To the Editor.
—Dr Maynard and colleagues1 used gastric tonometry to assess splanchnic oxygenation.2 They found gastric intramucosal pH (pHim), calculated from tonometer PCO2 and arterial HCO3- by the Henderson-Hasselbalch equation, to be the most reliable predictor of outcome. They further suggested that adequacy of splanchnic oxygenation may be the responsible discriminant between survivors and nonsurvivors. We agree that gastric Pco2, the directly measured variable, is a valid measure of gastric oxygenation when interpreted properly, and that it represents a major advance in intensive care unit monitoring. We would also argue that calculated pHim, viewed in isolation, is not.The physiological principle underlying tonometry is that ischemic tissue produces "metabolic acid" (lactate?) that neutralizes tissue bicarbonate. For each millimole per liter of HCO3- neutralized, tissue Pco2 increases by 1÷0.0306 (the latter number is the solubility coefficient of CO2), or 33 mm Hg;
Schlichtig R, Stremple JF. Measuring Splanchnic Oxygenation by Gastric Tonometry. JAMA. 1994;271(13):983-984. doi:10.1001/jama.1994.03510370035025