Gastric tonometry: Precision and reliability are improved by a phosphate buffered solution

Objective: To compare a phosphate buffered solution with normal saline as tonometric fluid in intramucosal Pco(2) measurement in humans. Design: Prospective, unblinded comparison. Setting: Postsurgical critical care unit of a university hospital. Patients: Six septic patients. Interventions: Two tonometric probes were positioned in the gastric lumen in each patient. One tube was used for conventional tonometry (saline-filled balloon), while phosphate buffered solution was instilled into the second tube. Measurements and Main Results: Pco(2) was determined with three blood gas analyzers (ABL 2 [Radiometer, Copenhagen, Denmark], Coming 288 [Ciba Coming Diagnostics GmbH, Neuss, Germany], and StatProfile 9 Plus [Nova Biomedical, Waltham, MA]). Eight parallel Pco(2) measurements per patient were evaluated, yielding a total of 48 measurements with each tonometric solution. Intrainstrumental comparison of the Pco(2) determinations demonstrated an increase of 12.3 + 9.9% for ABL 2, 31.0 +/- 12.9% for Ciba Coming 288, and 101.2 +/- 31.5% for StatProfile 9 Plus with the phosphate buffered solution. The Pco(2) values were decreased by the following amounts when the three instruments were compared, using the saline method: 14.2 +/- 8.2% (Ciba Coming 288 vs. ABL 2); 40.7 +/- 9.9% (StatProfile 9 Plus vs. ABL 2); and 30.9 +/- 9.35% (StatProfile 9 Plus vs. Ciba Coming 288). The difference in Pco(2) determination, resulting from the different instrument designs, were significant between the three blood gas analyzers (p <.001). In addition, the variance of the intramucosal Pco, values was significant between blood gas analyzers (p < .001) with normal saline as tonometric solution, but not with phosphate buffered solution. The coefficients of determination between Pco(2) Values in saline and phosphate buffered solution were r(2) = .85 for ABL 2, r(2) = .81 for Ciba Coming 288, and r(2) = .74 for StatProfile 9 Plus. When all 48 Pco, values were analyzed, the interinstrumental coefficients of determination within a method for saline (and for phosphate buffered solution in parentheses) were: r(2) = .83 (.92) between ABL 2 and Ciba Coming 288, r(2) = .72 (.92) between ABL 2 and StatProfile 9 Plus, and r(2) = .81 (.98) between Ciba Coming 288 and StatProfile 9 Plus. Conclusions: A considerable instrumental bias in Pco(2) analysis is observed when saline is used as tonometric fluid in gastric tonometry, thus preventing a reliable determination of intramucosal pH. The present in vivo data show that the accuracy and reliability of intramucosal pH measurement can be improved by the use of phosphate buffered solution as tonometric fluid.