Initially, the nutritional status of the patient has to be evaluated. During clinical examination, body weight, body condition, and muscle mass are evaluated, in addition to the presence of ascites, peripheral oedema, and mucosal or cutaneous lesions. The sick or injured patient will often present with adequate or even excessive fat stores while at the same time have significant muscle wasting. Careful examination is necessary, which involves palpation of skeletal muscle mass over bony prominences, such as the scapulae or vertebral column, to avoid misclassifying the patient as overweight or even obese. Hypoalbuminaemia, anaemia, or low blood urea nitrogen (BUN) levels may occur secondary to malnutrition. Some underlying diseases such as protein-losing enteropathy, protein-losing nephropathy, chylothorax, burns, and draining wounds cause excessive loss of body protein, requiring an aggressive nutrition plan to meet caloric and protein needs.

Enteral or Parenteral Nutrition?

Enteral feeding is always the preferred route of feeding, because it is physiologically sound, less costly, and safe. Advantages of enteral feeding include prevention of intestinal villous atrophy, maintenance of intestinal mucosal integrity (decreased risk of bacterial translocation), and preservation of GI immunologic function. However, contraindications for enteral feeding are uncontrolled vomiting, GI obstruction, ileus, malabsorption or maldigestion, or inability to protect the airway (animals in a coma, animals on the ventilator).

In these animals, parenteral nutrition allows provision of nutrients via intravenous administration of variable mixtures of amino acids, lipids, and dextrose, along with vitamins, minerals, and electrolytes. Relative contraindications for parenteral nutrition are sepsis, generalized thrombophlebitis, thromboembolic diseases, and a functional GI tract. Exclusive use of parenteral formulations for nutritional support should be performed with the understanding of the negative effects on the gastrointestinal barrier function and the development of mucosal atrophy.

Stepwise parenteral nutritional plan:

1.  When to start
For acutely ill or injured patients in a good condition, consideration for providing nutritional support should occur within 3–5 days of anorexia. In animals that are more debilitated, are growing, have inadequate muscle mass or adipose stores, that are not expected to consume food voluntarily within the next 2–3 days, or in cats which are at risk of hepatic lipidosis, intervention should be more immediate.

2.  How much to feed
The caloric goal is the provision of the resting energy requirement (RER). The RER is defined as the amount of energy (calories) needed to maintain homeostasis in the fed state in a thermoneutral environment. Most hospitalized patients are not expending much more energy than RER since they are caged. RER can be calculated by allometric formulas such as (70 x BW in kg) 0.75. An easier formula is RER = kg x 30+70 for dogs and cats. RER for a 6 kg cat is, therefore, RER = 6 kg x 30+70=250 kcal/day. “Illness factors” are no longer widely used in critical care nutrition. Overfeeding has risks and should be avoided. It can result in hyperglycaemia and hyperlipidaemia.

3.  Partial (peripheral) or total (central) parenteral nutrition
Several types of parenteral nutritional solutions are available. They differ in their osmolarity. Total parenteral nutrition (TPN) has an osmolarity of >800 mOsm/L and has to be administered via a large (central) vein because of the risk for phlebitis. Partial or peripheral parenteral nutrition can be given via either a central or a peripheral vein.

4.  Technical aspects of parenteral nutrition
Regardless of the chosen form of administration (central vs. peripheral), intravenous nutrition requires a dedicated catheter that is placed using an aseptic technique (clip, surgical scrub, and placed with sterile gloves). IV sets and extension sets should be changed every 24–48 hours, but otherwise not be manipulated and any disconnection should be avoided (e.g., the fluid bag is not disconnected if the patient needs to be moved or walked). If the parenteral nutrition has to be disconnected (e.g., occlusion of the venous catheter), the complete IV set and extension set have to be renewed and the patient has to be reconnected in as sterile a fashion as possible. An in-line 1.2-micron filter prevents fat embolization if the PN solution contains a lipid component. The fluid rate should be controlled by an infusion pump and parenteral nutrition fluids need to be taken into account when assessing fluid rates. Concurrent administration of other fluids or medications through the same catheter should be avoided, if this is not possible, compatibility must be guaranteed. Opened bags should be used within 24–48 hours and should be labelled with the date and time to ensure that they are discarded after 48 hours along with the entire tubing set. Due to the sensitivity of some nutrients to ultraviolet light, the solution bag and infusion lines should be kept covered. Amino acids and B vitamins are most susceptible to such degradation.

5.  Composition of the solution
Parenteral nutrition solutions are composed primarily of three base elements: amino acid solution, fat emulsion solution, and glucose (dextrose) solution. In commercially available human parenteral nutrition solutions, total parenteral nutrition solutions contain 40–50% glucose solution as an important component of energy. In contrast, human peripheral/partial parenteral nutrition contains less concentrated glucose (5–10%). In case of glucose intolerance (hyperglycaemia), partial instead of total parenteral nutrition solution should be administered. However, a higher fluid rate will be necessary to meet the energy requirement, and other fluids need to be reduced. The administration of B vitamins is important. With the exception of a small amount of vitamin B₁₂ in the liver, there is no storage of B vitamins in the body. These are important nutrients for metabolism and efficient use of energy, protein, fat, and glucose. Specific patient needs can be accommodated by adjusting the proportions of calories coming from protein, fat, and carbohydrate. For example, “low carb” solutions may be useful for patients with compromised pulmonary function and hypercapnia, and “low protein” solutions are used for patients with hepatic encephalopathy or renal disease. Provided that blood lipid clearance is normal, parenteral fat infusions are generally safe for patients with pancreatitis because exocrine pancreatic stimulation results from nutrients in the small intestine.
The preferred formulation of parenteral nutrition includes the use of individual amino acid solutions, lipid solutions, and dextrose solutions. Ideally, parenteral nutrition is adapted to the needs of the individual patient and the compound is freshly prepared in a hospital pharmacy.
The calculation and composition of the desired parenteral solution is carried out as follows:

a.  Calculation of energy requirements (RER).

b.  Protein requirements: Calculation of energy requirements provided by amino acids:

i.  Dogs require 15–20% of total calories should be supplemented with amino acids. The remaining calories are divided 50%:50% between dextrose and fat solutions.

ii.  Cats require 25–35% of total calories as proteins, the remaining calories are given as 60% lipids and 40% dextrose.

c.  According to the caloric density of the amino acid solution, the required amount (mL) of the amino acid solution is calculated.

d.  Calculation of lipid- and dextrose requirements and calculation of the amount of fluid according to the caloric density of the solution used.

Several commercially available human formulations are available and are appropriate for short-term use in dogs (1–3 days). If using a human commercial product, calories from protein should match species specific protein requirements. If prefabricated formulations are used, the required fluid amount is calculated based on the RER of the patient and caloric density of the solution. The calculated fluid dose may have to be adjusted in order to avoid over infusion of the patient.

6.  How to start
There are two reasons for starting slowly and gradually increasing the rate of parenteral nutrition: (1) refeeding syndrome and (2) possible hyperglycaemia. In patients with prolonged anorexia, gastrointestinal compromise, or metabolic derangements, the caloric goal should be achieved over several days. Typical feeding protocols might provide 30% to 50% RER on day 1, with the goal of reaching full RER over the next several days. In stable patients without prolonged anorexia (young, previously healthy dog with polytrauma in which enteral feeding is not possible) it is possible to start with 25% RER for 6 hours, increase to 50% RER in the next 6 hours and increase to 100% thereafter after 24 hours. However, it is important to closely check blood glucose levels to avoid hyperglycaemia. Blood glucose target ranges between 100–250 mg/dL (5.5–13.9 mmol/L).

7.  Monitoring
Before starting parenteral nutrition: Body weight; electrolytes: Na, K, P, Mg; blood glucose; haematocrit; albumin; BUN. After start of the infusion: Glucose check every 4 hours, cardiorespiratory parameters. After full infusion rate is archived, assess at least once daily for 2–3 days: Body weight; electrolytes: Na, K, P, Mg; haematocrit; catheter insertion site (q4h); thoracic auscultation; body temperature.
In addition, perfusion and hydration status must be assessed because, depending on the rate of the parenteral nutrition, the patient may need additional fluid therapy.

8.  Complications
Complications can be metabolic, mechanical, or septic. Mechanical complications of PN include catheter related problems such as catheter occlusion, accidental removal, or line disconnection. If the catheter is not 100% within the vessel, it must be removed and replaced as there is a very high risk for tissue necrosis if the hyperosmolar parenteral fluid is given perivascularly. Thrombophlebitis may manifest as swelling, redness, and pain at the catheter site. The most common metabolic complications are hyperglycaemia, lipaemia, hyponatraemia, and hypokalaemia. Hyperglycaemia is a common complication after initiation of PN, particularly in patients under physiologic stress associated with trauma, surgery, and sepsis, as well as in cats. Hypoglycaemia may occur upon abrupt discontinuance of PN, especially if voluntary eating or enteral support is not concurrent with PN withdrawal. TPN should be withdrawn over a 2–3-day period to prevent this complication. Alternatively, dextrose can be added to an infusion of crystalloids. Hypokalaemia, hypophosphataemia, hypomagnesaemia, and thiamin deficiency may occur upon initiation of enteral or parenteral nutrition, especially after a prolonged catabolic state. This condition is termed “refeeding syndrome” and results from a rapid rise in insulin and a shift of plasma potassium, magnesium, and phosphates into the intracellular compartment. Thiamin is a co-factor in carbohydrate metabolism. Profound weakness, gastrointestinal ileus, vomiting, diarrhoea, neuromuscular and cardiac rhythm disturbances can result. Hypophosphataemia can manifest as haemolysis. Hypomagnesaemia can result in tetany, seizures, panting, cardiac arrhythmias, and hypertension. Potassium phosphate, potassium chloride, and magnesium sulfate supplementation may be necessary. Hypertriglyceridaemia is seen with insulin resistance and catabolism associated with starvation. An increase in serum triglycerides may overwhelm liver metabolism and lead to steatosis. Hypertriglyceridaemia may require a decrease in parenteral nutrition application or adjustment of the composition of the solution. Other metabolic derangements include hyperammonaemia and bilirubinaemia. The administration of hyperosmolar solutions that are composed of lipid, amino acid and dextrose solutions provides an excellent growth media for bacteria. While PN related sepsis used to be a common complication, strict hygiene, and catheter monitoring decreases the likelihood of septic complications to 3–16%. Patients showing clinical signs of infection and sepsis should be actively examined for the likelihood of PN solution contamination or catheter related infection. Cultures of PN solution or catheters should guide antibiotic therapy.

References

References are available upon request.