Alterations in cerebral availability of metabolic substrates after severe head injury
Cerebral and systemic metabolism of oxygen, glucose, lactate, pyruvate, beta-hydroxybutyrate, acetoacetate, and amino acids were studied in 27 comatose patients during the first 7 days after a head injury. Systemic oxygen consumption was elevated initially and gradually returned toward normal over the week of study. In contrast, cerebral oxygen consumption was depressed and decreased further over the week of study. Cardiac output and cerebral blood flow were increased with respect to systemic and cerebral metabolic requirements, and remained elevated for the entire week of study. Systemic hyperglycemia and lactic acidosis were present. The injured brain often made a sizeable contribution to the lactic acidosis. The arterial concentrations of pyruvate, beta-hydroxybutyrate and acetoacetate were low. The early postinjury arterial amino acid profile was typically an increased level of alanine, taurine, glutamic acid, and a reduced concentration of valine, leucine, isoleucine, threonine, serine, ornithine, and arginine. At 3-4 days postinjury, as the early abnormalities were returning toward normal, glutamine, lysine, phenylalanine, tyrosine, and methionine became elevated. These late increases in amino acids occurred at the time of the peak in plasma catecholamine concentrations. The net cerebral flux of amino acids followed the same general pattern of evolution over time as did the arterial concentration of amino acids. On days when the availability of the individual amino acid was increased, the net cerebral flux tended to be positive; when the availability was decreased, the net cerebral flux was zero or there was a net efflux of the amino acid. There was a significant linear relationship between the arterial concentration and the net flux of 13 of the 17 amino acids studied. Unlike the fasting state in normal man, in which beta-hydroxybutyrate and acetoacetate are important metabolic substrates, cerebral metabolism after head injury is almost totally dependent upon the aerobic and anaerobic metabolism of glucose. This is at least in part due to injury-induced limitations in the variety of substrates that are available for the brain to extract.