The use of gastric tonometry for the measurement of splanchnic perfusion as a surrogate of tissue oxygen delivery is a well-established technique, but one that has not enjoyed widespread clinical usage. Automated air tonometry, together with the measurement of gastric to arterial or end-tidal alveolar differences in carbon dioxide tension, is starting to replace the use of saline tonometry and the measurement of intramucosal pH. Although this technique is in the early stages of validation, it is thought that it will be a more clinically useful tool.