Mural blood flow distribution in the large colon of horses during low-flow ischemia and reperfusion

Six horses were subjected to 3 hours of low-flow ischemia and 3 hours of reperfusion of the large colon. After induction of anesthesia, the large colon was exteriorized through a ventral midline celiotomy. Colonic blood flow was measured continuously, using Doppler ultrasonic flow probes placed on the colonic arteries supplying the dorsal and ventral colons and was allowed to stabilize for 15 to 30 minutes after instrumentation. Low-flow ischemia was induced by reducing colonic arterial blood flow to 20% of baseline (BL) flow. Colonic mucosal, seromuscular, and full-thickness blood flow were determined on a tissue-weight basis by injecting colored microspheres proximally into the colonic artery supplying the ventral colon. Reference blood samples were obtained at a known flow rate from the colonic artery and vein at a site more distal to the site of injection. Left ventral colon biopsy specimens were harvested at BL, 3 hours of ischemia, and 15 minutes of reperfusion. Blood and tissue samples were digested and filtered to collect the microspheres, and dimethylformamide was added to release the colored dyes. Dye concentration in blood and tissue samples was measured by use of spectrophotometry, and tissue-blood flow was calculated. Data were analyzed, using two-way ANOVA for repeated measures; statistical significance was set at P < 0.05. Doppler blood flow decreased to approximately 20% of BL, whereas microsphere blood flow ranged between 13.7 and 15.5% of BL at 3 hours of ischemia. Doppler-determined blood flow increased immediately on restoration of blood flow, reached 183% of BL at 15 minutes of reperfusion, and remained at or above BL throughout 3 hours of reperfusion. This reactive hyperemia was also detected, using the colored microspheres; blood flow increased to 242 and 327% of BL at 15 minutes of reperfusion in the mucosal and seromuscular layers, respectively. Mucosal blood flow was not different from seromuscular blood flow at any time, indicating relatively equal distribution of blood flow between these 2 layers. As determined from the venous reference samples, there was no evidence of arteriovenous anastomoses.