Changes in gut intramucosal pH and gut oxygen extraction ratio in a porcine model of peritonitis and hemorrhage.

Abstract

To establish the relationship between gut intramucosal pH and blood flow to the gut, gut oxygen delivery, and gut oxygen extraction ratio in a porcine model of peritonitis and hemorrhage. Prospective, controlled study. Experimental laboratory in a university teaching hospital. Thirty pigs of both sexes, weighing 15 to 22 kg. Animals were anesthetized, intubated, and mechanically ventilated. A flow probe was placed around the superior mesenteric artery for registration of blood flow. A tonometer was placed in the lumen of midileum for calculation of gut intramucosal pH. Hourly, for 5 hrs, blood samples were taken from mixed venous, mesenteric venous, and arterial blood. Five animals served as controls, ten animals had peritonitis induced by fecal instillation in the abdominal cavity, five were bled stepwise, five were bled rapidly (to a mean arterial pressure of 30 mm Hg), and five were bled rapidly and reinfused after 3 hrs. Both peritonitis and hemorrhage caused decreases in gut blood flow and intramucosal pH. In mild peritonitis, the intramucosal pH decrease preceded that of blood flow. In all experimental groups, oxygen delivery decreased over time; in both mild and severe peritonitis, this decrease was preceded by a decrease of intramucosal pH. Intramucosal pH correlated well with gut oxygen extraction ratio in peritonitis (r2 = .86). In hemorrhage, there was a correlation of r2 = .66, but in intramucosal pH of < 7.12, a further decrease was accompanied only by minor changes in extraction ratio. Since a reduction in blood flow was preceded by a decrease in intramucosal pH, low intramucosal pH in peritonitis cannot be explained by low flow alone. Gut oxygen delivery proved to be a poor indicator of gut acidosis (i.e., low intramucosal pH). In peritonitis, a decreasing intramucosal pH was associated with an increasing oxygen extraction ratio. In hemorrhage, this association had a sharp deflection point below which a further decrease in intramucosal pH occurred concomitantly with an unchanged gut oxygen extraction ratio. Increased extraction ratio was not sufficient, not even initially, to maintain aerobic metabolism (i.e., unchanged intramucosal pH).