The ability to provide specialized nutritional support (SNS) represents a major advance in medical therapy. Nutritional support, via either enteral or parenteral routes, is used in two main settings: (1) to provide adequate nutritional intake during the recuperative phase of illness or injury, when the patient's ability to ingest or absorb nutrients is impaired, and (2) to support the patient during the systemic response to inflammation, injury, or infection during an extended critical illness. SNS is also used in patients with permanent loss of intestinal length or function. In addition, an increasing number of elderly patients living in nursing homes and chronic care facilities receive enteral feeding, usually as a consequence of inadequate nutritional intake.
Enteral refers to feeding via a tube placed into the gut to deliver liquid formulas containing all essential nutrients. Parenteral refers to the infusion of complete nutrient solutions into the bloodstream via a peripheral vein or, more commonly, by central venous access to meet nutritional needs. Enteral feeding is generally the preferred route because of benefits derived from maintaining the digestive, absorptive, and immunologic barrier functions of the gastrointestinal tract. Small-bore pliable tubes have largely replaced large-bore rubber tubes, making placement easier and more acceptable to patients. Infusion pumps have also improved the delivery of nutrient solutions.
For short-term use, enteral tubes can be placed via the nose into the stomach, duodenum, or jejunum. For long-term use, these sites can be accessed through the abdominal wall using endoscopic, radiologic, or surgical procedures. Intestinal tolerance of tube feeding may be limited during acute illness by gastric retention or diarrhea. Parenteral feeding has greater risk of infection, reflecting the need for venous access, and a greater propensity for inducing hyperglycemia. However, these risks can generally be managed successfully by SNS teams. For the postoperative patient with preexisting malnutrition, or in trauma patients who were previously well nourished, SNS is cost-effective. In the most critically ill patient in the intensive care unit, SNS can enhance survival. Although enteral nutrition (EN) can be provided by most health care teams caring for hospitalized patients, safe and effective parenteral nutrition (PN) usually requires specialized teams.
Approach to the Patient: Requirements for Specialized Nutritional Support
Indications for Specialized Nutritional Support
Although at least 15–20% of patients in acute care hospitals have evidence of significant malnutrition, only a small fraction will benefit from SNS. For others, wasting is an inevitable component of a terminal disease and the course of the disease will not be altered by SNS. The decision to use SNS should be based on the likelihood that preventing protein-calorie malnutrition (PCM) will increase the likelihood of recovery, reduce infection rates, improve healing, or otherwise shorten the hospital stay. In the case of the elderly or chronically ill patient for whom full recovery is not anticipated, the decision to feed is usually based on whether SNS will extend the duration or quality of life. The decision-making process used to assess whether to use SNS is depicted in Fig. 76-1.
Decision-making for the implementation of specialized nutrition support (SNS). CVC, central venous catheter; PICC, peripherally inserted central catheter. (Adapted from chapter in Harrison's Principles of Internal Medicine, 16e, by Lyn Howard, MD.)
The first step in deciding to administer SNS is to consider the nutritional implications of the disease process. Is the condition or its treatment likely to impair food intake and absorption for a prolonged period of time? For example, a well-nourished individual can tolerate approximately 7 days of starvation while experiencing a systemic response to inflammation (SRI). The second step is to determine if the patient is already significantly malnourished to the degree that critical functions such as wound healing, immune responses, or ventilatory function are impaired (Chap. 75). An unintentional weight loss of >10% during the previous 6 months or a weight/height <90% of standard, when associated with physiologic impairment, represents significant PCM. Weight loss >20% of usual or <80% of standard reflects severe PCM. The presence or absence of SRI should be noted, since inflammation, injury, and infection increase the rate of lean tissue loss. SRI also has pathophysiologic effects that influence nutritional responses such as fluid retention and hyperglycemia, as well as impairment of anabolic responses to nutritional support.
Once it is determined that a patient is already or at risk of becoming malnourished, the next step is to decide whether SNS will impact positively on the patient's response to disease. In the end stages of many chronic illnesses with accompanying PCM, particularly those due to cancer or terminal neurologic disorders, nutrition may not reverse the PCM or improve quality of life. While the provision of food and water is part of basic medical care, nutrition delivered by tube or catheter, either enterally or parenterally, is associated with risk and discomfort. Thus, SNS should be recommended only when potential benefits exceed risks, and it should be undertaken with the consent of the patient. Like other life support measures, enteral or parenteral therapy is difficult to withdraw once started. Initiating nutrition support may be appropriate before a final prognosis can be determined, but this should not preclude its subsequent withdrawal. If preventing or treating PCM with SNS is appropriate, nutritional requirements and the method of delivery should be determined. The optimal route depends on the degree of gut function and somewhat on the available technical resources.
The timing of nutritional support is based on evaluation of the preexisting nutritional status, the presence and extent of SRI, and the anticipated clinical course. SRI is identified by the standard clinical signs of leukocytosis, tachycardia, tachypnea, and/or temperature elevation or depression. Although the degree of hypoalbuminemia provides an estimate of SRI severity, normal serum albumin levels will not be restored by adequate nutritional support until the SRI remits, even though nutritional benefits can be achieved by adequate feeding.
The SRI can be graded as severe, moderate, or mild. Examples of severe SRI include sepsis or other inflammatory conditions like pancreatitis requiring ICU care, multiple trauma with an Injury Severity Score > 20–25 or ...