Complications

Depression of Serum Folate Concentration

Tennant and colleagues^[181] reported acute depression of serum folate concentrations in patients fed preoperatively with parenteral nutrition solutions. Their studies seem to indicate that the amino acid component of the solutions may be at least partially responsible for the fall in serum folate level, as suggested by Krebs and coworkers.^[182] Tennant and colleagues^[183] cautioned that further investigation may be indicated in patients receiving postoperative parenteral nutrition after anesthesia exposure to nitrous oxide, which may alter serum methionine and folate concentrations.^{[184] [185]}

The most common errors are underestimating phosphate needs (40 to 100 mEq/day) and overestimating calcium requirements (3 to 8 mEq/day). The most common abnormality in patients receiving TPN is a mild-to-moderate elevation in results of liver function tests. This may be a result of fatty infiltration of the liver that occurs with excess glucose administration. In a patient who begins with normal indexes of liver function, twofold elevations in serum glutamic oxaloacetate (SGOT) and serum glutamic pyruvate transaminase (SGPT) levels and a progressive mild rise in alkaline phosphatase level are not uncommon. If an increase in bilirubin occurs and SGOT and SGPT levels continue to rise, a cause other than the TPN infusion should be sought. Unfortunately, maintenance or restoration of lean body mass in hypercatabolic patients can be difficult because of the inevitable increased breakdown of muscle protein supplying amino acids for gluconeogenesis. An attempt to induce anabolism by infusing large amounts of carbohydrate increases the metabolic rate, the conversion of carbohydrate into fat, and the rate of DVCO₂ ; it does not promote increased protein synthesis, and it results in self-defeating hypermetabolism.^[186] Hypercatabolic patients must be given adequate amino acids to achieve nitrogen equilibrium. Patients with persistent negative nitrogen balance should have the ratio of nitrogen to calories increased without increasing nonprotein calories beyond their use capabilities.^{[153] [186] [187]}

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Respiratory Complications of Nutrition Support and Relative Starvation

In 1976, Doekel and coworkers^[188] reported that normal volunteers fed 400 kcal/day of carbohydrate orally for 10 days experienced a decrease in ventilatory response to hypoxia, a measure of respiratory drive (see Fig. 77-10 ). Weissman and associates^[189] observed that 7 days of infusion of 400 kcal/day of 5% dextrose resulted in a reduced mean inspiratory flow, another measure of neuromuscular drive.

Arora and Rochester^[190] found that chronically ill, nutritionally depleted patients had significant reductions in respiratory muscle strength, as measured by maximal inspiratory and expiratory pressures; endurance, as measured by maximal voluntary ventilation; and vital capacity. High-protein nutritional support improves respiratory muscle strength and ventilatory drive.^{[152] [191]}

Excessive glucose administration leads to an increase in minute ventilation that is proportional to an increase in DVCO₂ caused by oxidation of carbohydrate (RQ = 1) or lipogenesis (RQ = 8).^[192] Infusion of glucose as a sole energy source can lead to increased respiratory work because ventilation increases in response to added CO₂ .

In summary, Weismann and Askanazi^[192] recommended that nonprotein calories be administered as 50% carbohydrate and 50% lipid. They recommended this balanced supply of energy sources for patients with poor respiratory function and for those who are septic and hypermetabolic. This approach leads to less DVCO₂ than occurs with methods supplying 100% of the nonprotein calories as glucose. The combined substrate technique causes less increase in energy expenditure and norepinephrine secretion in the septic and hypermetabolic patient^[193] and prevents essential fatty acid deficiency.^[150] Later work suggests that using glucose-based formula but reducing the total calories to that which the patient is actually consuming reduces the complications of overfeeding and hyperlipidemia. Essential fatty acids must be replaced with 500 mL of lipid three times per week. Hyperglycemia should be aggressively treated with insulin to improve outcomes. Long-term propofol infusions provide significant lipid calories, and nutritional support must be adjusted.^[194]

Hypoglycemia

The most common cause of hypoglycemia is a slowing or a cessation of infusion. The increased secretion of insulin that accompanies TPN when suddenly unopposed by exogenous glucose results in hypoglycemia. For this reason, TPN solutions should never be abruptly discontinued in the operating room without being replaced with a concentrated glucose solution, such as 10% dextrose solution.

Complications of Intravenous Fat Emulsions

Extended TPN using lipid emulsions may injure the bilirubin-transfer mechanism in the liver and lead to progressive cholestasis. This is usually a benign, readily reversible state, but continued TPN without a change in the constituents can cause liver injury with hepatocyte necrosis and periportal fibrous changes. In a study by Allardyce,^[195] patients with a high lipid intake had significant elevations of the serum bilirubin and alkaline phosphate levels at the completion of parenteral nutrition. Cholestatic jaundice is associated with administration of lipid emulsion in a dose of 3 g/kg/day in patients fed intravenously for more than 3 weeks. The condition improves, with a return of liver function to normal, when the dose of lipid emulsion is reduced or when intravenous feeding is discontinued. Lipid emulsion may be limited to 1 g/kg/day, and liver function may be evaluated twice weekly. It appears that the dose of lipid emulsion should be reduced if a progressive rise in serum alkaline phosphatase level occurs. Early use of intravenous fat emulsion in trauma patients is associated with increased septic complications compared with glucose-based intravenous feedings.^{[162] [196]}

Disorders of Water Metabolism

Bistrian and colleagues^[134] pointed out that the tendency to retain water is characteristic of many of the disease states for which TPN is employed, including postoperative states, shock resuscitation, congestive heart failure, oliguric renal failure, hepatic insufficiency, and severe malnutrition. The antinatriuresis that results from the hyperinsulinemia seen with glucose-based TPN can lead to serious water overload in a brief period. This can be minimized or avoided by matching total fluid losses plus 5 dL, by limiting sodium intakes to less than 40 mEq of sodium beyond loss replacement, and by limiting insulin response by the use of a mixed-fuel system.

Enteral Nutrition

Many patients who are candidates for enteral feeding are at increased risk for aspiration because of mechanical ventilatory support or altered consciousness. Trauma patients with pulmonary injuries such as contusions, aspiration at the scene, hemopneumothorax, or acute respiratory distress syndrome (ARDS) are at even greater risk of pulmonary aspiration of gastric contents. Advances in the field of tube feeding have made early enteral nutrition a valuable tool in initiating nutrition therapy through a feeding tube in the proximal jejunum. A feeding algorithm was developed by the American Society for Parenteral and Enteral Nutrition to assist in selecting the type and route of administration of nutritional support ( Fig. 77-22 ). Enteral nutrition may possess several advantages over parenteral nutrition for postoperative or post-trauma patients. In patients with a functional gastrointestinal tract, enteral feeding assists in maintaining a thicker gut barrier by nourishing the enterocytes at the local level, which protects the host from bacterial translocation from gut lumen to the blood. Jejunal tube feeding has advantages over gastric tube feeding, including faster metabolic recovery, less vomiting, and less risk of regurgitation and aspiration.

Enteral nutrition can be started at a rate of 10 to 40 mL/hour and increased by 20 mL every 8 hours to reach target rates over a period of 24 to 48 hours. Standard tube-feeding solutions provide 1 kcal/mL, whereas concentrated formulas provide 1.5 to 2 kcal/mL and contain a standard mix of vitamins and minerals that meet recommended daily allowance when 1700 to 2000 mL of solution are provided. Supplements such as glutathione, vitamin E, and β-carotene are important for providing food antioxidants. Additional supplements are needed

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Figure 77-22 Clinical decision algorithm governing assessments for route of nutrition support in adults. GI, gastrointestinal; PN, parenteral nutrition. (Data from Aspen Board of Directors: Clinical Pathways and Algorithms for Delivery of Parenteral and Enteral Nutrition Support in Adults. Silver Spring, MD, American Society for Parenteral and Enteral Nutrition, 1998.)