The metabolic alterations, nutritional and metabolic assessment, and nutritional requirements of
critically ill patients are discussed, and
parenteral nutrition support
therapies are reviewed. Physiological alterations in the metabolism of the injured or septic patient are mediated through the interactions of neuroendocrine, cardiovascular, toxic, and
starvation responses. These responses cause mobilization of nutritional substrates in an effort to maintain vital organ function and immune defenses. A patient's nutritional status can be determined from anthropometric measurements,
creatinine excretion rate, and evaluations of
protein stores and immune reserves and function;
body weight is a poor
indicator.
Nitrogen-balance calculations are also useful for determining the adequacy of nutritional intake and the degree of metabolic stress. Early assessments of nutritional status may assist in identifying those patients for whom
nutritional support interventions are needed. Nutritional requirements are altered by the metabolic responses to injury and
sepsis. Studies suggest that use of nutrient solutions enriched for
branched-chain amino acids may enhance
nitrogen retention and that energy expenditures in injured or septic patients are only moderately elevated. Most nonprotein calories in parenteral nutrient solutions are provided as
glucose, but
lipids are an important source of energy in the
critically ill patient who has high energy requirements or
carbohydrate intolerance; however, clearance of
lipids may be decreased. Fluid,
electrolyte, and
mineral status must be evaluated frequently.
Critically ill patients have unique nutritional requirements, and
parenteral nutrition support
therapies for these patients are being investigated and refined.