Characteristics of time-dependent PCO2 tonometry in the normal human stomach

Factors that affect PCO2 measurement in balloon saline during gastrointestinal tonometry are unclear. They include carbon dioxide diffusion rate, correction factors for calculation of equilibrium PCO2 from measurements at saline dwell times that are shorter than needed for full equilibration, role of blood-gas analyser bias during ex vivo PCO2 measurements in saline, and normal values for intragastric PCO2 (PiCO2) and intramucosal pH (pHi) at equilibrium, and their differences from blood values. In a laboratory study, normal PCO2 changes in a saline-filled tonometer balloon placed in a saline bath at constant PCO2 were described by a non-linear model, with a half-time of mean 4.4 min and 95% equilibration at mean 83 min. In a study in 20 healthy volunteers, PiCO2 build up in a saline-filled tonometer balloon placed in the stomach, measured at dwell times of 10, 20, 30 and 60 min, was slightly (P < 0.05) slower than in vitro, with a half-time of mean 5.8 min and 95% equilibration at mean 110 min. Correction factors to derive equilibrium PiCO2 at short dwell times and independently from blood-gas analyser bias were calculated. The factors differed (P < 0.05) from those currently provided by the manufacturer. Normal threshold values (mean) were: equilibrium PiCO2 < or = 6.6 kPa, pHi > or = 7.33, PiCO2 to blood PCO2 difference < or = 1.1 kPa and pH difference > or = -0.06. PiCO2 did not differ from, and was directly related to, blood PCO2. These values provide a reference base for other studies and show that gastric mucosal PCO2 depends on alveolar ventilation if blood flow is adequate.