CALCULATION OF NUTRITIONAL REQUIREMENTS

Once a decision is made that nutritional support is indicated and that the GI tract can be used, the number of calories (kilocalories), grams of protein,

Figure 12-30 Percutaneous gastroduodenostomy tube placement.

and milliliters of water required by the animal is calculated.

Energy Requirements

There are several ways to estimate the amount of calories required by a patient. Illness energy requirements may be estimated by using a multiple of resting energy requirements or some fraction of maintenance energy requirements. In addition, energy requirements may be calculated using a lin­ear or exponential formula. These formulas are presented in Table 12-4.

Although estimates overlap in patients weigh­ing between approximately 2 and 20 kg (5 to 45 lb), in larger dogs the linear formula often overestimates

Figure 12-31 Use of a surgically placed enterostomy feeding tube in an adult Boston terrier.

energy requirements. As can be seen from the table, maintenance energy requirements are approximately two times resting energy require­ments (RERs) for dogs and one and one-half times RERs for cats. In many patients that are ill, the stress of illness results in increased RERs; how­ever, because of lack of physical activity, energy requirements during illness for most patients are less than maintenance. Numerous reports of criti­cally ill humans indicate that metabolic rates are increased over RER: 25% to 35% postoperatively; 35% to 50% with trauma or cancer; 50% to 70% with sepsis; and 70% to 100% or more with major burns or head trauma. Energy requirements of critically ill humans rarely approach normal main­tenance energy requirements. Based on data derived from humans, various factors have been suggested for estimating energy requirements of ill or injured animals. In most critically ill animals, the goal of nutritional support is to maintain body weight and body condition. Therefore a practical alternative to using a complex factoring system is to multiply the RER by a factor of 1.5 for dogs and cats. Alternatively, using the formula 100(BW_k ^0.75) provides the same results as multi­plying the RER by 1.5. The clinician should notice that this represents estimated maintenance energy requirements in cats.

Protein Requirements

After energy requirements are calculated, protein requirements should be estimated. Dietary protein is necessary to facilitate recovery from illness and injury. Protein requirements of ill or injured patients vary on a patient-by-patient basis.Variables include dietary energy content; dietary protein quantity and quality; and individual patient differences reflect­ing species, life stage, and type, degree, and stage of injury or illness. Protein requirements of critically ill dogs and cats have not been determined. Unless a patient has compromised renal or hepatic function, intake of dietary protein should meet maintenance protein requirements. Adequate protein intake is necessary to facilitate recovery to provide structural protein for repair of injured tissues, and to optimize immune function. For maintenance, diets should provide a minimum of 4 g of protein per 100 kcal of metabolizable energy (16% of energy as protein) for dogs and 6 g of protein per 100 kcal of metabo­lizable energy (25% of energy as protein) for cats. Taking into account availability, digestion, and absorption of enterally provided protein, require­ments may be estimated at 4 g/BW_k for dogs and 6 g/BW_k for cats. In patients with liver or renal failure, less protein should be provided, and in patients with hypoalbuminemia or protein loss through the GI tract or through loss of protein-rich fluids (e.g., peritonitis), protein intake should be increased; protein intake should be decreased with protein-losing nephropathy. Protein of high bio­logic value should be used in critically ill patients. Special importance has been ascribed to metabo­lism of specific amino acids in nutritional support, and they have been added to commercially available convalescent diets. Arginine is an essential amino acid for dogs and cats and is important in wound healing, immune function, and promoting a positive nitrogen balance.

Fluid Requirements

It is important to maintain adequate hydration when providing nutritional support. Fluid is pro­vided in prepared liquid diets or added to canned pet foods to make a gruel for enteral nutrition. Dogs and cats require 50 to 100 ml of water per kilogram of body weight for daily maintenance, depending on environmental temperature, type of food, and level of activity. Canned pet foods typically contain

Figure 12-32 Placement of an enterostomy tube. A, Make a 1- to 1.5-cm linear incision in the seromuscular layers of the antimesenteric border of the selected jejunal segment; use the tip of a scalpel blade to puncture a hole in the aboral aspect of the seromuscular incision. B and C, Place the distal end of the feeding tube through the incision; lay the exiting portion of the tube in the 1- to 1.5-cm seromuscular incision and construct a “tunnel” by inverting the seromuscular layer over the tube with three or four Cushing sutures of 4-0 absorbable material. (From Fossum et al.: Small animal surgery, ed 2, St. Louis, 2002, Mosby.)

Figure 12-33 A, The catheter is exteriorized through a separate stab incision in the body wall. B and C, The jejunum is attached to the peritoneum with three to four simple interrupted sutures of 3-0 or 4-0 absorbable suture material. (From Fossum et al.: Small animal surgery, ed 2, St. Louis, 2002, Mosby.)

Figure 12-34 Combination of enterostomy feeding tube (red rubber feeding tube in foreground) and gastrostomy feeding tube (mushroom-tipped feeding tube in background) in an adult male West Highland white terrier with acute pancreatitis and hiatal hernia.

70% to 85% water. Normal daily fluid requirements are approximately equal to daily caloric require­ments. Patients affected with diseases associated with excessive fluid losses (e.g., polyuria, diarrhea, vomit­ing, and third spacing of fluids) require more than calculated normal fluid amounts. Abrupt changes in body weight usually reflect hydration status; there­fore fluid intake can be adjusted to maintain body weight. Fluid requirements should be met as part of nutritional support. A general recommendation is to mix water and canned diet in a 1:1 ratio to meet maintenance energy requirements for most patients.

Vitamins and Minerals

Little is known about vitamin and mineral status in critically ill patients, although deficiencies have been observed in veterinary medicine. One study in humans indicated that micronutrient deficiency was common (64% of 284 patients examined) in a wide variety of illnesses. Use of well-balanced diets should provide adequate amounts of micro­nutrients, especially when pet foods are used. Human enteral products may not provide ade­quate amounts. Oversupplementation should be avoided because excess vitamins and minerals may compromise the patient (e.g., iron supplementa­tion may worsen bacterial infection and oxygen free radical production).