CHARACTERIZATION OF THE HEMODYNAMIC AND METABOLIC ALTERATIONS IN THE LARGE COLON OF HORSES DURING LOW-FLOW ISCHEMIA AND REPERFUSION

Effects of low-flow ischemia and reperfusion of the large colon on sys temic and colonic hemodynamic and metabolic variables were determined in horses. Twenty-four adult horses were randomly allocated to 3 group s: sham-operated (n = 6), 6 hours of ischemia (n = 9), and 3 hours of ischemia and 3 hours of reperfusion (n = 9). Low-flow ischemia was ind uced in groups 2 and 3 by reducing colonic arterial blood flow to 20% of baseline. Heart rate, arterial blood pressures, cardiac index, pulm onary artery pressure, right atrial pressure, and colonic blood flow w ere monitored. Arterial, mixed-venous, and colonic venous blood gas an d oximetry analyses; PCV; and blood lactate and pyruvate and plasma to tal protein concentrations were measured. Data were recorded, and bloo d samples were collected at baseline and at 30-minute intervals for 6 hours; additionally, data were collected at 185, 190, and 195 minutes (corresponding to 5, 10, and 15 minutes of reperfusion in group-3 hors es). There were no differences among groups at baseline or across time for any systemic hemodynamic or metabolic variable. Colonic blood flo w did not change across time in group-1 horses. Colonic blood flow sig nificantly (P < 0.05) decreased to 20% of baseline at induction of isc hemia in horses of groups 2 and 3 and remained significantly decreased throughout the ischemic period in horses of groups 2 (6 hours) and 3 (3 hours). Colonic blood flow significantly (P < 0.05) increased above baseline by 5 minutes of reperfusion in group-3 horses. Colonic oxyge n,delivery and oxygen consumption, and colonic venous pH, Po-2 percent age saturation of hemoglobin, and oxygen content were significantly (P < 0.05) decreased within 30 minutes after induction of ischemia in ho rses of groups 2 and 3; colonic venous Pco(2), colonic oxygen extracti on ratio, and lactate and pyruvate concentrations were significantly ( P < 0.05) increased by 30 minutes of ischemia. These alterations conti nued throughout ischemia, but within 5 minutes of reperfusion in group -3 horses, these variables either returned to baseline (pH, Pco(2), la ctate, pyruvate), significantly (P < 0.05) increased above baseline Po -2, oxygen content, % saturation of hemoglobin), or significantly (P < 0.05) decreased below baseline (colonic oxygen extraction ratio). Col onic oxygen consumption remained decreased during reperfusion in group -3 horses. Colonic mucosal ischemia-reperfusion injury observed in thi s model of ischemia was associated with local colonic hemodynamic and metabolic alterations in the presence of systemic hemodynamic and meta bolic stability. Reactive hyperemia was observed at restoration of col onic blood flow in group-3 horses and persisted during reperfusion. Co lonic venous metabolic alterations were corrected at reperfusion, indi cating adaptation of the colon to the return of blood now and oxygen d elivery with resultant decrease in anaerobic, metabolism. The early al terations in these variables may simply represent a washout of metabol ic by-products.