Arterial-jugular vein free amino acid levels in patients with head injuries: important role of glutamine in cerebral nitrogen metabolism.

Abstract

Traumatic brain injury is the single largest contributor of trauma center deaths. Injury to the brain cannot be considered as an isolated event, affecting only this organ. Profound hypoglutaminemia commonly seen in patients with head injuries may be caused by the diminished release of glutamine from the brain to the systemic circulation. To assess this hypothesis, we have simultaneously measured the free amino acid (AA) levels in systemic arterial (A, radial artery), cerebral venous (JV, jugular bulb), and systemic venous (PA, pulmonary artery) plasma in 11 adult patients with severe head injuries once within 48 hours of the initial injury before starting nutritional support and again after 3 to 4 days of enteral feeding. Cerebral organ (A-JV) changes of AA levels were compared with whole body systemic (A-PA) changes. Arterial total AA levels when compared with reported normal values are diminished by 46% in patients with isolated severe injuries. Cerebral outflow of glutamine is 6% of the total AA output compared with 73% in normals. The systemic outflow of glutamine in patients with brain injuries is 28% of total AA flow. Despite this high systemic output, significant hypoglutaminemia persists. Feeding for 3 days did not appreciably change the arterial plasma AA levels except that of glutamate and citrulline. A significant (p = 0.01) linear relationship between glutamine (product) and glutamate (precursor) was seen in JV samples but not in A or PA samples. The ratio of plasma glutamine to glutamate was decreased significantly only in JV during nutritional support, and this was caused mainly by an increase in glutamate levels. This may be owing to defective amidation to glutamine, inasmuch as gamma aminobutyric acid (GABA) levels were only minimally affected. Nutritional support improves the net release of glutamine from the brain. This suggests that supplementing the diet with glutamine may be beneficial to support systemic requirements in patients with severe head injuries.