Reasons for performing study: D(+)-xylose absorption tests are commonly performed when investigating suspected small intestinal malabsorption in the horse. The test involves the administration of a D(+)-xylose solution via a nasogastric tube followed by serial blood sampling to determine its rate of absorption. In some horses, nasogastric intubation cannot be safely performed without prior administration of a sedative. Due to its short duration of action, the α2 agonist xylazine is commonly used for this purpose. However, α2 agonists have also been reported to influence the rate of gastric emptying as well as small intestinal motility patterns.
Objective: To evaluate if prior sedation with xylazine would influence the rate of absorption of D(+)-xylose in 6 normal Standardbred horses in a randomised cross-over study.
Methods: D(+)-xylose was administered by nasogastric intubation at a dose rate of 0.5 g/kg bwt given as a 10% solution with water while xylazine was administered iv at a dose rate of 0.5 mg/kg bwt. A heparinised blood sample was collected prior to administration of D(+)-xylose (and xylazine when used) and then at 30, 45, 60, 75, 90, 120, 150, 180 and 240 min following administration. Samples were immediately analysed using a modified colorimetric micro method. The cumulative amount of D(+)-xylose absorbed at each time point with and without prior sedation were. The significance rate was set at P<0.05.
Results: The study failed to demonstrate a statistically significant difference in the amount of D(+)-xylose absorbed between sedated and unsedated animals, although there was a tendency for a less rapid initial uptake with prior sedation.
Conclusion: This study suggests that prior sedation with xylazine will not significantly affect the result of a D(+)-xylose absorption test in the normal horse.
Chronic weight loss resulting from small intestinal malabsorption is a well recognised clinical problem in the horse. This can be the result of different processes such as infiltrative inflammatory cell disorders, oedema and villous atrophy, all of which may interfere with intestinal absorption (Roberts 1985, 2000). With the former, the predominant cellular infiltrate will generally categorise the type of inflammatory bowel disease although, regardless of this, the clinical presentation is often similar (Schumacher et al. 2000). This typically includes weight loss despite a normal to increased appetite and hypoalbuminaemia, which eventually may become sufficiently low for the horse to develop ventral oedema (Roberts 2000; Schumacher et al. 2000).
The D(+)-xylose absorption test is commonly performed during the investigative process to help assess the small intestinal absorptive capacity. It offers some advantages over the oral glucose absorption test as D(+)-xylose concentration in urine and plasma is low under normal dietary conditions making it an easily detectable marker (Merritt et al. 1986). In addition, D(+)-xylose is not metabolised by the mucosa and there is no effect of insulin (Roberts 2000). In contrast, blood glucose levels may also reflect abnormalities of glucose metabolism and regulation, which may make interpretation more difficult (Sweeney 1987).
The administration of the D(+)-xylose solution is performed via nasogastric intubation. In a small number of horses, this procedure can only be safely carried out following sedation. It has been suggested that the α2 agonist xylazine, a commonly used sedative in the horse, may influence the rate of gastric emptying as well as small intestinal motility (Adams et al. 1984; Merritt et al. 1989, 1998; Doherty et al. 1999) although the degree and duration of this may be dose dependent.
The purpose of this study was to test the hypothesis that prior sedation with xylazine administered at a dose rate of 0.5 mg/kg bwt would not significantly influence the rate of absorption of D(+)-xylose in 6 healthy Standardbred horses.
Materials and methods
Six normal Standardbred horses (5 mares and one gelding) from the Norwegian School of Veterinary Science research and teaching herd were used in this study. The median age was 15.5 years (range 4–23 years). Median bodyweight was 546 kg (range 501–665 kg) with the body condition score ranging from 6–8 (Henneke et al. 1983). All horses had been on the same level of feeding and from the same feed source at least 4 weeks prior to commencing the study. The diet consisted of hay only fed 3 times daily constituting approximately 1.5% of each horse's bodyweight. This remainedunchanged throughout the study period, which included a 2 week wash-out period. Prior to each test, the horses were physically examined and a blood sample collected and submitted for haematology and biochemistry evaluation.
On the day prior to the experiment, an i.v. catheter (Secalon)1 was aseptically placed and feed withheld for approximately 16 h. On the day of the experiment the horses were weighed to accurately estimate the dose of D(+)-xylose and of xylazine when selected. The dose rate of xylazine (Narcoxyl)2 was 0.5 mg/kg bwt administered i.v., while that of D(+)-xylose3 was 0.5 g/kg bwt dissolved in water to make a 10% solution. If selected for sedation, the effect of this was allowed to take place prior to nasogastric intubation and D(+)-xylose administration, usually 4–5 min.
Heparinised blood samples were collected prior to administration of D(+)-xylose (and xylazine if used) and then at 30, 45, 60, 75, 90, 120, 150, 180 and 240 min following administration. Prior to collecting the sample to be submitted for analysis, 10 ml of blood were collected and discarded. Samples were immediately analysed using the modified colorimetric micromethod described by Eberts et al. (1979).
Random selection meant that 3 horses were selected for sedation and 3 not in the first part of the experiment and vice versa following a wash-out period of 2 weeks.
Dunnett's method (Dunnett 1955) was used to analyse the data when comparing D(+)-xylose concentration in plasma between sedated and the unsedated controls. Significance rate was set at P<0.05.
The study protocol was approved by the Norwegian National Animal Research Authority.
The mean amount of D(+)-xylose absorbed at each time point with and without prior sedation was compared but failed to detect a significant difference at any of these. However, there was a tendency for the D(+)-xylose absorption curve to have a slightly steeper initial increase in the unsedated animals as well as having a more rapid decrease than with prior sedation (Fig 1).
Apart from one horse, the results of analyses of all blood samples taken prior to the experiments were within reference range. One horse had a slightly low total protein (58 g/l, reference range 60–65 g/l) and albumin concentration (28 g/l, reference range 30–42 g/l) but was otherwise clinically normal.
The present study supported the hypothesis that prior sedation with xylazine at a dose rate of 0.5 mg/kg bwt would not significantly affect the rate of D(+)-xylose absorption in this group of horses.
The dose rate of xylazine was similar to that used by Merritt et al. (1989, 1998) when demonstrating its effect on proximal small intestinal motility patterns. Although there were some discrepancies regarding the exact effect, it was concluded by Merritt et al. (1998) that xylazine was likely to mildly reduce duodenal motility patterns for the first 30 min when given at a dose rate of 0.5 mg/kg bwt. In contrast, Doherty et al. (1999) demonstrated that xylazine given at 1.0 mg/kg bwt to 6 adult ponies resulted in a significant reduction in the rate of gastric emptying of a liquid marker but proposed that the effect of xylazine on both gastric emptying as well as small intestinal transit time may be dose dependent. Therefore it seems reasonable to assume that the effect of administering xylazine at a dose rate of 0.5 mg/kg bwt is either of too short duration and/or does not affect gastric emptying or intestinal transit time sufficiently to significantly alter the rate of D(+)-xylose absorption.
However, it was noted that the initial rate of absorption in the current study was somewhat slower with a more prolongedpeak absorption creating a flatter curve with prior sedation supporting the previously reported effect of xylazine on gastric emptying of a liquid marker and proximal small intestinal motility (Adams et al. 1984; Merritt et al. 1989, 1998; Doherty et al. 1999).
There are several other factors that may influence the D(+)-xylose absorption test in the horse. Diet has been one such factor as described by Jacobs et al. (1982), who demonstrated a significantly lower D(+)-xylose absorption in horses fed a high energy concentrate feed compared to when feeding the same horses a low energy alfalfa chaff diet. The reason for this is unclear but may relate to the fact that D(+)-xylose shares a common carrier with glucose across the intestinal mucosa, although glucose has a higher affinity for this and will, therefore, be absorbed preferentially (Caspary 1972). Feeding a high energy diet will release a high concentration of hexose sugars that will probably be absorbed in preference to D(+)-xylose, resulting in a flatter absorption curve. Additionally, in the study by Jacobs et al. (1982), the D(+)-xylose absorption tests had been performed one week after changing the diet from a high to a low energy diet and vice versa. It was proposed that changes in the intestinal bacterial flora may also have influenced the D(+)-xylose absorption test (Jacobs et al. 1982). In the present study, the horses were fed a low energy diet prior to, and throughout, the experiment to help eliminate any detrimental dietary effects on D(+)-xylose absorption.
Food deprivation has been reported to alter the D(+)-xylose absorption curve in healthy mares (Freeman et al. 1989; Ferrante et al. 1993). Although there was a discrepancy in the total amount of D(+)-xylose absorbed between the 2 studies, both concluded that prolonged starvation (>36 h) was likely to affect the peak concentrations of the D(+)-xylose absorption curve negatively. It was suggested that factors such as delayed gastric emptying and decreased rate of absorption of D(+)-xylose across the small intestinal mucosa may be involved (Freeman et al. 1989; Ferrante et al. 1993). In the present study, the horses were starved for 16 h prior to administration and hence this is unlikely to have influenced the D(+)-xylose absorption curve negatively.
In the present study, there was a reasonably wide age spread in the group of horses and a bias towards females. Merritt et al. (1986) demonstrated that D(+)-xylose absorption test can be used even in foals although in animals aged <3 months the results need to be compared to age matched animals. This difference gradually disappears with age, which is probably the result of increased bacterial colonisation of the stomach and intestinal tract and/or changes in their substrate requirements (Merritt et al. 1986). To our knowledge, there are no documented reports on changes in rate of absorption in the older horse. Additionally, as all 6 horses in the current study were otherwise clinically healthy, it seems unlikely that their age should influence the test results. Similarly, although there was a bias towards females in the current study, there is no reported link indicating altered D(+)-xylose absorption rates based on gender and so it is reasonable to assume that this did not influence the results.
The dose rate and concentration of the D(+)-xylose solution was based on the results of previous investigators (Roberts 1974; Bolton et al. 1976; Roberts and Norman 1979). Roberts and Norman (1979) concluded that 0.5 g/kg bwt D(+)-xylose given as a 10% solution should discriminate normal from abnormal absorption even using different assay methods when they re-evaluated the test also with regards to different grade, dose and concentration of D(+)-xylose.
In conclusion, this study failed to demonstrate a significant effect of prior sedation with xylazine on the D(+)-xylose absorption test in the normal horse. Further studies are needed to evaluate the effect on sedation on D(+)-xylose absorption in horses with suspected small intestinal malabsorption.
Conflicts of interest
C. Fintl has not received any research grants in the last two years. CF Ihler has received two grants in the last two years funded by the Norwegian School of Veterinary Science and Norwegian Research Council respectively. Both grants are each of approximately £10000 and both involve work on anthelmintic resistance and are collaborative research projects with the Norwegian Veterinary Institute.
Source of funding
The Norwegian School of Veterinary Science funded the study.
We wish to thank Marianne Bakken Tollefsen, Erik Bjørsvik and Jens Rønnebeck for technical assistance. Åse Risberg was instrumental to the original study design.