The human body responds to surgical trauma, infection, and critical illness in a complex manner. The response is designed to provide energy and essential substrates for reparative processes while protecting the host from microbial infection and optimizing vital organ function. Compounds are released from peripheral tissues and taken up by visceral organs for use in these functions, which expedites recovery once the primary pathologic process has been controlled. The flow of substrates is initiated and maintained by the neuroendocrine milieu as well as a multitude of inflammatory mediators. The catabolic response appears well orchestrated but favors—sometimes too exuberantly—abundant delivery. Hence, the response may be deleterious when prolonged or severe. There may be significant degradation of lean body mass, which can result in debility and death if nutritional and cardiopulmonary reserves are exhausted. The magnitude and extent of the catabolic state is proportional to the incipient injury. One goal of surgeons and others who care for seriously ill patients is to support the salutary components of the response to injury while mitigating the detrimental elements. This can be accomplished in part through appropriate nutritional management of the critically ill patient.
This chapter will focus on the metabolic responses to starvation, traumatic injury, and sepsis, with emphasis on the nutritional management of the surgical patient.
METABOLIC RESPONSE TO STARVATION
Basal Metabolic Rate & Resting Energy Expenditure
Daily energy expenditure is comprised of basal metabolic rate (BMR), the thermic effect of exercise, and thermogenesis of food intake. BMR, the major component of daily energy expenditure, is the amount of energy expended under complete rest, shortly after awakening, in a fasting state for 12-18 hours. BMR depends on age, gender, and body size. It is proportional to lean body mass—not fat—and correlates roughly with body surface area. The thermic effect of exercise is the energy used in physical activity. BMR increases by approximately 20%-30% in those who are ambulating but may increase significantly more in those who are vigorously exercising. In contrast, ventilated patients who are paralyzed have very low activity levels because the ventilator performs the work of breathing and there is little muscle activity. The thermogenic effect of feeding relates to the increase in BMR that follows food intake. The digestion and metabolism of nutrients by mouth increase the metabolic rate and accounts for 5%-10% of the daily energy expenditure, depending on the amount and composition of the diet.
Resting energy expenditure (REE), which often, but incorrectly, is used synonymously with BMR, represents the amount of energy expended 2 hours after a meal under conditions of rest and thermal neutrality. REE is generally 10% higher than the BMR. Those suffering from starvation, but otherwise healthy, usually have an REE less than 90% of predicted values and are therefore hypometabolic. This affords an advantage for the starved individual, because it decreases the need for exogenous calories and therefore ...