Tissue oxygenation in low flow states and during hypoxemia

Abstract

Tissue oxygenation during ischemia and hypoxia is critical to cellular viability. Adequate tissue oxygenation reflects a balance between oxygen delivery (Do2) and oxygen demand. At critical levels of Do2, oxygen consumption (Vo2) declines due to supply dependency, indicating inadequate tissue oxygenation and impaired cellular metabolism. The mechanism by which Do2 is compromised (hypoxia vs. ischemia) determines the level of delivery at which Vo2 becomes supply dependent. Tonometry is a method used to assess the adequacy of gut oxygenation. Studies using tonometric monitoring of intramural pH have shown that tonometry detects early tissue anaerobiosis and net adenosine triphosphate (ATP) hydrolysis. A decrease in intramural pH strongly correlates with the onset of supply dependency in tissue Vo2. Inadequate intestinal oxygenation determined by tonometry has predicted clinical outcomes and complications in patients at risk. Gut tonometry may provide an early indication of inadequate tissue oxygenation. This early recognition is necessary to mitigate adverse consequences, including ATP catabolism, the production of reactive oxygen metabolites, and the activation of the inflammatory process, which can lead to progressive cellular dysfunction and cell death. (Crit Care Med 1993: 21:S44-S49)