The role of antacids in gastrotonometry: An in vitro study

Objective: To evaluate in vitro the value of antacids as an alternative to H₂ blockers to raise gastric juice pH for reliable PCO₂ tonometry in the human stomach. Design and setting: Laboratory study in a university hospital. Interventions: A bath was filled with 0.1 M hydrochloric acid at pH 1.3 and CO₂ was gassed through the acid to simulate an intragastric environment. Experiments were performed in duplicate. Both the manual saline technique and the new semi-continuous, semi-automated air technique of PCO₂ tonometry were used to evaluate the effects of adding i) aluminium oxide/magnesium hydroxide (Antagel®) and ii) sodium bicarbonate on the PCO₂. The latter technique was done to simulate intragastric CO₂ production following buffering of gastric acid by bicarbonate in the mucosa, or in the saliva or pancreatic/duodenal juice entering the stomach, an effect that can be prevented in vivo by prior administration of H₂ blockers. Endpoints: Changes in tonometrically determined PCO₂ after the addition of alkali. A secondary endpoint was the difference between the manual saline and the semicontinuous air tonometry techniques. Measurements and main results: The mean pH increased from 1.3 to 4.0 after adding 40 ml aluminium oxide/magnesium hydroxide. The subsequent addition of sodium bicarbonate, dosed to increase pH above 6.5, caused a steep rise in PCO₂ from 34 and 39 mmHg to 134 and 99 mmHg, for each experiment respectively, as measured by conventional tonometry, and from 37 and 38 mmHg to 116 and 115 mmHg, respectively, for the air technique. These changes were transient. Air tonometry detected changes in PCO₂ more rapidly than the manual technique. Conclusions: Adding antacids elevates the PCO₂ in hydrochloric acid and does not prevent CO₂ generation after the addition of sodium bicarbonate, even though the pH is raised from about 1 to 4. The clinical use of antacids instead of H₂ blockers to prevent spurious PCO₂ elevations following buffering of acid by bicarbonate, and thereby increase the reliability of gastric tonometry, should be discouraged.