Preoperative Total Parenteral Nutrition

Total parenteral nutrition (TPN) should be administered to patients who are severely malnourished with nonfunctioning gastrointestinal tracts. Dextrose and lipid formulas are used to provide nonprotein calories, usually in a 70:30 ratio. The caloric values of TPN substrates can be found in Table 2. The amount of dextrose administered should be 4–6 mg/kg/min. However, in patients with chronic obstructive pulmonary disease or diabetes, it is recommended to keep the dextrose administration at 4 mg/kg/min or less. Blood sugars must be monitored and kept tightly controlled between 85 and 120 mg/dl.

Table 2 Caloric Value of Parenteral and Enteral Nutrients

Intravenous fat can be utilized to supplement nonprotein calories; however, there is data to suggest that preoperative lipid therapy should be limited to less than 30% of the total calories. Lipids are administered as a 20% emulsion and depending upon caloric needs anywhere from 100 to 250 ml may be prescribed daily.

Protein is administered as a free amino acid solution at 1.5 g/kg of body weight daily to promote protein anabolism, and should not be calculated as a source of calories. Adequate nonprotein calories must be administered to support protein synthesis in a 150/1 calorie to nitrogen ratio, along with multivitamins and trace elements as part of the nutritional regimen.

While providing preoperative parenteral nutrition for patients with gastrointestinal dysfunction, it is important to consider fluid requirements. A more dilute solution may be needed in patients with large fluid losses, while more concentrated solutions will be necessary in patients who have volume restrictions due to heart failure, renal failure, or hepatic insufficiency.

With protein-calorie malnutrition there is loss of the intracellular ions potassium, magnesium, and phosphorus, and a gain in sodium and water. During refeeding, sodium balance may become markedly positive and cause water retention. Potassium, phosphorus, and magnesium levels may drop precipitously upon initiation of nutritional support. It is important to monitor electrolytes and fluid balance to avoid the risk of refeeding syndrome. In addition, potassium and magnesium deficiencies must be corrected if anabolism is to occur (Table 3).

Table 3 Electrolyte Requirements, Compartments, and Sequelae of Abnormal Levels

Trace minerals are inorganic compounds, and vitamins are complex organic compounds that regulate metabolic processes (Table 4). The majority act as coenzymes or as essential elemental constituents of enzyme complexes regulating the use of carbohydrates, proteins, and fats. Iron, zinc, copper, chromium, selenium, iodine, and cobalt are known to be necessary for health in man. However, in malnourished and seriously ill patients, requirements for zinc and selenium should be assessed and replenished as necessary.⁴

Table 4 Vitamins and Minerals

If the patient’s fluid and electrolyte status stabilizes on parenteral support with blood glucose levels in good control, the patient may be discharged home on cyclic overnight feedings while awaiting surgery. The parenteral cycle is gradually decreased from 24 to 18 hours and then to 14–16 hours daily. A permanent access port will be needed for home care.

Preoperative Enteral Nutrition

Enteral nutritional support is the delivery of nutrients into the gastrointestinal tract and may require a temporary or permanent feeding tube. Enteral feeding is the preferred route of nutritional support and should be used whenever possible. Surgical patients benefit from enteral nutrition due to the maintenance of the gut-associated lymphoid tissue, enhancement of mucosal blood flow, and maintenance of the mucosal barrier.

The initial gastrointestinal barrier function is provided by mucous containing lactoferrin and lysozyme, both of which are effective, nonspecific inhibitors of microbial growth. Normal, undisturbed bacterial flora exert a similar effect. Epithelial tight junctions form the next line of nonspecific defense, with junctional integrity being energy dependent, and at least partially reliant on the presence of intraluminal energy substrates. Specific intestinal immunity is governed by the gut-associated lymphoid tissue (GALT). The inductive sites in the Peyer’s patches provide an interface between antigen-presenting cells and circulating lymphocytes. Animal studies have demonstrated improved immunity in enterally fed groups.⁵

Patients may have inadequate appetite or gastrointestinal function to maintain optimal nutrition on oral intake alone. Enteral feeding has been used successfully to meet the nutritional needs of patients with a wide range of surgical diseases including cancer, inflammatory bowel disease, and pancreatic disorders. However, its use is contraindicated in cases of bowel obstruction, persistent intolerance, hemodynamic instability, major gastrointestinal bleeding, and inability to access the gastrointestinal tract safely.

Once it has been decided to administer enteral nutrition, the optimal type of enteral access must be selected. Factors that determine the choice of enteral access include which components of the gastrointestinal tract are available, how long a course of enteral therapy is planned, whether the patient is at risk for aspiration, and finally, the nutritional status of the patient. When available, the gastric route is usually preferred. Postpyloric feeding into the duodenum or jejunum may be indicated when there is early satiety, gastric pathology or a risk of aspiration. Nasogastric and nasoenteral tubes are recommended for short-term feeding because of their ease of placement, low cost and low complication rate. Percutaneous endoscopic gastrostomy has become one of the most common methods for placing gastrostomy tubes. Interventional radiology can also place feeding tubes percutaneously into the stomach as well as in the jejunum. If these less-invasive techniques are not successful, feeding tubes may be placed by open or laparoscopic techniques; however, this approach is less desirable in the preoperative patient with severe malnutrition.

The appropriate selection of enteral formulation requires knowledge of the physiologic mechanisms of the digestion and absorption of each macronutrient. Sources of carbohydrate found in enteral formulas range from simple sugars to starches. The larger molecular weight of starches exert less osmotic pressure in the intestinal lumen, are less sweet and require more time for digestion prior to absorption. Different enteral formulas contain variable amounts of carbohydrates that can range anywhere from 28% to 70% of total calories. Patients with diabetes or carbon dioxide retention due to chronic obstructive pulmonary disease should be given formulas with fewer carbohydrate calories. (See “Preoperative Total Parenteral Nutrition” section.)

Many enteral formulas now contain fiber, which may be soluble or insoluble. Insoluble fiber improves colonic function and bowel transit time, but there is no nutritional benefit or requirement. In contrast, soluble fiber binds to cholesterol and bile salts, and thus lowers serum cholesterol levels. Colonic bacteria digest soluble fiber and produce short-chain fatty acids that are utilized by the colonocyte as a fuel source.

Enteral formulas contain fats derived from corn, soy, and safflower oil. Fat serves as a concentrated energy source and enhances the flavor of enteral formulas without increasing osmolality. The absorption of fat-soluble vitamins requires the intake of a minimum of 15–25 g of fat per day. Linoleic or linolenic acid must be provided to prevent essential fatty acid deficiency. Because omega-6 fatty acids have been shown to have immunosuppressive effects due to the production of inflammatory end-products, omega-3 fatty acids have been added to some enteral products. Medium-chain triglycerides may be a useful caloric source for patients with fat malabsorption, as they do not require pancreatic lipase for hydrolysis, are absorbed without micelle formation, and do not require carnitine for transport into the mitochondria.

Enteral protein may be in the form of intact protein such as casein, partially hydrolyzed oligopeptides, or crystalline L-amino acids. Intact and protein hydrolysates require further digestion by pancreatic and brush-border pancreases into short peptides and amino acids. These nutrients are then freely absorbed by the enterocyte, primarily in the proximal intestine. Patients with malabsorption may benefit from enteral protein in the form of short peptides and free amino acids. Preoperatively, patients should be given 1.5 g/kg/day to support protein synthesis.

Currently, there are over a hundred enteral products on the market. Most formulas have a caloric density of 1-2 kcal/ml, are lactose-free, and provide the recommended daily allowances of vitamins and minerals in less than 2 liters of formula per day. The majority of patients tolerate standard enteral formulas; however, elemental formulas may be necessary in patients with malabsorption. Recently, excellent results with improved immune function and surgical outcomes have been obtained with the preoperative administration of immunoenhancing formulas.⁶ Other disease specific formulations have been created for patients with liver disease, renal failure, pulmonary insufficiency, and diabetes. Formulas for patients with liver disease and encephalopathy contain a higher percentage of protein in the form of branched-chain amino acids with almost no aromatic amino acids. Renal failure formulas have very low levels of potassium and phosphorus. However, hepatic and renal formulas have a very low protein content, which must be considered. Patients with advanced pulmonary disease need to receive most of their calories as fat in order to decrease carbon dioxide production. Diabetic formulas also contain additional calories as fat, but also contain soluble fiber to decrease blood sugar levels. It is important to note that all of these specialty formulas are very expensive, and should only be used when a standard formulation with an appropriate nutrient profile has failed.

It is important to monitor patients on enteral feeding for improvement in nutritional status, gastrointestinal tolerance, and fluid and electrolyte balance. When gastric feedings are administered, it is important to monitor gastric residual volumes. Increased residual volumes lead to vomiting, aspiration, pneumonia, prolonged hospital stays, and mortality. Another complication that can occur in the malnourished patient is diarrhea, which may precipitate fluid and electrolyte abnormalities. Diarrhea may be due to medications, formula composition, or infections. If infectious causes are eliminated and there are no offending medications, the formula should be changed to one that is more elemental to increase absorption. If necessary, medications can be tried in incremental dosages to slow down intestinal transit time.

Because most formulas only contain 65% water, it may be necessary to administer hypotonic enteral fluid boluses to achieve satisfactory fluid and electrolyte balance. Electrolyte levels should be monitored to avoid hyperosmolar states and to replete serum levels of potassium, phosphorus, and magnesium during refeeding. Patients may develop fluid retention and electrolyte imbalances that can result in life-threatening cardiac dysrythmias. Blood sugars must be carefully monitored, and hyperglycemia should be treated to avoid the increased risk of infections and the development of hyperosmolar states.

Preoperative enteral feedings have been demonstrated to decrease postoperative complication rates by 10%–15% of controls, but there is debate over the length of therapy needed to achieve this. The literature supports a course of enteral feedings for 5–20 days prior to surgery. Recently, it has been shown that there may be an advantage to utilizing immune-enhancing formulas, either alone as preoperative supplements, or in combination with postoperative support.⁶