Dexmedetomidine preconditioning reduces ischemia-reperfusion injury in equine model of large colon volvulus

Background: Large colon volvulus is a common cause of colic in horses with high morbidity and mortality when not promptly resolved surgically. More treatment options are needed to improve the outcome of these cases by ameliorating the damage caused by ischemia and reperfusion injury to the large colon.

Objective: To determine the effect of preconditioning with dexmedetomidine prior to induction of ischemia-reperfusion (IR) injury in a large colon volvulus model in the horse.

Study Design: Randomized prospective experimental blinded study.

Methods: Horses received either a dexmedetomidine (DEX) or saline (CON) constant rate infusion (CRI) immediately following induction of anesthesia. Venous, arterial, and transmural occlusion of a section of the large colon was then performed for three hours, after which the ligatures and clamps were removed to allow for reperfusion for further three hours. Full thickness biopsies of the large colon were taken at baseline, 1 and 3 hours of ischemia, and at 1 and 3 hours of reperfusion for histological analysis.

Results: The severity of crypt epithelial loss (DEX= 2.1 (0.8-2.8), CON= 3.1(2.5-4), p = 0.03) and mucosal hemorrhage was decreased (DEX= 2.1 (1.3-3), CON= 3.5(2.5-4), p = 0.03) in group DEX compared to group CON. Crypt length remained longer (DEX= 369.5±91.7 µm, CON= 238.5±72.6 µm, p = 0.02) and interstitium to crypt (I:C) ratio remained lower (DEX= 1.4 (1-1.7), CON= 2.6 (1.8-5.9), p = 0.03) in group DEX compared to group CON during reperfusion.

Main Limitations: Clinical applicability of pharmacologic preconditioning is limited.

Methods
Horses were anesthetized and prepared for aseptic celiotomy, and a 20-cm ventral midline incision was made. The large colon was exteriorized and placed on a plastic drape.  Approximately 15-20 minutes following induction of anesthesia, a 1- cm full thickness wedge biopsy of the antimesenteric border of either the dorsal or ventral colon (randomly assigned) was obtained as a baseline sample and the biopsy site was closed. A 40-cm area of dorsal and ventral colon centered over the pelvic flexure was then subjected to ischemia using Payr intestinal clamps to create transmural compression and arterial and venous ligation.  The colon, the colonic vasculature, and the associated mesentery was then surgically divided at the pelvic flexure so that the two ends no longer communicated to avoid the possibility of biopsy acquisition during ischemia affecting the reperfused tissue. 

Each colonic segment was assigned randomly to either ischemic sampling or reperfusion sampling groups. A similar biopsy was obtained from the ischemic sampling portion of colon after one and three hours of ischemia. The colon was returned to the abdomen and the body wall temporarily closed with towel clamps between each biopsy. 

After 3 hours of ischemia the ligatures and clamps were removed from the reperfusion assigned colonic segment to allow for reperfusion. Sampling took place similarly after one and three hours of reperfusion from the portion of colon assigned to reperfusion sampling. Biopsy sites were closed in two layers. The animals were humanely euthanized under general anesthesia after the final sample was collected.

Biopsy samples were fixed in 10% formalin, prior to routine trimming in a plane parallel to the outer longitudinal muscularis, followed by histoprocessing, microtomy and staining with hematoxylin-eosin (H&E). The samples were histologically analyzed using brightfield microscopy by a board-certified pathologist who was blinded to the treatment.

To incorporate a broad range of morphologic characteristics that may be associated with colon viability following IR injury, the specimens were analyzed and scored using the quantitative and semi-quantitative scoring system.