[PCO2 tonometry of the stomach]

Gastrointestinal luminal tonometry is a minimally invasive technique for measuring gastrointestinal ischaemia. Mucosal ischaemia leads to excessive production of tissue CO2 and thus to an increase of luminal PCO2. For this measurement, a nasogastric catheter is introduced with at its end a balloon permeable for CO2, this balloon is filled with air or liquid. After CO2 has diffused from the tissue into the lumen of the balloon, the PCO2 in the liquid or air is determined. Due to uncertainties about physiological background, methodology and clinical usefulness tonometry is not yet widely applied. The recent introduction of automated airtonometry, replacing the laborious and error-prone manual saline technique, makes tonometry more reliable and easier applicable in the clinical situation. Reliable measurements require inhibition of gastric acid production and measurement in a fasting condition. The lumen-blood PCO2 gradient is the most reliable parameter of gastrointestinal mucosal ischaemia. In the past intraluminal pH--calculated from the intraluminal PCO2 measured by tonometry and the bicarbonate concentration in the blood--has been the parameter most often used. Tonometric parameters are reliable indicators of morbidity and mortality in critically ill patients. The effect of 'tonometry-guided' treatment on the morbidity and mortality is still a matter of debate. Other than using tonometry as a global ('hemodynamic') monitoring device, selective monitoring of the regional perfusion of the digestive tract--such as for diagnostic purpose in suspected chronic ischaemia due to splanchnic arterial disease--is a promising new application area.