• The fit, previously well-nourished patient undergoing elective surgery could be fasted for up to a week postoperatively without apparent adverse effect on outcomes, provided that fluid and electrolyte needs are met. On the other hand, it is well established in multiple studies that malnourished patients benefit from nutritional repletion via either enteral or parenteral routes prior to surgery.
  
• The indications for total parenteral nutrition (TPN) are narrow, including those patients who cannot absorb enteral solutions (small bowel obstruction, short gut syndrome, etc.); partial parenteral nutrition may be indicated to supplement enteral nutrition (EN), when EN cannot fully provide for nutritional needs.
  
• TPN will generally require a venous access line with its catheter tip in the superior vena cava. The line or port through which the TPN solution will be infused should be dedicated to this purpose, if at all possible, and strict aseptic techniques should be employed for insertion and care of the catheter.
  
• In the patient with critical illness, discontinuing an EN infusion may require multiple potentially dangerous adjustments in insulin infusions and maintenance of intravenous fluid rates. Meanwhile, the evidence is sparse that EN infusions delivered through an appropriately-sited gastrointestinal feeding tube increases the risk of aspiration pneumonitis.
  
• Regardless of whether the TPN infusion is continued, reduced, replaced with 10% dextrose, or stopped, blood glucose monitoring will be needed during all but short, minor surgical procedures.

Issues related to nutrition tend to be far removed from the usual concerns of the surgical anesthesiologist. On the other hand, appropriate nutritional support has been recognized in recent years to be of key importance for favorable outcomes in patients with critical illness, a large fraction of whom will require surgical services. Severe malnutrition causes widespread organ dysfunction and increases perioperative morbidity and mortality rates. Nutritional repletion may improve wound healing, restore immune competence, and reduce morbidity and mortality rates in critically ill patients. This chapter does not provide a complete review of nutrition in the patient undergoing surgery or with critical illness, but rather offers the framework for providing basic nutritional support in such patients. We consider, for example, whether enteral nutrition (EN) or parenteral nutrition (PN) will best meet the needs of an individual patient. This chapter also briefly reviews the conditions under which the ongoing nutritional needs of patients may come into conflict with anesthetic preferences and dogmas, such as the duration that patients must not receive EN before undergoing general anesthesia.

Maintenance of normal body mass, composition, structure, and function requires the periodic intake of water, energy substrates, and specific nutrients. Nutrients that cannot be synthesized from other nutrients are characterized as “essential.” Remarkably, relatively few essential nutrients are required to form the thousands of compounds that make up the body. Known essential nutrients include 8-10 amino acids, 2 fatty acids, 13 vitamins, and approximately 16 minerals.

Energy is normally derived from dietary or endogenous carbohydrates, fats, and protein. Metabolic breakdown of these substrates yields the adenosine triphosphate required for normal cellular function. Dietary fats and carbohydrates normally supply most of the body’s energy requirements. Dietary proteins provide amino acids for protein synthesis; however, when their supply exceeds requirements, amino acids also function as energy substrates. The metabolic pathways of carbohydrate, fat, and amino acid substrates overlap, such that some interconversions can occur through metabolic intermediates (see Figure 32-4). Excess amino acids can therefore be converted to carbohydrate or fatty acid precursors. Excess carbohydrates are stored as glycogen in the liver and skeletal muscle. When glycogen stores are saturated (200-400 g in adults), excess carbohydrate is converted to fatty acids and stored as triglycerides, primarily in fat cells.

During starvation, the protein content of essential tissues is spared. As blood glucose concentration begins to fall during fasting, insulin secretion decreases, and counterregulatory hormones, such as glucagon, increase. Hepatic and, to a lesser extent, renal glycogenolysis and gluconeogenesis are enhanced. As glycogen supplies are depleted (within 24 h), gluconeogenesis (from amino acids) becomes increasingly important. Only neural tissue, renal medullary cells, and erythrocytes continue to utilize glucose—in effect, sparing tissue proteins. Lipolysis is enhanced, and fats become the principal energy source. Glycerol from the triglycerides enters the glycolytic pathway, and fatty acids are broken down to acetylcoenzyme A (acetyl-CoA). Excess acetyl-CoA results in the formation of ketone bodies (ketosis). Some fatty acids can contribute to gluconeogenesis. If starvation is prolonged, the brain, kidneys, and muscle also begin to utilize ketone bodies efficiently.

The previously well-nourished patient undergoing elective surgery could be fasted for up to a week postoperatively without apparent adverse effect on outcomes, provided fluid and electrolyte needs are met. The usefulness of nutritional repletion in the immediate postoperative period is not well defined, but likely relates to the degree of malnutrition, number of nutrient deficiencies, and severity of the illness/injury. Moreover, the optimal timing and amount of nutrition support following acute illness remain unknown. On the other hand, malnourished patients may benefit from nutritional repletion prior to surgery.

Modern surgical practice has evolved to an expectation of an accelerated recovery. Accelerated recovery programs generally include early enteral feeding, even in patients undergoing surgery on the gastrointestinal tract, so prolonged periods of postoperative starvation are no longer common practice. All well-nourished patients should receive nutritional support after 5 days of postsurgical starvation, and those with ongoing critical illness or severe malnutrition should be given nutritional support immediately. The malnourished patient presents a different set of issues, and such patients may benefit from both preoperative and early postoperative feeding. Clearly, the healing of wounds requires energy, protein, lipids, electrolytes, trace elements, and vitamins. Depletion of any of these substrates may delay wound healing and predispose to complications, such as infection. Nutrient depletion may also delay optimal muscle functioning, which is important for supporting increased respiratory demands and early mobilization of the patient.