Tonometry is based on the principle that at equilibrium the partial pressure of a diffusible gas such as CO2 is the same in both the wall and lumen of a viscus.
Because the mucosal and systemic bicarbonate concentrations are equal, a modified Henderson-Hasselbalch equation may be used to calculate the intramucosal pH: pHi = 6.1 + log [HCO3−/0.03 x PCO2].
It is of special value when used in the gastric lumen because splanchnic circulation is one of the vascular beds that is subject to early blood flow redistribution in shock states.
Because the introduction of a nasogastric tube is almost routine in critically ill patients, the measurement of gastric carbon dioxide can be an easy method to monitor tissue perfusion.
by Gutierrez and colleagues, has shown that therapeutic interventions guided by gastric tonometry improved survival in critically ill patients.