Tonometry is a relatively non-invasive technique that measures gut mucosal PCO2 by allowing the equilibration of CO2 partial pressures between a fluid-filled balloon and the gastrointestinal mucosa (Fig 1). The physiological basis of gastrointestinal tonometry as a monitor of hypoxia is the increase in tissue CO 2 production that accompanies anaerobic metabolism (Gutierrez.., ..a.n.,d.., ..ยง..r.o.w.Q.., 1995).

Fig. 1 Schematic representation of a balloon tonometer next to the mucosal surface. After some time, the partial pressures of CO 2 in the mucosa and in the fluid contained in the balloon come into equilibrium. The vulnerability of the mucosa to hypoxic insults is related to the countercurrent microcirculation of the villus.

Another important aspect governing the response of the gastrointestinal tract to hypoxia is the mucosal microvasculature pattern, which consists of arterioles running in parallel to venules. This microvascular arrangement results in a countercurrent circulation of blood, characterized by the diffusion of oxygen from the arterioles to the adjacent venules. An opposite phenomenon occurs with CO2, as this gas diffuses from the venules to the arterioles. Therefore tissue concentrations of O 2 are lowest, and those of CO2 are highest, at the tip of the countercurrent system. Countercurrent vascular systems are characteristic of the renal and splanchnic microcirculation and fare poorly during conditions of hypoperfusion or hypoxia. Thus they manifest the harmful effects of hypoxia or hypoperfusion earlier than other tissues.

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