Tissue CO2 has been measured in various tissues, including subcutaneous, sublingual, small and large bowel and, most notably, the stomach. Gastric tonometry was in vogue as a research tool 15 to 20 years ago, prompted by the identification that an increasing gastric-arterial or gastric-end-tidal Pco2 gap or a falling gastric intramucosal pH (placing the values into a Henderson-Hasselbalch equation) were predictive of complications and mortality in patients admitted to intensive care or undergoing major surgery.57,58 The original technique involved inserting saline into a semipermeable gastric luminal balloon, allowing Pco2 equilibration with the gastric mucosa over 30 min, and then aspirating the saline and measuring the Pco2 level in a blood gas analyzer. Gastric feed had to be interrupted premeasurement, and gastric acid suppressants were required. This proved too cumbersome for regular use. An automated, semicontinuous air tonometric technique using infrared spectrophotometry to measure gastric CO2 levels offered faster equilibration times and ease of use. However, a multicenter study failed to confirm adequate prognostic capability,59 so the product was commercially abandoned. Despite this checkered history, utility in early identification of tissue hypoperfusion has been demonstrated in animal shock models using alternative locations, such as subcutaneous tissue and bladder.50,60 Potentially, with newer technology offering user friendliness and increased accuracy, this concept could be gainfully revitalized.