blood temperature fell by 0.67 ± 0.09 °C/h in the control patients and by 0.38 ± 0.06 C/h in the amino acid-treated patients. Anaesthesia during 34 ± 4 min before surgery reduced the pulmonary oxygen uptake by 145 ± 9 ml/min in the control patients and by 81 ± 10 ml/min in the amino acid-treated patients, corresponding to reductions in total energy expenditure of 47 W in the control group and 26 W in the amino acid-treated group. The difference, 21 W, illustrates the thermogenic action of the amino acids. This value may be compared with that of 4 W, observed in unanaesthetized individuals subjected to 30 min of identical amino acid infusions. 4. At awakening after the anaesthesia, the oxygen consumption rose to 71 ± 21% above the pre-anaesthesia level in the amino acid-treated patients, who, without shivering, rapidly returned to normothermia, whereas in the control patients the oxygen uptake remained slightly below the pre-anaesthesia level, despite sustained hypothermia and vigorous shivering. 5. It is concluded that general anaesthesia augments five-fold the thermic effect of amino acids and that peroperative infusion of amino acids effectively prevents anaesthesia-induced hypothermia.