Stressful conditions such as surgery are often characterized by hypermetabolism, depletion of protein stores, impaired immune function, and delayed recovery. Provision of adequate nutrition to the surgical patient is vital in order to ensure optimal outcomes. Traditionally, nutrition was regarded as adjunctive care designed to provide nutrients to support the patient during the perioperative state. Recently, nutrition has evolved to become a medical intervention, specifically designed to attenuate the metabolic response to stress, to prevent oxidative cellular injury, and to modulate the immune response. Enteral nutrition (EN), the provision of nutrients via the gastrointestinal (GI) tact, nonvolitionally through a feeding tube or catheter is recommended for patients who cannot meet their nutrient needs through voluntary oral intake. The surgeon is often faced with serious decisions about what, how, and when to feed these patients. Recent technological advancements in enteral formulations and equipments have made it possible to provide EN to a variety of patients in many different settings. This chapter reviews the evidence for the use of EN, timing of feedings, specific nutrients, and the various aspects that are important to consider with this intervention.

Historically, it was thought that the GI tract was quiescent following surgical intervention and that the primary role of the gut was digestion, absorption, and secretion. It is now evident that the gut is an important metabolically active organ and plays a vital role in nutrient transport, exposure of nutrients to absorptive mucosa, prevention of stasis and bacterial overgrowth, as well as immune regulation. As a result, efforts have been focused on using the GI tract whenever possible.

The question of route of feeding, that is, the use of enteral versus parenteral feedings is largely academic primarily due to the physiological benefit associated with using normal digestive and absorptive pathways. In practice, if the GI tract is functional, accessible, and safe to use, EN is preferred over parenteral nutrition (PN).  PN can be used in conjunction with EN. The two are not mutually exclusive. However, enterally supplied nutrients experience first-pass metabolism in the liver, which promotes their efficient utilization. The presence of
nutrients in the small intestine supports the functional integrity of the gut by maintaining tight junctions between the intraepithelial cells, stimulating blood flow, and inducing the release of trophic endogenous agents (e.g., cholecystokinin, gastrin, bombesin, and bile salts). Luminal nutrients also help to maintain normal intestinal pH and microbial flora, while specific nutrients contained in enteral formulas, such as glutamine and short-chain fatty acids, serve as a fuel source for the intestine. Luminal nutrients are potent stimulators of enterocyte growth and intestinal adaptation. From a practical standpoint, enteral formulas can mimic the normal diet and supply intact nutrients such as fiber, whole proteins, dipeptides, and specialized fatty acids, which cannot be supplied parenterally.

The beneficial effects of EN when compared to PN are well documented in numerous prospective randomized controlled trials involving a variety of patient populations, including major surgery, trauma, burns, head injury, and acute pancreatitis (Table 1). The most consistent beneficial outcome from the use of EN compared with PN is a reduction in infectious morbidity. A reduction in mortality has not been clearly demonstrated. Other outcome variables include significant reductions in hospital length of stay and cost of nutrition intervention.

EN should be considered after assessment of nutritional risk (see Chapter 2) and the ability of the patient to consume adequate nutrition. The presence and degree of malnutrition should be established since malnourished patients tend to have higher rates of morbidity and mortality and longer hospitalizations than adequately nourished patients.

EN provided orally or delivered via a feeding tube is the method of choice for those individuals with adequate digestive and absorptive capacity of the GI tract, but have clinical conditions in which oral intake is impossible, inadequate, or unsafe to use. Specific indications for EN include psychiatric disorders, severe dysphagia or esophageal obstruction, neurologic impairment, major burns or trauma, organ system failure, ration or chemotherapy, acquired immunodeficiency syndrome, and low output enterocutaneous fistulas. However, determining which patients should receive a feeding tube is more complex and requires consideration of several factors including the patient’s clinical status, diagnosis, prognosis, risk–benefit ratio, discharge plans, quality of life, ethical considerations, and the patient/family wishes.

Although few, there are some contraindications to enteral feeding, which relate primarily to the presence and degree of malnutrition, the patient’s ability to consume adequate nutrition by mouth, and the integrity and functional capacity of the GI tract. These can be considered as relative or absolute contraindications. EN, either supplementation via mouth or via enteral feeding tube, is not indicated in those individuals who are well nourished, are able to eat by mouth, or do not have a functional GI tract that can be safely accessed (Table 2). However, some of the potential barriers to EN can be circumvented with careful selection of enteral access device, formula, and route of administration.

The underlying metabolic responses to early enteral feeding and the derived clinical beneficial outcomes have been well described for the intensive care unit (ICU) patient. Secretory IgA, gut-associated lymphoid tissue (GALT), and mucosal-associated lymphoid tissue (MALT) are stimulated by enteral feeds and help fight infection locally in the gut and at distant sites as well. In a systematic review and analysis of 12 randomized prospective controlled trials, Lewis et al. showed a significant reduction in infections and hospital length of stay with the use of immediate postoperative tube feeding or aggressive early oral nutrition versus standard therapy. EN has even been successfully used in trauma patients with an open abdomen. Although GI motility is impaired in critically ill postoperative patients, the use of prokinetic agents alone or in combination and opiate antagonists and a multifaceted change in clinical practice aided the delivery of adequate EN support to critically ill and postoperative patients. Recent EN and PN clinical guidelines for critically ill patients were published by the European Society of Parenteral and Enteral Nutrition (ESPEN) and the American Society of Parenteral and Enteral Nutrition (ASPEN) jointly with the Society of Critical Care Medicine (SCCM). The guideline publications have extensive bibliographies containing all the studies that were analyzed and graded for levels of evidence by a multidisciplinary group of clinical experts in the field. Grades of recommendations were made based on levels of evidence with Grade A recommendations in the ESPEN guidelines based on either a meta-analysis of randomized controlled trials or at least one randomized controlled trial and Grade B recommendations were based on either a well-designed controlled trial without randomization or a well-designed nonexperimental descriptive study. The lowest grade was C while Grade A recommendations for the ASPEN/CCM guidelines were supported by at least two large randomized trials with clear-cut results and a low risk of false-positive and/or false-negative error and Grade B recommendations were supported by just
one such large randomized trial. The lowest possible recommendation grade was E, which was similar to the ESPEN Grade C. In the ESPEN surgery enteral nutrition publication, there were 36 guidelines developed of which 12 were category A. In the ESPEN intensive care enteral nutrition publication, there were 21 guidelines of which 6 were category A. The ASPEN/CCM guidelines were developed for both EN and PN in the adult ICU patient for whom there were 72 guidelines 11 of which were graded A or B. Only those recommendations with strong levels of supportive evidence are reviewed in this section. For those patients with severe malnutrition (weight loss >10% in the preceding 6 months, BMI <18.5 kg/m², and Subjective Global Assessment of C or serum albumin <3.0 mg%) consideration should be given to providing nutrition support for 10 to 14 days prior to major surgery. Both sets of guidelines made strong well-supported recommendations to initiate normal food intake or enteral feeding early after GI surgery and that EN is the preferred route of nutrition support over PN. Tube feedings should be started within 24 hours after surgery for patients undergoing major head and neck surgery or major GI surgery for cancer. In addition, it should be initiated early in patients with severe trauma and in those patients who came to surgery malnourished. Strong consideration should be given to inserting a jejunostomy or nasojejunal feeding tube at the time of major abdominal surgery to facilitate postoperative EN. The presence of bowel sounds, passage of flatus, or passage of stool is not required to start EN. Immune-modulating enteral solutions containing arginine, nucleotides, and omega-3 fatty acids are superior to standard enteral formulas in significantly reducing duration of mechanical ventilation, infectious morbidity, and hospital length of stay for patients having major elective surgery, trauma patients, and surgical ICU patients. Glutamine should be added to enteral formulas given to burn and trauma patients. These guidelines and the supporting literature favor the early institution of postoperative EN for surgical patients in the ICU setting; however, there is no uniformity of agreement and no clear definition of “early.” Ultimately, the surgeon has to make a clinical decision to start early EN based not only on these recommendations, but also on each patient’s condition and special circumstances. Relative contraindications to EN include expectation of early resumption of oral intake in a previously well-nourished patient, mechanical intestinal obstruction, irresolvable severe diarrhea, severe short-bowel syndrome (<100 cm of small intestine), and severe GI bleeding. Hypoperfusion associated with low flow states had previously led to the belief that an underperfused intestine would not be able to tolerate enteral feedings. However, more recently, it has been proposed that EN is possible in some patients with low flow states and may even be beneficial. In both of these publications, however, it is noted that the patient must be carefully and frequently monitored for signs of ischemic bowel clinically, radiologically, and by laboratory studies. However, the early diagnosis of ischemic bowel is very difficult and EN should be withheld until the patient is stable. Despite the advantages of EN over PN, parenteral nutrition should not be avoided when EN support cannot be given or when EN cannot meet energy and protein requirements. When energy and protein requirements cannot be met by enteral feeding alone a combined EN and PN support program should be considered to prevent the negative outcomes of prolonged cumulative negative energy balance.