Endotoxemia causes ileal mucosal acidosis in the absence of mucosal hypoxia in a normodynamic porcine model of septic shock

Objective: To evaluate the hypothesis that splanchnic ischemia and mucosal hypoxia are responsible for lipopolysaccharide-induced intramucosal acidosis in pigs. Design: Prospective, randomized, unblinded study. Setting: Surgical research laboratory at a large, university-affiliated medical center. Subjects: Anesthetized, mechanically ventilated swine. Interventions: Pigs were infused with lactated Ringer's solution (12 mL/kg/hr) and, starting at 30 mins, 25-mL boluses of dextran-70 (maximum 15 mL/kg/hr) to maintain cardiac output at 90% to 110% of the baseline value for each pig. Ileal mucosal hydrogen ion concentration was measured tonometrically. A segment of distal ileum was exteriorized, opened, and placed on a platform to permit measurement of mucosal PO₂, using an array of Clark-type microelectrodes and a computerized data acquisition and analysis system. Mucosal perfusion was measured using laser-Doppler flowmetry. The control group (n = 4) received no further interventions. Pigs in the lipopolysaccharide group (n = 6) were infused with 150 μg/kg of Escherichia coli lipopolysaccharide over 60 mins. To assess the effect of mucosal acidosis on mucosal PO₂ in nonendotoxemic animals, intramucosal hydrogen ion concentration, mucosal PO₂, and mucosal perfusion were measured in pigs rendered hypercarbic through deliberate hypoventilation (hypercarbia group; n = 4). Measurements and Main Results: Infusion of lipopolysaccharide resulted in a significant increase in intramucosal hydrogen ion concentration. However, in the lipopolysaccharide group, mucosal perfusion did not change significantly and mucosal PO₂ increased significantly. In the hypercarbia group, hypercarbia was associated with significant increases in both intramucosal hydrogen ion concentration and mucosal PO₂. Conclusions: Mucosal hypoxia is not responsible for lipopolysaccharide-induced mucosal acidosis in this normodynamic pig model of septic shock. A rightward shift of the oxyhemoglobin dissociation curve (the Bohr effect) can explain the increase in mucosal oxygenation observed in endotoxemic pigs.