Page 623 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Parenteral Nutrition 611

This exponential equation will more accurately estimate and 7.5 kg of which is fat). Therefore one can take the
the animal’s true requirements across all body weights. ideal weight plus 25% of the excess weight (to account
However, for animals weighing between 3 and 25 kg, for the extra lean body mass) as the weight to use for cal-
the following linear formula gives a reasonable approxi- culation of RER. Using the 30-kg dog and ideal weight of
mation of energy needs: 20 kg from the example above, the adjusted body weight
to use for calculation of RER would be 20 kg þ (25%
RER ¼ 30 ðcurrent body weight in kilogramsÞþ 70 10 kg) or 20 kg þ 2.5 kg ¼ 22.5 kg. Thus the RER for
this overweight dog would be 723 kcal/day.
In the past, the RER was multiplied by an illness factor
One should avoid using the linear equation for animals
between 1.0 and 2.0 to account for increases in metabo-
smaller than 3 kg or larger than 25 kg because the linear
lism associated with different conditions and
equation will underestimate or overestimate these 1,10,42
injuries. Recently, there has been less emphasis on
animals’ energy requirements, respectively (Figure 25-5).
these subjective illness factors, and current
For animals that are underweight, the authors recom-
recommendations are to use more conservative energy
mend using the animal’s current weight for the RER cal- 15,28
estimates to avoid overfeeding. Overfeeding can
culation. The goal of parenteral nutrition should not be
result in metabolic and gastrointestinal complications,
weight gain, which can be achieved after the animal’s
hepatic dysfunction, and increased carbon dioxide pro-
underlying disease has been treated and the animal is able 5,6
duction. We have shown in cats receiving TPN that
to tolerate enteral or oral feedings. Overfeeding for the
animal’s current weight also increases the risk for meta- those in which the RER was multiplied by an illness factor
bolic complications (see Complications section). For were more likely to develop hyperglycemia than those in
animals that are overweight, one should feed an appropri- which energy requirements were provided at or below
21
ate number of calories to prevent weight loss because seri- RER. Critically ill cats, in particular, are at high risk
ously ill or injured animals lose lean body mass rather than for hyperglycemia due to a variety of hormonal alterations
17
fat. There are a number of ways to calculate parenteral that are similar to those identified in critically ill people.
Avoiding the development of hyperglycemia has
nutrition requirements in markedly overweight animals
been shown to be beneficial in certain populations of
(i.e., >25% above ideal body weight). One is to use the 27,69
critically ill people. Newer recommendations for
animal’s current body weight for the RER calculation
nutritional support in critically ill people emphasize the
while carefully monitoring body weight to ensure that
need to avoid overfeeding and reduce the risk of
the animal does not lose or gain weight. Another option 40,47,60 16,69 67
hyperglyceamia. In cats and in dogs, hyper-
is to use the assumption that 25% of excess weight is lean
glycemia is associated with reduced survival and longer
tissue and the remaining 75% is metabolically inactive fat
hospitalization times. To reduce the risk of hyperglycemia
(i.e., if a dog’s ideal weight is 20 kg and it weighs 30 kg, it
and other complications, the authors use RER as an initial
has 10 kg of excess weight, 2.5 kg of which is lean tissue
estimate of a critically ill patient’s energy requirements.
Further adjustments are made based on the animal’s
2500 response to feeding, body weight, and changes in the
underlying condition. Indirect calorimetry can accurately
assess the caloric needs of individual patients, but it is
Resting energy requirement 1500 Exponential nique may be more commonly used in the future, partic-
Linear
2000
rather cumbersome to use in a clinical setting. This tech-
ularly for patients that are difficult to manage on
nutritional support. Studies using indirect calorimetry
support the hypothesis that the application of illness
1000
factors in calculating energy expenditure in clinical
time, the key to successful nutritional support is vigilant
500 patients overestimates energy needs. 57,72 At the current
monitoring after therapy has been initiated to ensure that
provision of calories is adjusted as necessary.
0
0 10 20 30 40 50 60 70 80 Other Nutrient Requirements
Weight (kg)
Figure 25-5 Comparison of resting energy requirements (RERs) After calorie requirements are determined, one must also
as calculated using a linear equation [(30 body weight) þ 70] address protein requirements. Animals require a nitrogen
versus an exponential equation [70(body weight) 0.75 ]. Note that the source and essential amino acids. These are typically
equations yield similar results for animals weighing between 3 and provided parenterally by an amino acid solution. Essential
25 kg. For animals that weigh more than 25 kg, the linear equation fatty acids (linoleic acid in the dog, linoleic and
overestimates the animal's RER. arachidonic acids in the cat) are also required. These