Nutritional support of the critically ill patient

Injury or stress generates a vigorous metabolic response designed to establish the metabolic priorities required for the repair of injured tissues. In this condition, hormones commonly found to be elevated in the plasma include glucagon, catecholamines, glucocorticoids, growth hormone, aldosterone, and antidiuretic hormone. This hormonal profile results in rapid lysis of body protein, an increased rate of fat oxidation, and water and salt conservation. Rates of gluconeogenesis and ureagenesis are accelerated and may result in significant losses in lean body mass, a process that, if allowed to progress, will adversely affect patient survival. Exogenous nutrients provided to the critically ill patient may be poorly tolerated and may result in complications. Dextrose and intravenous fat emulsions provide the major sources of parenteral, nonprotein energy. These energy sources may not be metabolized efficiently in these patients, even though energy expenditure in this condition is increased significantly. Measurement of urinary nitrogen losses yields evidence useful in assessing the patient's degree of stress. In this manner, the patient's energy and protein requirements may be estimated. Formulations of amino acids, including the branched-chain amino acids, in higher concentrations have been reported to have anticatabolic effects and may improve the maintenance of lean body mass in stressed individuals. The stressed patient is prone to metabolic complications and, therefore, requires more careful monitoring of fluid, electrolyte, and acid-base balance, as well as renal, pulmonary, and liver function. Nutritional status is difficult to assess, since negative nitrogen balance may persist and the visceral proteins such as transferrin become altered in stress and, therefore, may not respond to nutritional intervention alone. The goal of nutritional therapy is the preservation of lean body mass by the safe and efficacious provision of metabolic substrate, thus improving patient survival.