Alternatively, insulin can be administered as a bolus or CRI to maintain normal glucose concentrations. The subcutaneous route is preferred over the intravenous route for bolus dosing, as slower absorption occurs and variation in blood glucose tends to be more moderate. A CRI is the preferred method of administration. There are different types of insulin; the following doses apply to regular insulin. Doses of insulin needed are often high as transient insulin resistance is present in critically ill patients [62, 66, 67].
A starting point that is well tolerated is 0.07 IU/kg/h of regular insulin [68] and is derived from a retrospective study in foals [59]. Other authors describe much lower starting doses of 0.0016 IU/kg [62]. Maximal effect is seen after approx. 90–120 minutes of infusion and alterations to dosing should therefore be made slowly and gradually. Blood glucose concentrations should be monitored more often when insulin is introduced (every 2–4 h) to prevent hypo‐ or hyperglycemia. When insulin is administered as subcutaneous bolus doses, dramatic swings in blood glucose levels are often seen and changes in PN rate and insulin doses should not be made simultaneously to avoid such swings. Increasing insulin doses by 50% is a reasonable approach when hyperglycemia persists. If hypoglycemia occurs, a bolus of 0.25–0.5 ml/kg 50% dextrose should be administered over 2–3 minutes. Blood glucose should be monitored every 30 minutes for the next 90 minutes to assess whether hypoglycemia recurs. Once a stable state of insulin and PN rate is reached, blood glucose monitoring can be decreased to twice daily. If adequate calories cannot be provided through glucose‐containing solutions due to persistent hyperglycemia, addition of lipids should be considered. Addition of lipids provides essential fatty acids and allows for provision of more calories without increasing the glucose load and decreases the risk of thrombophlebitis as the solution becomes more iso‐osmotic.
To prevent hypoproteinemia, the following steps should be followed. If PN has to be administered for prolonged periods of time (>48–72 h in adults, >24 h in foals), proteins should be added to glucose‐only solutions to avoid muscle wasting. Proteins provide essential and non‐essential amino acids. The caloric benefit of amino acids is controversial. Some authors recommend excluding the calories provided from amino acids from calculations to spare proteins from anabolism. However, this could lead to underestimating the caloric content of a solution by 15–20% and could lead to overfeeding. Physiologically, it is more likely that calories are used as needed (for production of proteins and all other processes), and providing an overall correct amount of calories is more useful than the concept of protein sparing [69]. Adequate protein provision should be monitored through daily measurement of Blood Urea Nitrogen (BUN) and serum protein and serum albumin. Solutions should be protected from direct sunlight to avoid denaturation of proteins. Commercial aminoacid solutions are usually hypertonic (up to 2,000 mosm/l) and safe infusion rates with gradual increase and the use of infusion pump need to be calculated individually.
Figure 6.5 Blood from a patient with severe hyperlipemia as a consequence of reduced energy intake.
To avoid hyperlipidemia and its consequences, serum triglycerides should be measured before initiation of lipid containing PPN and monitored daily thereafter (reference range 0.1–0.5 mmol/L). If hyperlipidemia and hypertriglyceridemia are not controlled, they can lead to hepatic lipidosis. Visual inspection for signs of lipemia (see Figure 6.5) is not sensitive enough. The administration of lipid containing parenteral nutrition should be reduced or discontinued when triglycerides levels in blood increase above the reference range. Monitoring during PPN administration should include regular assessments of serum and urine glucose, triglycerides, electrolytes and BUN as explained above. Once stable levels are reached, once daily monitoring is adequate. Additionally, respiratory function should be monitored with blood gas analysis, as glucose administration leads to endogenous production of CO2. Foals should also be weighed daily to ensure anabolic state and adequate weight gain. The catheter site should be monitored four times daily for signs of thrombophlebitis. Parenteral nutrition should be discontinued gradually to avoid rebound hypoglycemia.
Diagnosis and clinical signs
Clinical signs are vague and often masked by the underlying condition necessitating parenteral nutrition. Blood glucose, protein, triglyceride and cholesterol measurements need to be used for diagnosis.
Treatment
Treatment of hyperglycemia: see above under Prevention
Treatment of hyperlipemia: discontinue lipid solution. Exogenous insulin therapy can be considered in severe cases to stimulate the hormone sensitive lipase. In high‐risk patients such as ponies, donkeys and obese individuals, as well as animals with pre‐existing hypertriglyceridemia, administration of lipid‐containing solutions should be avoided.
Treatment of electrolyte abnormalities: see earlier in this chapter.
Treatment of hypoproteinemia: adjustment of the parenteral solution to ensure that adequate amounts of amino acids are included.
Expected outcome
Hyperglycemia has been shown to be detrimental and to cause increased morbidity and mortality in human and equine patients. Recommendations are to maintain serum glucose concentrations within narrow margins and avoid hypo‐ or hyperglycemia [27]. Outcome of hyperlipidemia depends on the severity but can be fatal. Outcome of electrolyte abnormalities depends on severity but is usually good. Outcome of hypoproteinemia and muscle wasting depends on severity and the underlying disease necessitating parenteral nutrition.
Literature on parenteral nutrition in horses is mostly available in the form of case reports, retrospective case series and conference proceedings [60, 62, 65, 67, 70–79]. There are few controlled studies available [70, 76, 80, 81]. Therefore, most information on application but also complications of parenteral nutrition is extrapolated from human medicine. This represents challenges as parenteral nutrition in humans differs from those in an equine setting with regards to administration, types of fluid, duration of therapy and metabolic side effects.
Complications Due to Withholding of Enteral Feeding
Definition
Withholding of enteral nutrition and use of parenteral nutrition leads to a decrease in gut mass and structural protein, decreased motility and digestive function and loss of mucosal integrity. Development of gastric ulcers also can occur.
Risk factors
Foals are at particular risk of developing gastric ulcers.
Pathogenesis
Enterocytes need food. Enteral feeding is also less expensive, more physiological, improves immunity including gastrointestinal immunity and is easier and safer. Neonatal foals that receive PPN or TPN without additional enteral feeding are prone to development of gastric ulcers during that period and after re‐feeding [70].
Prevention
Some human studies suggest that the route of administration is not as important as providing calories in itself; recommendations are still to institute early use of enteral nutrition if possible [82]. Current recommendations of the American Society for Parenteral and Enteral Nutrition are to avoid parenteral nutrition when
