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. Des ign: Prospective, unblinded comparison. Setting: Postsurgical critical care unit of a university hospital. Patients: Six septic patients. In terventions: Two tonometric probes were positioned in the gastric lume n in each patient. One tube was used for conventional tonometry (salin e-filled balloon), while phosphate buffered solution was instilled int o the second tube. Measurements and Main Results: Pco(2) was determine d with three blood gas analyzers (ABL 2 [Radiometer, Copenhagen, Denma rk], Coming 288 [Ciba Coming Diagnostics GmbH, Neuss, Germany], and St atProfile 9 Plus [Nova Biomedical, Waltham, MA]). Eight parallel Pco(2 ) measurements per patient were evaluated, yielding a total of 48 meas urements with each tonometric solution. Intrainstrumental comparison o f the Pco(2) determinations demonstrated an increase of 12.3 + 9.9% fo r ABL 2, 31.0 +/- 12.9% for Ciba Coming 288, and 101.2 +/- 31.5% for S tatProfile 9 Plus with the phosphate buffered solution. The Pco(2) val ues were decreased by the following amounts when the three instruments were compared, using the saline method: 14.2 +/- 8.2% (Ciba Coming 28 8 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 P co(2) determination, resulting from the different instrument designs, were significant between the three blood gas analyzers (p <.001). In a ddition, the variance of the intramucosal Pco, values was significant between blood gas analyzers (p < .001) with normal saline as tonometri c solution, but not with phosphate buffered solution. The coefficients of determination between Pco(2) Values in saline and phosphate buffer ed 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 a nalyzed, 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) betwe en 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 d etermination of intramucosal pH. The present in vivo data show that th e accuracy and reliability of intramucosal pH measurement can be impro ved by the use of phosphate buffered solution as tonometric fluid.