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HOW TO FEED THE PATIENT
After total parenteral nutrition (TPN) was estab-lished as a feasible approach for feeding patients lacking a functional gut, physicians extended the practice of TPN to many circumstances where “logic” and “clinical experience” suggested that it would be better than EN. For example, one such indication was in the patient with acute pancreatitis, where, in the 1970s, many clinicians thought that a period of TPN would put the gut and pancreas at “rest,” allowing for resolution of pain and weight loss. Unfortunately, “logic” and “clinical experience” were incorrect. Now, the worldwide consensus expressed in clinical practice guidelines is that patients with acute pancreatitis (and indeed all oth-ers with functioning guts) will have worse outcomesif TPN is provided, rather than EN. Today, the indications for TPN are narrow and includepatients who cannot absorb enteral solutions (small bowel obstruction, short gut syndrome, etc.); partial PN may be indicated to supplement EN, in cases in which EN cannot fully provide for nutritional needs. In the latter circumstance, recent evidence suggests that the decision to add supplemental PN should be made only after a week’s time in previously well-nourished patients. Earlier initiation of supplemen-tary PN in previously well-nourished patients, as had been supported by 2009 European guidelines, resulted in worse outcomes in a large randomized clinical trial; however, these results are not firmly established, as smaller randomized clinical trials have suggested findings to the contrary. The diver-gent results from these recent trials may be associ-ated with the type of parenteral formulations being used, types of patients being studied, timing of par-enteral nutritional administration, and treatment in the control groups. Thus, further studies are needed to better define patients that may benefit from PN, as well as the optimal timing of nutritional support and formulations for feeding. In short, EN should be the primary mode of nutritional support, and PN should be used when EN is not indicated, not tolerated, or insufficient.
There was a time when nearly every physician who took care of critically ill patients was in the position of frequently ordering TPN for patients. This is no longer the case, given that EN is now so much more widely employed. As a consequence, many hospitals and health systems insist that a nutrition support team take responsibility for those rarer patients who require TPN.
In general, patients with critical illness should undergo whatever initial hemodynamic resuscita-tion they require before initiation of nutritional support (either EN or PN). Absorption, distribu-tion, and metabolism of nutrients require tissue blood flow, oxygen, and carbon dioxide removal. Adequate tissue blood flow requires an adequately resuscitated patient. Nutrient delivery to ischemic tissues may cause tissue damage by increasing car-bon dioxide and oxidant production while consum-ing energy. Patients with critical illness who require EN will usually require placement of a feeding tube. Feeding tubes may be placed into the stomach in patients with adequate gastric emptying and low risk of aspiration. In patients with delayed gastric emptying or those at high risk of aspiration, feeding tubes are best placed into the small intestine. Ide-ally, the tip of such tubes will be sited within the small intestine, either by transpyloric placement ofnasoenteral tube or directly into the jejunum dur-ing abdominal surgery (via a percutaneous route), reducing the likelihood of gastric distention and regurgitation. Patients who are unable to eat, but require EN over long periods of time, will often undergo percutaneous endoscopic placement of gastrostomy (PEG) tubes (the tips of such tubes can be placed distal to the pylorus). One should con-firm that the tips of all feeding tubes are appropri-ately placed before initiating feeds to reduce the likelihood that EN solutions will be accidentally infused, say, into the tracheobronchial tree or abdominal cavity.TPN will generally require that a venous access line be placed with the catheter tip in the superior vena cava. Peripheral PN can support the nutritional requirements of the patient, but necessitates the use of larger volumes of fluids dueto the requirement for lower osmolarities than used with central PN and increases the risk of phlebitis. The line or port through which the TPN solution will be infused should be dedicated to this pur-pose, if at all possible, and strict aseptic techniques should be employed for insertion and care of the catheter.
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