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EQUINE VETERINARY EDUCATION / AE / FEBRUARY 2018
(see Table 1 for an example calculation of dextrose supplementation). After 24–48 h of dextrose-containing fluids, the nutritional
plan should be revisited to determine whether the foal can tolerate enteral nutrition. In addition to vital parameters and hydration status the presence or absence of gastrointestinal motility, frequency of faecal output and consistency of faeces should be taken into consideration in the assessment. If continued parenteral nutrition is required a more complete solution that contains amino acids and possible lipids should be administered. Since the metabolic response to injury and sepsis is the increased protein degradation in muscle tissue, the addition of a protein source reduces this catabolic response. Protein supplementation provides essential and nonessential amino acids. The most commonly used protein solutions provide approximately 4.0 kcal/g (16.7 kJ/g) of protein. A commercially premixed standardised combination of a 50% dextrose solution and 8.5% amino acid solution is available and has been utilised in the author’s practice. Another option is to purchase the solutions separately and aseptically mix in a sterile parenteral nutrition bag. The following formula is well tolerated by the foal: 1500 ml 50% dextrose and 1500 ml 8.5% amino acids. The caloric density of this solution is 1.02 kcal/ml (4.27 kJ/ml). Because the combination of dextrose and amino acids is hypertonic and potentially harsh on the vein, the solution should be diluted in an isotonic solution or sterile water for administration. The greatest benefit to the addition of lipids is providing
more calories than just dextrose or dextrose and amino acids. Lipid emulsions contain primarily long-chain triglycerides and provide a concentrated source of calories at 9–11 kcal/g (38–46 kJ/g). Another benefit is that lipid emulsions are isotonic and help reduce total osmolarity when added to glucose and amino acid mixtures. Critical illness may result in protein catabolism and muscle wasting as a result of the release of cytokines and catabolic hormones. Thus it is important to provide at least 100–200 nonprotein calories (carbohydrate and lipids) per gram of nitrogen in the parenteral nutrition formula to avoid the use of amino acids for energy (Hansen 1990). Solutions composed of 10 g/kg bwt/day of dextrose, 2 g/
kg bwt/day of amino acids and 1 g/kg bwt/day of lipids are well tolerated by the foal. The volume of each nutrient is calculated based on the proportion of energy derived from dextrose and lipids as well as the grams of protein that the foal requires. Once the final volume is determined, an hourly infusion rate is calculated by dividing the total volume in millilitres by 24 h. Several basic formulas have been described for use in the foal and can easily be prepared using aseptic technique (Table 2). Premade mixtures of i.v. multivitamins can be added to parenteral nutrition. These products contain the fat soluble vitamins A, D and E, which are solubilised in an
TABLE 1: Example calculaton of dextrose supplementation
50 kg foal Goal: 4 mg/kg bwt/min 50 kg 9 4 mg/kg bwt/min = 200 mg/min 200 mg/min 9 60 min/h = 12,000 mg/h 5% dextrose = 50 mg/ml 12,000 mg/h divided by 50 mg/ml = 240 ml/h Infusion rate = 240 ml/h of 5% dextrose
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TABLE 2: Formulations of total parenteral nutrition (TPN) for the foal Composition
Caloric density
900 ml of 50% dextrose 1400 ml 8.5% amino acids 900 ml 20% lipids
1500 ml 50% dextrose 2000 ml 8.5% amino acids 500 ml 20% lipids
1.19 kcal/ml (4.98 kJ/ml) 1.08 kcal/ml (4.52 kJ/ml)
aqueous medium, permitting i.v. administration. The B complex vitamins including thiamine, folic acid, pantothenic acid and niacin are also found in these commercial vitamin products. Thiamine (vitamin B1) is a component of thiamine pyrophosphate and is an essential cofactor in carbohydrate metabolism. Vitamin B complex can be added directly to the parenteral nutrition solution. Supplemental electrolytes can be added the maintenance crystalloid fluids. Parenteral nutrition can safely be administered by an i.v.
catheter placed aseptically in the jugular vein although proper catheter management is extremely important. The solutions used for parenteral nutrition are hypertonic and can cause injury to the vascular endothelium resulting in phlebitis or thrombosis. Multi-lumen catheters allow for one lumen to be dedicated to infusion of the solution thus minimising the risks of contamination. Parenteral nutrition should be
delivered at a constant rate using an infusion pump to avoid fluctuations in glucose delivery and metabolic complications and the actual volume of parenteral nutrition delivered to the case should be carefully monitored and recorded. Parenteral nutrition administration should be started at one quarter of the target rate. This is to allow for insulin and other physiological parameters to adapt to the solution. The rate should then be gradually increased every 4–6 h, depending on the blood glucose values, until the target rate is reached (see Table 3 for example calculation of TPN). In the early stage of parenteral nutritional therapy the foal must be frequently monitored. In addition to the physical examination and vital signs, blood glucose concentrations should be monitored. Initially the blood glucose concentration should be monitored every 2–4 h until the patient is at the target rate of parenteral nutrition then the frequency can be decreased. Blood glucose should be maintained between 5.0–10.0 mmol/l. The frequency of monitoring blood glucose concentration depends on the age and stability of the patient with the very critically ill neonatal foal often requiring more frequent monitoring. Urine output should be monitored constantly with intermittent monitoring of urine glucose. In some cases glucosuria and diuresis are observed when blood glucose levels exceed 10 mmol/l indicating the administration rate should be adjusted. Some critically ill foals, usually
TABLE 3: Example calculation of total parenteral nutrition
80 kg foal, 4-week-old foal Goal: 100 kcal/kg bwt/day (418 kJ/kg bwt/day) 80 kg 9 100 kcal/kg bwt/day = 8000 kcal/day (33,472 kJ/day) Using the first formula in Table 2 (1.19 kcal/ml) (4.98 kJ/ml) 8000 kcal/day divided by 1.19 kcal/ml = 6722 ml/day 6722 ml/day divided by 24 h/day = 280 ml/h Infusion rate = 280 ml/h
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