SUMMARY AND RECOMMENDATIONS
●Parenteral nutrition is a mixture of solutions that contain dextrose, amino acids, electrolytes, vitamins, minerals, and trace elements. Lipid emulsion may be infused separately or added to the mixture (ie, a three-in-one mixture).
●Parenteral nutrition should be prescribed by a multidisciplinary team with advanced knowledge about how to tailor the composition and infusion rate to the needs of each patient, rather than by individual clinicians. Guidelines exist that describe safe practices for parenteral nutrition.
●For critically ill patients receiving parenteral nutrition, we suggest the inclusion of multivitamins and trace elements (Grade 2B). They are typically already included as a component of the parenteral nutrition, in the multivitamin, and multi-trace element solutions that are usually given in unit doses. The optimal mixture of nutrients has not been determined.
●Routine monitoring of parenteral nutrition includes measurement of fluid intake and output, as well as selected laboratory studies.
●Patients who receive parenteral nutrition support are at risk for infection, adverse metabolic effects, and complications related to venous access.
Dextrose — Dextrose-containing stock solutions are available in a variety of concentrations, most commonly 40, 50, and 70 percent. The percentage of calories that is contributed by dextrose is titrated according to individual factors, such as severity of illness, the caloric needs of the patient, and the patient's ability to tolerate fluid volume.
The caloric contribution of dextrose in medical solutions is 3.4 kcal/gm, which differs from dietary carbohydrate (4 kcal/gm). The reason for the difference is that water contributes to the weight of the dextrose-hydrate that is used to prepare parenteral nutrition.
Amino acids and electrolytes — Amino acid solutions contain most essential and nonessential amino acids. The exceptions are arginine and glutamine. While these are thought to be conditionally essential in critical illness, due to blockade of their metabolic pathways in catabolism, research does not support their supplementation. The caloric contribution of amino acids is approximately 4 kcal/gm. The buffer of the amino acid solution contains electrolytes, usually in small quantities. Amino acid solutions with large amounts of electrolytes are used infrequently because they limit the degree to which parenteral nutrition can be customized. It is occasionally necessary to use amino acid solutions with further electrolyte restriction (usually 15 percent solution), particularly in patients with renal failure when electrolytes and phosphate levels are difficult to control.
The amino acid stock solutions come in concentrations ranging from 5.5 to 15 percent. Higher concentrations are useful for minimizing volume and electrolytes delivered to patients. Enteral protein, peptide, and arginine supplementation is discussed separately.
Enrichment of parenteral nutrition with branched chain amino acids has been studied, but there is insufficient evidence to recommend this approach. A meta-analysis of four randomized trials (202 patients) that compared branched chain amino acid-enriched parenteral nutrition to standard parenteral nutrition found a trend toward decreased mortality among patients who received branched chain amino acids (24 versus 36 percent, relative risk 0.58, 95% CI 0.26-1.28) . However, the meta-analysis was limited by differences among the trials and an imprecise estimate of the magnitude of effect. Other meta-analyses have found no difference in infection rate, ICU length of stay, or hospital length of stay.
Lipids — Lipids are provided as an emulsion that may be infused separately or added to the mixture (three-in-one mixture). In the United States, lipid emulsion consists of long-chain omega-6 triglycerides derived from soybean and safflower oils and then emulsified using egg phospholipids and glycerin. The caloric contribution of a typical lipid emulsion is 2 kcal/mL in 20 percent emulsion and 1.1 kcal/mL in 10 percent emulsion. Use of intravenous fat emulsions should be done with care in patients with prior allergy to eggs as very rare allergic reactions have been reported.
Other types of lipids (eg, refined olive, soybean, and fish oil emulsions), have been evaluated and are available in many countries. However, they have not conclusively demonstrated any clinical meaningful benefits among patients receiving parenteral nutrition．
Vitamins and trace elements — Numerous clinical trials have examined the effect of antioxidants on critically ill patients when given as single nutrients (eg, selenium) or as a combination of nutrients (selenium, copper, zinc, vitamin A, vitamin C, vitamin E, and N-acetylcysteine). The trials administered the antioxidants in various ways, including as a separate intravenous infusion, as a component of parenteral nutrition, as a component of enteral nutrition, and orally.
A meta-analysis of 15 randomized trials (1647 patients) found that critically ill patients who received vitamins and trace elements, either alone or in combination, had a lower mortality rate than patients who did not receive vitamins or trace elements (20 versus 27 percent, relative risk 0.76, 95% CI 0.65-0.88). Similar meta-analyses showed improvement in the duration of mechanical ventilation, but no differences in infectious complications, hospital length of stay, or ICU length of stay.
Few of the trials looked specifically at patients receiving parenteral nutrition. Rather, most enrolled a heterogeneous sample of patients receiving either parenteral or enteral nutrition and did not look for differences in the effects of the vitamins and trace elements in these subgroups. Given their safety, it seems reasonable to provide vitamins and trace elements to most critically ill patients, regardless of the type of nutrition support that they are receiving. The optimal mixture of vitamins and trace elements is yet to be determined. Enteral vitamin and trace element supplementation is discussed separately.
Glutamine — Glutamine is a precursor for nucleotide synthesis and an important fuel source for rapidly dividing cells, such as gastrointestinal epithelia. Despite evidence that indicates that parenteral glutamine may be beneficial to patients who are receiving parenteral nutrition , glutamine is only available in the United States as a non-sterile powder. Enteral glutamine supplementation is discussed separately.