Glucose Management and Surgical Nutrition

Protein

After major stress, patients are in an obligate protein catabolic state in which nitrogen excretion increases from a basal level of 5 to 7 g/day to a level of 8 to 11 g/day. Negative nitrogen balance occurs despite adequate delivery of glucose and amino acids. Protein wasting lasts until the wounds are closed, adequate circulating volume has been established, pain is well controlled, and infection is eliminated. This may take several days to several weeks. Some energy needs during stress are met through gluconeogenesis with a net catabolism of 300 to 500 g/day of lean body mass. Most energy in a stress situation is derived from fat, and replacement of amino acids is needed to minimize the negative nitrogen balance. In general, 10% to 15% of the daily calories should be delivered as protein (optimally, 1.5 to 2.5 g/kg/day). The most efficient use of protein occurs when a ratio of 150 to 250 nonprotein calories (carbohydrates and fats) for each protein calorie is given. This may increase to a ratio of 400 to 500 to 1 in settings of severe illness or hepatic or renal decompensation.

Nitrogen balance is defined by the following formula:

where 24-hour UUN is urinary urea nitrogen excretion (in g/day), 4 is obligatory loss (in g/day), and 6.25 is the number of grams of protein that yield 1 g N ₂ . The goal of therapy is +4 g/day balance.

Lipid

Recovery from injury is fueled mostly by fats. There is net increased lipolysis despite increased insulin concentrations. In the liver, there is increased free fatty acid synthesis. The fatty acids are used primarily for oxidation. Ketogenesis is increased in minor injury or stress, but is nearly absent in major injury or stress. The high fat content of the average American diet makes it unnecessary to supplement fats routinely, but for prolonged nutritional replacement, fats are the most concentrated source of calories. Replacement of lipids is indicated to supply calories and to avoid depletion of essential fatty acids.

Carbohydrate

One of the hallmarks of injury or stress is hyperglycemia. This sets stress apart from fasting or starvation. Blood sugar rises in response to catabolic neuroendocrine hormones. Initially, the hyperglycemia is a result of decreased circulating insulin secondary to β-cell insensitivity to glucose. Soon, insulin levels become normal or elevated, and persistent hyperglycemia is probably due in part to peripheral resistance to insulin. Despite the insulin resistance, the increased glucose in the blood stream allows greater uptake of glucose into the tissues during stress. Healing wounds must be thought of as obligate glucose-requiring tissues like brain and RBCs. This requirement may be because of the large number of infiltrating WBCs in the wound.

Enteral

If the patient has adequate bowel function, the enteral route is best. It allows physiologic use of the bowel, decreases bacterial translocation across the bowel wall (which may be a significant source of sepsis), and allows normal glucose use. The delivery of enteral feeds depends on the patient.

Parenteral

In patients in whom the bowel is not functioning properly or in whom the absorptive surface is insufficient to maintain adequate nutrition, enteral feeds may be supplemented with or replaced by parenteral delivery of nutrition. This may be delivered as complete nutritional replacement, in which case it is termed total parenteral nutrition (TPN). TPN uses a hypertonic solution delivered through a central venous catheter. TPN consists of adequate glucose, protein, and lipid replacement to facilitate wound healing and anabolism ( Table 14.4 ). Another approach does not require central venous access and is termed peripheral parenteral nutrition (PPN). The fluids used in PPN are of a low enough osmolarity to allow delivery through peripheral veins, but they will not deliver adequate calories when used alone. They must be coupled with enteral feeds or with body stores of nutrients. Each hospital has specific guidelines as to safe and effective delivery of parenteral nutrition. The concentrations of glucose and lipid must be increased slowly to avoid complications of hyperglycemia and hyperlipidemia; the electrolytes and the blood glucose concentrations must be followed closely; and the central access must be well protected from infection, as the nutrients delivered make this site more vulnerable to bacterial and yeast infection. Often, printed TPN delivery orders may be followed. Consult with your resident, the pharmacy, or the specific dietary department before starting or stopping TPN.

When adequate calories are delivered with TPN, the concentration of glucose is high, and the pancreas becomes acclimated to the rate of glucose delivery. The maximum rate can reach 5 to 8 mg/kg/minute. This puts a patient at risk for hypoglycemia if the TPN infusion is discontinued abruptly. If a central line becomes unusable, a peripheral line may be used for concentrations as high as 10% dextrose in water for a short period of time. Periodic glucose checks must be made to ensure adequate blood sugar levels. When TPN is no longer needed, the infusion is reduced slowly over 1 to 2 days, and care is taken to make sure that adequate blood sugar levels are maintained.

Vitamins and trace elements should be provided as directed by the TPN guidelines in your institution. In general, an ampule of multivitamins should be added to the formulation per day. Additional vitamin K (10–20 mg) should be administered to the patient, intramuscularly (IM) or subcutaneously (SC) once per week. Additional vitamin and mineral supplementation is outlined in Table 14.5 .

Electrolyte administration will vary with each patient; reasonable guidelines are listed in Table 14.6 . The patient must be followed closely with daily electrolyte, phosphorus, and glucose levels until the administration of TPN has stabilized (usually 1–2 weeks) and then 3 times per week thereafter. Weekly liver function tests and triglyceride levels should be monitored.

Complications of TPN delivery are common and can be severe. They are divided into physical, metabolic, and infectious complications. The physical complications are related to placement and maintenance of the central line. The metabolic complications are influenced by the formulation of the TPN solution, the rate of delivery, and the underlying biochemistry of the patient. The infectious risks are nearly inevitable with long-term administration but can be minimized by close adherence to sterile technique during placement and by following the TPN line protocols of your institution. A summary of complications is listed in Table 14.7 .