Parenteral Nutrition
PN' major advantages are its abilities to supply nutrition when the enteral route is unavailable and tailor the types of nutrition provided for each individual animal. PN has been shown to decrease weight loss, particularly lean body mass, and improve wound healing, immune function, and outcome in human and animal studies when the enteral route cannot be used.49 PN can be used to provide partial nutritional support or total nutritional support.
In the adult horse it is most commonly used to supply partial nutrition when oral intake is insufficient or the oral route cannot be used. Horses with proximal enteritis, colitis, postoperative ileus, esophageal lacerations, or obstructions can receive nutritional support until resolution of the underlying problem allows reinstitution of adequate enteral feeding (1.5% body weight or ~23 kcal/kg day). Recumbent or dysphagic animals at risk for aspiration pneumonia and individuals with preexisting protein calorie malnutrition; increased energy demands (late gestation, early lactation, and young, growing animals); metabolic derangements resulting in hyperlipemia; and completely anorexic animals or those with prolonged decreased feed consumption (a peripheral vein or combined with other components. Lipids come in 10% and 20% emulsion and are composed principally of safflower and soybean oil, egg yolk phospholipids, and glycerin. Lipids should be added to the solution last to avoid destabilization of the emulsion due to an acidic environment. Some clinicians mix PN solutions in isotonic crystalloids to provide both fluid support and nutritional support. When doing so, it is best to separate the lipid component, as the presence of divalent cations can result in destabilization of the emulsion. Some clinicians are reluctant to use lipids in PN solutions due to the costs and risks associated with destabilization, as well as metabolic consequences that may develop (hyperlipemia, fatty liver, lipid thromboembolism). It can be difficult to provide total energy requirements without lipids, especially in insulin-resistant patients or patients with systemic inflammation, as these individuals appear to have a harder time regulating blood glucose. Lipids are calorie dense, containing 9 to 11 kcal/g. Although recommendations vary, lipids can be added to provide 30% to 60% of the nonprotein calories in a PN solution. The addition of lipids to PN is beneficial in patients with persistent hyperglycemia or hypercapnia, reducing the dependency on glucose as the principal energy source. Lipids should be avoided in horses with known hyperlipemia or in horses suspected of being lipid intolerant. Lipid intolerance can be seen in patients with systemic inflammation, sepsis, or underlying metabolic derangements, and triglyceride levels should be monitored regularly.Amino acid preparations are available in several concentrations; 8.5% and 10% solutions are most commonly used in veterinary medicine. They are approximately twice the osmolarity of plasma and are acidic. Amino acid preparations should be mixed with dextrose before adding the lipid component to prevent destabilization of the lipid emulsion. Solutions containing both essential and conditionally essential amino acids, including glutamic acid, would seem to be preferable, though no data on the benefits of conditionally essential amino acids in horses are available. As already discussed, while resting energy requirements should be used when calculating caloric needs for adult animals, protein requirements should be determined using maintenance requirements of approximately 0.7 g/kg/ day or higher. Although there are no clear data on the benefits of a higher protein supplementation, current recommendations in humans are to provide protein in excess of the normal daily requirement because of the higher protein catabolism seen with injury or inflammation. The ratio of nonprotein calories to nitrogen (in the final solution) is recommended to be 120 to 150 : 1 in healthy humans and 80 to 90 : 1 in acutely ill humans to limit the use of protein as an energy source.
The ideal ratio in equids is unknown; consequently, human targets are typically used. This can be calculated using the conversion of 1 g nitrogen to 6.2 g protein. Protein provides ~4 kcal of energy/g protein, and 100 mL (10 g) of 10% amino acid solution contain 40 kcal.Additional components may be added to PN or given separately including electrolyte solutions and vitamin and mineral supplements. The vitamin and mineral requirements of the critically ill horse are unknown, but published recommendations for healthy horses (NRC 2007) can be used when determining feeding rates. Multivitamin supplements for humans are available and can be added directly to PN solutions. If possible, however, it is best to limit the amount of additives to PN solutions to avoid contamination or lipid destabilization. On the basis of information in humans, it is strongly recommended to supplement water-soluble B vitamins daily as deficiencies can exacerbate problems associated with refeeding.56 Though horses and ruminants synthesize some of the B vitamins in their gastrointestinal tract, chronic inappetence or disruption of the normal flora may affect synthesis. Parenteral B vitamins are light sensitive; when added to crystalloids, the fluids need to be protected from sunlight. Some vitamins are best given orally (vitamins C and E) or added to separate crystalloid solutions (B vitamins). Vitamin C is also produced by the horse, and a need for supplementation has not been documented. However, some authors recommend oral supplementation in sick horses with recommendations of 10 to 20 g/horse/day.57 Fat-soluble vitamins such as E, D, and A are stored in body tissue and generally do not need daily supplementation. However, the antioxidant benefits of vitamin E may make it worth providing oral supplementation early in illness. Doses of oral vitamin E vary significantly from 500 international units (IU)/horse/day to 40 lU/kg/day. Minerals, if required, are best supplemented in separate crystalloid solutions because divalent cations may destabilize lipid emulsions. Although trace mineral requirements for sick animals are not known, trace mineral supplementation is rarely performed except in patients receiving PN as their sole nutritional source for prolonged periods (>7 days). See Table 50.6 for a PN formulation for adult horses.