Veterinary Surgery, 17, 1, 10-14, 1988

Morphologic Effects of Experimental Distention of Equine Small Intestine

DOUGLAS ALLEN, JR., DVM, MS, DiplomateACVS, NATHANIEL A. WHITE II, DVM, MS, DiplomateACVS, and DAVID E. P/LER, DVM, PhD, DiplomateACVP

The morphologic effects of induced intraluminal hydrostatic pressures (IHPs) of 0, 9, and 18 cm H 2 0 were evaluated in 33 isolated equine jejunal segments. Fifteen segments were distended with Tyrode's solution for 1 hour and nine segments for 4 hours. Tyrode's solution was added as needed to maintain the prescribed pressures. Nine other segments were left undisturbed for 4 hours after the initial distention period. On decompression of the intestinal segments, progressive peristaltic contractions resumed in all segments. Evaluation of intestinal sections by light microscopy and transmission electron micros- copy showed edema of the villi and submucosa and separation of the epithelial cells adjacent to the basement membrane in all segments. The epithelial cell necrosis found in ischemic intestine was not seen. This study indicates that the necrosis found at the villous tips in distended sections of small intestine remote from the site of obstruction cannot be reproduced by IHP increases of 4 hours duration.

HE SMALL INTESTINE of the horse is involved frequently T in lesions causing colic. These lesions include simple obstruction, strangulation obstruction, nonstrangulating in- farction, and enteritis.'.2 A feature common to all is disten- tion of the intestine proximal to the primary lesion. Disten- tion occurs because of increased intraluminal content arising from saliva, swallowed air, gas from bacterial fermentation, ingested fluids, and secretions from the stomach, pancreas, and intestine. 3-7 The accumulation of intraluminal contents is accompanied by an increase in the intraluminal hydro- static pressure (IHP).

Marked increases in IHP reduce blood flow to the intes- tine, especially to the mucosa of the distended ~ e g m e n t . ~ Low blood flow to the small intestinal segment leaves the villous tip unperfused and leads to epithelial necrosis.' Similary , total ischemia of small intestinal segments in horses causes mucosal epithelial necrosis involving the villi.'

In clinical cases of equine colic, mucosal damage proxi- mal to the obstructed site is more severe in strangulation obstructions than in simple obstructions. '" Recent clinical data also indicate that intestinal mucosal lesions are more

severe, peritoneal proteins are higher, and prognosis for survival is poorer with significant increases of IHP." In that clinical study, the mean IHP was 6.3 cm H 2 0 for horses that survived and 15 cm H 2 0 for those that died.

The current study was initiated to determine the effect of experimentally increased IHP on small intestinal mucosal morphology.

Materials and Methods

Thirty-three jejunal segments from 1 1 clinically healthy adult horses were studied. The horses were anesthetized with guaifenesin* and thiamylal sodium? and positioned in dorsal recumbency. Anesthesia was maintained with halo- thane in oxygen in a semiclosed system with an intermittent- positive-pressure ventilator. Acetated Ringer's solution was administered (10 ml/kg/hour intravenously).

* Guaifenesin, R . W . Gruff & Co., Inc., Old Greenwich, CT 06870. t Bio-tal, Bio-Ceutic Division, Boehringer lngleheim Animal Health,

Inc., St. Joseph. MO 64502

From the Departments of Large Animal Medicine (Allen, White), and Veterinary Pathology (Tyler), College of Veterinary Medicine, University

Supported in part by a research grant from the American College of Veterinary Surgeons. Reprint requests: Douglas Allen, Jr. , DVM, MS, The University of Georgia, College of Veterinary Medicine, Department of Large Animal

of Georgia, Athens, Georgia.

Medicine, Athens, GA 30602.

i n

ALLEN, WHITE, AND TYLER 11

A 40 cm ventral midline incision was extended cranially from the umbilicus to expose the abdominal viscera. The intestine was followed proximally from the ileum until four jejunal arcades were passed. Each of the next three vascular arcades supplied one of the intestinal segments used in the study. Three jejunal segments approximately 10 cm long were isolated from adjacent intestine by transection. The ends of each isolated segment were secured with umbilical tape over a polyvinyl chloride (PVC) cap (Fig. 1). The transected ends of the jejunum were closed with an inverting continuous suture pattern to preclude abdominal soilage. Each PVC cap was secured to a metal rod that extended 10 cm down from a Plexiglas table device. The caps provided influx and efflux ports accommodating the male end of an intravenous infusion set. Once the isolated intestinal seg- ments were secured to the table device and the tubes were in place (Fig. 2 ) , the segments were replaced into the abdomi- nal cavity with the horizontal part of the device resting on the horse's abdomen (Fig. 3). Tyrode's solution, adjusted to pH 7.4 and heated by a fluid infusion warmer to 37"C, was used to distend the segments. Before distention, the lumen of each bowel segment was lavaged with 100 ml of Tyrode's solution. Water manometers were attached to fixed three- way stopcocks 10 cm lower than the top of the horizontal part of the Plexiglas device to ensure that the pressures observed in the manometers accurately reflected intra- luminal pressure. A bubble level was placed on top of the Plexiglas device to ensure accuracy in measurement of Fig, I . Attachment of a small intestinal segment to PVC caps.

Fig. 2. Plexiglas frame used to support intesti- nal segments in situ. Intestinal pressure was monitored with water manometers.

12 EQUINE SMALL INTESTINE EXPERIMENTAL DISTENTION EFFECTS

Manorneter-

Bubble level

Fig. 3. Intestinal segments attached to support frame and placed into abdominal cavity.

the fluid column in the manometers. The three bowel seg- ments were infused with Tyrode's solution to the prescribed pressures, with the distal segment serving as 0 cm H20 control. The middle segment was distended to an IHP of 9 cm H20 , and the proximal segment was distended to an IHP of 18 cm H20.

The intestinal segments from five horses were distended for 1 hour and the segments from three horses for 4 hours. Tyrode's solution was added to maintain the prescribed pressures because the IHP dropped during the initial course of the distention. The segments from three horses were left undisturbed for 4 hours after the initial distention.

At the end of the period of distention, the intraluminal fluid was extracted from each intestinal segment and intesti- nal motor activity was observed. A transmural specimen 1 x 1 cm was taken from each intestinal segment and imme- diately placed in Trump's fixative. One part of each speci- men was embedded in paraffin, sectioned at 5 p m , and stained with hematoxylin and eosin. A second portion from each specimen was processed for evaluation of mucosal cell plasma membranes, intercellular attachments, and intracell- ular organelles by transmission electron microscopy. Each specimen evaluated by light microscopy was graded for lesions as described el~ewhere. ' ,~ The horses were euthana- tized without recovery from anesthesia.

Results

After the intraluminal fluid was removed, each intestinal segment appeared morphologically normal and spontaneous segmental contractions began immediately. Histologically, there was edema formation in the 9 and 18 cm H 2 0 samples, with the edema being more pronounced at the higher pres- sure. The lamina propria and central lacteals of the villi had pronounced accumulations of edema fluid (Fig. 4). Necrosis

or lifting of the epithelium from the lamina propria at the villous tips was not observed. There was no difference in the histologic appearance of similar pressure groups between the 1 and 4 hour distention times.

Evaluation of the specimens with transmission electron microscopy reinforced the observations made with light mi- croscopy. Edema fluid accumulated perivascularly and within the interstitial matrix in the 9 and 18 cm H 2 0 speci- mens. There was separation of individual epithelial cells by edema fluid, but no disruption of the epithelial basement membrane was observed and there were no degenerative changes associated with mitochondria or cell nuclei. No differences were seen between the three experimental groups. The morphologic changes found in the biopsy speci- mens were related to edema in the lamina propria and central lacteals without epithelial necrosis or related ultrastructural changes.

Discussion

The small intestine is normally an absorbing organ, with the net transport of fluid and electrolytes from the bowel lumen toward the mucosal vascular space.I2 The mecha- nism of electrolyte and fluid transport involves the active movement of electrolytes from lumen to plasma with a pas- sive diffusion of water. 1 3 , l4 With obstruction, the small intestine functionally converts from absorption to secretion and distention ensues.

The earliest ultrastructural and histologic changes ob- served in intestine made ischemic for 5 to 10 minutes in- cluded mitochondral swelling and dilation of superficial mu- cosal capillaries with extravasation of fluid leading to edema. Deterioration of superficial epithelium, basement membranes, and related lamina propria were also ob- served.10.11.15 Although interstitial edema developed from increased intraluminal pressure in the current study, the degeneration of mitochondria, basement membranes, and epithelial cells reported in ischemia did not occur after 4 hours of intestinal distention.

Previously, increases in IHP have been suggested as causing reduced blood flow leading to intestinal i ~ c h e m i a . ~ However, experimental distention of the small intestine of dogs by IHP up to 25 cm H 2 0 for I hour caused no compro- mise of intestinal microcirculation. l 6 The intestinal mucosal flow and total wall flow declined at IHP above 25 to 40 cm H20. Investigations have indicated that IHP resulting from clinical distention can be used as a prognostic aid for survivability in horses with small intestinal obstruction. In one study, IHP proximal to the obstructed site in horses that lived ranged from 4.5 to 10.0 cm H 2 0 (mean, 6.3 cm H20), whereas IHP was 8 to 21 cm H20 (mean, 15 cm H20) in those that died. l2 Intestinal lesions proximal to the obstruc- tion were more severe in the horses with higher IHP and were consistent with the lesions described with experimental intestinal ischemia.', I s

ALLEN, WHITE, AND TYLER 13

- - - - - - - - - - - - - __ __ . - - -I - ~ ~ ~ ~ _ _ _ _ -

Fig. 4. Histologic appearance of intestinal segments subjected to distention. Filling of the central lacteals with a looseness of the lamina propria are characteristic of edema. (Hematoxylin and eosin; original magnification, x 40). A. 9 cm H20 distention pressure. 6. 18 cm H20 distention pressure

Lesions in the small intestine resulting from natural cases of small intestinal obstruction ''3 I were not reproduced in the current study by increased intraluminal pressure for 4 hours. The changes in the mucosa in the current study were consistent with those found in other experimental models used to evaluate the effects of luminal distention, l 5 These changes were attributable to increased capillary filtration.

The fluid accumulation in the lamina propria in the cur- rent study can be explained as a physiologically enhanced capillary filtration in response to luminal distention. It has been demonstrated in cats that 92% of IHP from 0 to 25 mm Hg is reflected onto the mucosal lymphatics, and presum- ably onto the lamina propria. l 7 When intestinal distention occurs and tissue pressure exceeds venous pressure, capil- lary hydrostatic pressure increases, enhancing capillary fil- tration, and fluid accumulates in the lamina propria. Investi- gations using other animal models have shown that as IHP increases from 0 to 20 mm Hg, lymph flow increases con- comitantly because of increased capillary f i l t ra t i~n.~, '' As tissue pressure increases, the transmural vascular pressure decreases, causing relaxation of the arteriolar precapillary sphincter.17 This in turn causes an increase in capillary

hydrostatic pressure and capillary filtration, and edema

By eliminating factors that are found commonly in clini- cal cases of small intestinal obstruction, such as endotoxe- mia, acidemia, and hypovolemia, we were able to evaluate more accurately the effects of intraluminal pressure in- creases on isolated small intestinal segments in vivo. The information gained in this study indicates that artificial ele- vation of IHP for 4 hours or less in isolated intestinal seg- ments will not cause the histologic or ultrastructural damage to mucosal epithelium that is found in clinical intestinal obstructions or in experimental intestinal ischemia. The edema and venous compression may act in concert with other factors to contribute to the lesions found in naturally occumng cases of small intestinal obstruction.

occurs.

References 1 . Tennant B , Wheat JD, Meagher DM. Observation on the causes and

evidence of acute intestinal obstruction in the horse. Proc Am Assoc Equine Pract 1972;25 1.

2. Huskamp B . The diagnosis and treatment of acute abdominal condi- tions in the horse. The various types and frequencies as seen at the Animal Hospital in Hochmoor. Proc Equine Colic Res Symp 1982;261.

14 EQUINE SMALL INTESTINE EXPERIMENTAL DISTENTION EFFECTS

3. Watson DW, Sodeman WA. The small intestine. In: Sodeman WA, Sodeman WA Jr, eds. Pathologic physiology: mechanisms of dis- ease, Philadelphia: WB Saunders, 1974:734.

4. Chui CC, Gallavan RH. Blood flow and intestinal motility. Fed Proc 1980;41(6):2090.

5. Robertson JT. Conditions of the stomach and small intestine. Vet Clin N Am 1982;4(1):105.

6. Jones RS. Intestinal obstruction. In: Sabaston DC, ed. Textbook of surgery. Sabaston. Philadelphia: WB Saunders, 1977:1002.

7. Shields R. The absorption and secretion of fluid and electrolytes by obstructed bowel. Br J Surg 1965;52(10):774.

8. Chui CJ, McArdle AH, Brown R, et al. Intestinal mucosal lesions in low flow states. Arch Surg 1970;101:478.

9. White NA, Moore JN, Trim CM. Mucosal alterations in expenmen- tally induced small intestinal strangulation obstruction in ponies. J Am Vet Med Assoc 1980;41(2):193.

10. Meschter CL, Tyler DE, White NA, et al. Histologic findings in the gastrointestinal tract of horses with colic. Am J Vet Res 1986; 47:598.

I I . Allen D, White NA, Tyler DE. Factors for prognostic use in equine obstructive small intestinal disease. J Am Vet Med Assoc 1986; 189(7):777.

12. Grace RH. The handling of water and electrolytes by the small bowel following the relief of intestinal obstruction. Br J Surg 1971; 58( 10):760.

13. Frizell RA, Field M, Shultz SG. Sodium-coupled chloride transport by epithelial tissues. Am J Physiol 1979;236:Fl.

14. Schultz SG, Frizell RA. An overview of intestinal absorptive and secretory processes. Gastroenterology 1972;63( I): 161.

15. Whitehead R. The pathology of ischemia of the intestines. Pathol Ann 1976;ll: 1 .

16. Hanson KM. Hemodynamic effects of distention of the dog small intestine. Am J Physiol 1973;252(2):456.

17. Granger DN, Kvietys PR, Mortillaro NA, et al. Effects of luminal distention on intestinal transcapillary fluid exchange. Am J Physiol 1980;239:G5 16.

18. Swabb EA, Hynes RA, Marmone WG, et al. Intestinal filtration secre- tion due to increased intraluminal pressure in rabbits. Am J Physiol I982;242:G65.