Daniel L. Chan,     Hertfordshire, United Kingdom

Patient Selection

As with any intervention in critically ill animals, nutritional support carries some risk of complications. This risk likely increases with the severity of the disease, and the clinician must therefore consider many factors in deciding when to institute nutritional support. Of utmost importance is the patient’s cardiovascular status, which must be stable before initiation of any nutritional support. Processes such as gastrointestinal motility, digestion, and nutrient assimilation are altered when perfusion is reduced. Feeding under such circumstances is likely to result in complications. Other issues that should be addressed before nutritional support begins include patient hydration, electrolyte imbalances, and abnormalities in acid-base status.

In animals that have been stabilized, careful consideration must be given to the appropriate time to initiate nutritional support. A previously held notion that nutritional support is unnecessary until 10 days of inadequate nutrition have elapsed is outdated. Commencing nutritional support within 3 days of hospitalization (sometimes as early as within the first 12 hours), even before determining the diagnosis of the underlying disease, is a more appropriate goal in most cases; however, other factors should also be considered as discussed in the next section.

Nutritional Assessment

Indicators of malnutrition in animals that have been proposed include unintentional weight loss (typically greater than 10% of body weight), poor hair coat quality, muscle wasting, signs of inadequate wound healing, and hypoalbuminemia. However, these abnormalities are not specific to malnutrition and often occur as late complications of a variety of systemic diseases. A greater emphasis is placed on evaluating overall body condition rather than simply noting body weight. Body condition scores (BCSs) have been shown to be reproducible, reliable, and clinically useful in nutritional assessment. Fluid shifts may significantly impact body weight, but BCSs are not affected by fluid shifts and therefore are helpful in assessing critically ill animals. More recently, lean muscle loss has also been evaluated in cats and this may become a component of nutritional assessment in small animals (Michel et al, 2011).

In light of the limitations to assessing nutritional status, it is crucial to identify early risk factors that may predispose patients to malnutrition such as anorexia of greater than 5-days’ duration, serious underlying disease (e.g., severe trauma, sepsis, peritonitis, acute pancreatitis), and large protein losses (e.g., protracted diarrhea, draining wounds, burns). Nutritional assessment also identifies factors that can impact the nutritional plan such as specific electrolyte abnormalities; hyperglycemia, hypertriglyceridemia, or hyperammonemia; or comorbid illnesses such as renal, cardiac, or hepatic disease, including various forms of neoplasia. In the presence of such abnormalities the nutritional plan should be adjusted accordingly to limit acute exacerbations of any preexisting condition.

Finally, since many of the techniques required for implementation of nutritional support (e.g., placement of most feeding tubes, intravenous catheters for parenteral nutrition) necessitate sedation or anesthesia, the patient must be properly evaluated and stabilized first. When the patient is deemed too unstable for general anesthesia, temporary measures of nutritional support that do not require anesthesia (e.g., nasoesophageal tube placement, placement of peripheral catheters for parenteral nutrition) should be considered.

Nutritional Plan

Nutrition should be provided as soon as it is feasible, with careful consideration of the most appropriate route of nutritional support. Providing nutrition via a functional digestive system is the preferred route of feeding, and particular care should be taken to evaluate if the patient can tolerate enteral feedings. Even if the patient can only tolerate small amounts of enteral nutrition, this route of feeding should be pursued and supplemented or augmented with parenteral nutrition (PN) as necessary to meet the patient’s nutritional needs. However, if an animal demonstrates complete enteral feeding intolerance, some form of PN should be provided. Implementation of the devised nutritional plan also should be gradual, with the goal of reaching target level of nutrient delivery within 48 to 72 hours. Adjustments to the nutritional plan are made on the basis of frequent reassessment and the development of any complications.

Calculating Nutritional Requirements

Based on indirect calorimetry studies in dogs, there has been a recent trend toward formulating nutritional support simply to meet resting energy requirements (RERs) rather than more generous illness energy requirements (IERs). For many years clinicians used to multiply the RER by an illness factor between 1.1 and 2 to account for purported increases in metabolism associated with different disease states. However, now less emphasis is being placed on these extrapolated factors, and the current recommendation is to use more conservative energy estimates (i.e., start with the animal’s RER) to avoid overfeeding and its associated complications. Examples of complications resulting from overfeeding include gastrointestinal intolerance, hepatic dysfunction, and increased carbon dioxide production. Although several formulas are proposed to calculate the RER, a widely used allometric formula can be applied to both dogs and cats of all weights. The formula most commonly used by the author is:

Hospitalized dogs should be supported with 4 to 6 g of protein per 100 kcal (15% to 25% of total energy requirements), whereas cats are usually supported with 6 g or more of protein per 100 kcal (25% to 35% of total energy requirements). In most cases estimation of protein requirements is based on clinical judgment and recognition that in certain disease states (e.g., peritonitis, draining wounds) protein requirements are markedly increased.

Parenteral Nutritional Support

PN, formerly referred to as total PN (TPN) or partial PN (PPN), is the intravenous delivery of nutrients (e.g., dextrose, amino acids, lipid emulsion, vitamins, minerals, electrolytes) to patients. This can be achieved via a central vein (central PN [CPN]) or a peripheral vein (peripheral PN [PPN]). Factors that influence how PN should be administered include feasibility of central venous access and the osmolarity of the PN solution, with the recommendation to use the central route when the osmolarity of solution exceeds 850 mOsm/L.

Crystalline amino acid solutions are an essential component of PN. The importance of supplying amino acids relates to the maintenance of positive nitrogen balance and repletion of lean body tissue, which may be vital in the recovery of critically ill patients. Supplementation of amino acids may support protein synthesis and spare tissue proteins from being catabolized via gluconeogenesis. The most commonly used amino acid solutions (e.g., Travasol, Aminosyn II) contain most of the essential amino acids for dogs and cats, with the exception of taurine. However, because PN is typically not used beyond 10 days, the lack of taurine does not become a problem in most circumstances. Amino acid solutions are available in different concentrations from 4% to 15%, but the most commonly used concentration is 8.5%. Amino acid solutions are also available with and without electrolytes.

Lipid emulsions are the calorically dense component of PN and a source of essential fatty acids. Lipid emulsions are isotonic and available in 10% to 30% solutions (e.g., Intralipid, Liposyn III). These commercially available lipid emulsions are made primarily of soybean and safflower oil and provide predominantly long-chain polyunsaturated fatty acids, including linoleic, oleic, palmitic, and stearic acids. The emulsified fat particles are comparable in size to chylomicrons and are removed from the circulation via the action of peripheral lipoprotein lipase. Side effects attributed to lipid emulsions include liver dysfunction and immune suppression. Newer lipid emulsions with fewer side effects have been developed, and one such product composed of soybean oil, medium chain triglycerides, olive oil, and fish oil (i.e., SMOF lipid) is available in Europe and may become more widely distributed in the future (Goulet et al, 2010). There is a persistent misconception regarding the use of lipids in cases of pancreatitis. Although hypertriglyceridemia may be a risk factor for pancreatitis, infusions of lipids have not been shown to increase pancreatic secretion or worsen pancreatitis and therefore are considered safe. However, the one exception is in cases in which serum triglyceride concentrations are severely increased, indicating a clear failure of triglyceride clearance. Although specific data regarding the maximal safe level of lipid administration in veterinary patients are not available, it would seem prudent to maintain normal serum triglyceride concentrations in patients receiving PN. Another concern surrounding the use of lipids in PN is their purported immunosuppressive effects via impairment of the reticuloendothelial system, particularly in PN solutions containing a high percentage of lipids. Despite in vitro evidence supporting the notion that lipid infusions can also suppress neutrophil and lymphocyte function, studies have not yet correlated lipid use and increased rates of infectious complications.

Electrolytes, vitamins, and trace elements also may be added to the PN formulation. Depending on the hospital and the individual patient, electrolytes can be added to the admixture, included as part of the amino acid solution, or left out altogether and managed separately. Because B vitamins are water soluble, they are more likely to become deficient in patients with high-volume diuresis (e.g., renal failure, diabetes mellitus), and supplementation could be considered. Since most animals receive PN for only a short duration, fat-soluble vitamins usually are not limiting; therefore supplementation is not typically required. The exception is in obviously malnourished animals in which supplementation may be necessary. Trace elements serve as cofactors in a variety of enzyme systems and can become deficient in malnourished patients as well. In people receiving PN, zinc, copper, manganese, and chromium are routinely added to the PN admixture. These are sometimes added to PN admixtures for malnourished animals, but their compatibility with the solution must be verified.

The addition of other parenteral medications to the PN admixture is possible; however, their compatibility also must be verified. Drugs that are known to be compatible and sometimes added to PN include heparin, insulin, potassium chloride, and metoclopramide. Although the addition of insulin to PN is often necessary in people, the hyperglycemia seen in veterinary patients with PN usually does not require insulin administration, except for patients with known diabetes mellitus.