Augmented Thermic Effect of Amino Acids under General Anaesthesia Occurs Predominantly in Extra-Splanchnic Tissues

Abstract

1. Intravenous infusion of amino acid mixtures stimulates human oxidative heat production more effectively under general anaesthesia than in the unanaesthetized state. To analyse the splanchnic and extra-splanchnic regional distribution of this stimulation, whole body and splanchnic oxygen uptake, blood flow and blood temperatures were measured by a catheterization technique in 14 patients undergoing isoflurane anaesthesia for abdominal surgery. During the anaesthesia period, a mixture of 19 amino acids (240 kJ/h) was infused intravenously into seven of the patients while the others served as controls, receiving isovolumic infusions of a nutrient-free saline solution. 2. Whole body oxygen consumption fell by ≈ 40% during anaesthesia and surgery in the controls. Approximately 80–95% of the reduction occurred in the extra-splanchnic tissues. The splanchnic oxygen uptake fell by 31 ± 6% in the controls during the initial phase of anaesthesia, after which it returned to the pre-anaesthesia level. The initial reduction of the controls' splanchnic oxygen consumption accounted for only ≈23% of the simultaneous anaesthesia-induced reduction in pulmonary oxygen uptake. No initial reduction of the splanchnic oxygen uptake was observed in the patients treated with amino acid. 3. The amino acid infusion stimulated the whole body oxidative heat production by ≈18 W during anaesthesia and surgery and by ≈70 W at the emergence from anaesthesia. Approximately 74% of the stimulation occurred in the extra-splanchnic tissues. At awakening, the splanchnic oxygen uptake rose to ≈64% above the pre-anaesthesia level in the amino acid group. 4. During the entire period of anaesthesia, the whole body heat content fell by 282 ± 68 kJ in the controls and by 57 ± 25 kJ in the amino acid group. Amino acid treatment thus prevented ≈80% of the anaesthesia-induced reduction in whole body heat content. 5. During anaesthesia and surgery, cardiac output was ≈25% and ≈6% below the baseline, pre-anaesthesia levels in the control and amino acid groups, respectively. At awakening, it rose to ≈44% above baseline in the amino acid group while in the controls it remained unchanged. In both groups the splanchnic blood flow was unaffected by anaesthesia or by amino acid infusions.