Introduction

Critically ill animals undergo several metabolic alterations that put them at high risk for the development of malnutrition and its associated complications. More importantly, because of the seriousness of many disorders encountered in the critically ill patient, nutritional support is often given a low priority and poor nutritional support further compromises patients in the intensive care unit. Whilst cardiovascular stabilization does take priority in critically ill animals, it is important to remember that during periods of nutrient deprivation, a sick or traumatized patient will preferentially catabolize lean body mass (stressed starvation) when they are not provided with sufficient calories. This is in contrast to a healthy animal deprived of food, where energy is derived from stored fat sources (simple starvation). The loss of lean body mass negatively impacts immune function, wound healing, and perhaps, overall survival. Inadequate calorie intake is commonly due to a loss of appetite, an inability to eat or tolerate feedings, and gastrointestinal dysfunction (e.g., ileus, vomiting) that accompanies many disease processes. Because malnutrition can occur quickly in these animals, it is important to provide nutritional support by either enteral or parenteral nutrition if oral intake is not adequate.

Rather than focusing on improving appetite, which is commonly the only attempt at nutritional support, a more appropriate approach would be to simply ensure adequate nutritional intake--i.e., placement of a feeding tube or instituting parenteral nutrition. Whenever possible, the enteral route should be used because it is the safest, most convenient, and most physiologically sound method of nutritional support. Enteral feeding also promotes gastrointestinal perfusion, attenuates gastrointestinal permeability, and promotes mucosal immunity. However, many if not most critically ill animals actually have significant gastrointestinal dysfunction and are unable to tolerate enteral feedings or are unable to utilize nutrients administered enterally. In such cases, parenteral nutrition should be considered, however, this is seldom available in general practice. Ensuring the successful nutritional management of critically ill patients involves selecting the right patient, making an appropriate nutritional assessment and implementing a feasible nutritional plan.

Nutritional Assessment

As with any medical intervention, there are always risks of complications associated with nutritional therapies. Exacerbation of cardiovascular dysfunction, electrolyte and acid-base disturbances, hyperglycemia, hypercarbia are but a few of the possible complications. Minimizing such risks involves appropriate patient selection and patient assessment. Nutritional assessment identifies malnourished patients that require immediate nutritional support and also identifies patients at risk for developing malnutrition in which nutritional support will help to prevent malnutrition. Indicators of overt malnutrition include recent unintentional weight loss of at least 10% of body weight, poor haircoat quality, muscle wasting, signs of poor wound healing, and hypoalbuminemia. However, these abnormalities are not specific to malnutrition and are not present early in the process. Factors that predispose a patient to malnutrition include anorexia lasting longer than three days, serious underlying disease (e.g., trauma, sepsis, peritonitis, pancreatitis, and significant gastrointestinal surgery), and large protein losses (e.g., protracted vomiting, diarrhoea, or draining wounds). Nutritional assessment also identifies factors that can impact the nutritional plan, such as cardiovascular instability, electrolyte abnormalities, hyperglycemia, hypertriglyceridemia or concurrent conditions such as kidney or hepatic disease that may impact the nutritional plan (e.g., reduction in protein in severely azotemic patients or patients with signs of hepatic encephalopathy). Appropriate laboratory analysis should be performed in all patients to assess these parameters. Before implementation of any nutritional plan, the patient must be cardiovascularly stable, with major electrolyte, fluid, and acid-base abnormalities corrected. Commencing nutritional therapies before these abnormalities are addressed may lead to further complications.

Goals of Nutritional Support

By providing adequate energy substrates, protein, essential fatty acids, and micronutrients, the body can support wound healing, immune function, and tissue repair. A major goal of nutritional support is to minimize metabolic derangements and catabolism of lean body tissue. During hospitalization, repletion of body weight is not a priority as this will only occur when the animal is recovering from a state of critical illness. Therefore, gain of body weight is not a goal whilst the animal is hospitalized for the majority of cases. However, continued weight loss during hospitalization is of particular concern and should be addressed. With amelioration of underlying condition, the animal should start voluntarily consuming calories. This must be continually assessed.

Nutritional Plan

Proper diagnosis and treatment of the underlying disease is the key to the success of nutritional support. Based on the nutritional assessment, a plan is formulated to meet energy and other nutritional requirements of the patient and at the same time address any concurrent condition requiring adjustments to the nutritional plan. For each patient, the best route of nutrition should be determined--enteral versus parenteral nutrition. This decision should be based on the underlying disease and the patient's clinical signs. Whenever possible, the enteral route should be considered first. If enteral feedings are not tolerated or the gastrointestinal tract must be bypassed, then parenteral nutrition should be considered. Nutritional support should be introduced gradually and reach target levels in 48-72 hours.

In animals with obvious gastrointestinal dysfunction, e.g., decreased GI motility, prokinetic agents such as metoclopramide may be useful. Antiemetics such as maropitant may also improve food intake in certain animals. With regards to appetite stimulants, it is the author's opinion that they have no place in the nutritional management of hospitalized critically ill patients. The only means of insuring adequate caloric intake is through effective nutritional support (i.e., tube feeding or parenteral nutrition). Appetite stimulants could be used once the patient is recovering from its disease and at home.

Calculating Energy Requirements

The patient's resting energy requirement (RER) is the number of calories required for maintaining homeostasis while the animal rests quietly. The RER is calculated using the following formula:

RER = 70 x (body weight in kg)^0.75

For animals weighing between 2 and 30 kg, the following linear formula gives a good approximation of energy needs:

RER = (30 x (body weight in kg) + 70

Traditionally, the RER was then multiplied by an illness factor between 1.1-2.0 to account for purported increases in metabolism associated with different conditions and injuries. Recently, there has been less emphasis on these subjective illness factors and current recommendations are to use more conservative energy estimates (approximately RER) to avoid overfeeding. Overfeeding can result in metabolic and gastrointestinal complications, hepatic dysfunction, increased carbon dioxide production, and weakened respiratory muscles.

It should be emphasized that these general guidelines should be used as starting points, and animals receiving nutritional support should be closely monitored for tolerance of nutritional interventions. Continual decline in body weight or body condition should prompt the clinician to reassess and perhaps modify the nutritional plan (e.g., increasing the number of calories provided by 25%).

Enteral Nutrition

The enteral route of nutritional support is usually the preferable route. Enteral nutrition is safer and less expensive than parenteral nutrition, and helps to maintain intestinal structure and function. Even with the use of feeding tubes, patients can easily be discharged for home-care with good owner compliance. Although the enteral route should be utilized, if at all possible, there are contraindications to its use. Contraindications include persistent vomiting, severe malabsorptive conditions, and an inability to guard the airway. Feeding tubes commonly used in dogs and cats include nasoesophageal, esophagostomy, gastrostomy, and jejunostomy tubes. In most cases, naso-oesophageal tubes are only effective for a few days (typically less than 5 days). Esophagostomy tubes are in the author's opinion, the most useful and effective feeding tube available and should be considered in the majority of patients within the intensive care unit that are at risk of malnutrition.

Parenteral Nutrition

Parenteral nutrition (PN) is more expensive than enteral nutrition and is only for in-hospital use. Indications for PN include protracted vomiting, severe malabsorptive disorders, and severe ileus. The use of PN in dogs with acute pancreatitis remains controversial as there is increasing evidence in other species supporting enteral feeding in the face of acute pancreatitis. Part of the difficulties in using PN in general practice is due to the requirement of a dedicated (usually centrally placed) catheter that is placed using aseptic technique. Multi-lumen catheters are often recommended for PN because they can remain in place for longer periods of time as compared to normal jugular catheters and provide other ports for blood sampling and administration of additional fluids and IV medications. Most PN solutions are composed of a carbohydrate source (dextrose), a protein source (amino acids), and a fat source (lipids). Formulation of PN solutions is ideally individualized to each patient and this makes its use in practice difficult. Some authors use generic fixed PN formulations which limits its use in some patients. In most cases, it is easiest to have a local human hospital formulate PN for veterinary patients. There is also increased use of ready-made, commercially available amino acid and dextrose solutions in veterinary patients, however, further studies are warranted to evaluate the feasibility of their use.

Monitoring and Reassessment

Possible complications of enteral nutrition include mechanical complications such as obstruction of the tube or early tube removal. Metabolic complications include electrolyte disturbances, hyperglycemia, volume overload, and gastrointestinal problems (e.g., vomiting, diarrhoea, cramping, bloating). Possible complications with PN include sepsis (low risk), thrombophlebitis, and metabolic disturbances, such as hyperglycemia, electrolyte shifts, hyperammoniemia, and hypertriglyceridemia. Frequent monitoring of vital signs, catheter-exit sites, and routine biochemistry panels may alert clinician of developing problems. The discontinuation of nutritional support should only begin when the patient can consume approximately 75% RER without much coaxing. In patients receiving PN, transitioning to enteral nutrition should occur over the course of at least 12-24 hours, depending on patient tolerance of enteral nutrition.

References

References are available upon request.