Determination of gastric bicarbonate secretion in the human without acid suppression

Gastric CO2/HCO3 was determined in absence of simultaneous inhibition of acid secretion by intra- and extragastric pCO2/pH measurements in 23 persons and calculated using the equation of Henderson-Hasselbalch. pCO2 was measured with use of a new electrode. The characteristics of the CO2 selective electrode membrane were tested in vitro. The CO2 selective membrane proved to be stable against 0.1 n HCl, gastric fluid with a pH of 1, and bile as well as mechanical irritations. 96.8 +/- 0.52% of given amounts of bicarbonate were detected in test fluids ranging in pH from 2.5-7.1. In gas and fluid, a linear relationship between given and measured bicarbonate of y = 0.97 x +1.03 with a correlation coefficient of 0.99 was observed. In vivo at pH less than 6, the intragastric pCO2 was significantly higher than that in venous blood. CO2 diffusion could be prevented by equilibration of intragastric pCO2 with venous pCO2. pCO2 was then measured in fluid as well as in reaspirated gas. Thus 94.6 +/- 8.8%, HCO3- that had been instilled intragastrically, could be detected. Without equilibration and resultant volume increase of the gas phase, bicarbonate secretion was 296.4 +/- 41.9 mumol/h under basal conditions and 520.8 +/- 252.8 mumol/h after acid stimulation. With equilibration, a bicarbonate secretion of 574.4 +/- 66.7 and 754.4 +/- 81.2 mumol/h was observed during basal and stimulated acid secretion. Our results indicate that bicarbonate measurements in absence of acid suppression require determination of CO2 in the fluid as well as in the gas phase.