The pH/Pco₂Method for Continuous Determination of Human Gastric Acid and Bicarbonate Secretion A Validation Study

Background: The present paper describes and evaluates a methodologic approach for registration of the fast, interdigestive, motility-related changes in gastric acid and bicarbonate outputs seen in man. Methods: The technique is based on continuous gastric luminal perfusion and measurements of pH and PCO2 in gastric effluent and concomitant intragastric/duodenal manometry. Fourteen healthy volunteers participated. Results: Direct acid secretory estimations from pH recordings, corrected for hydrogen ion activity, correlated closely with values obtained by conventional titration. After intragastric infusion of bicarbonate, 96 +/- 5% of the newly measured steady-state value was registered virtually instantaneously provided that corrections for the PCO2 electrode time constant and the perfusion/aspiration time were made. In the neutral pH range (pH 5-7), practically full quantitative recovery of intragastrically infused bicarbonate was obtained. In the acid pH interval (pH 2-5) the recovery was significantly lower (53 +/- 6%; p < 0.01). With an aspirate without air admixture and during high perfusion rates (31 and 46 ml/min), full recovery of bicarbonate was obtained also at an acid pH, whereas a reduced perfusion rate (16 ml/min) significantly (p < 0.05) reduced the recovery rate. Conclusions: With the pH/PCO2 technique both acid and bicarbonate assessments have a close to on-line time resolution. Acid output is measured accurately, but the method potentially underestimates actual bicarbonate levels in the acid pH range, a combined effect of diffusion of CO2 into air bubbles in the aspirate and into the gastric mucosa from the lumen. A high gastric perfusion rate minimizes this source of error. The pH/PCO2 technique is well suited for studies of the interaction between secretion and motility in the human stomach.