ObjectiveTo compare a phosphate buffered solution with normal saline as tonometric fluid in intramucosal PCO2measurement in humans.DesignProspective, unblinded comparison.SettingPostsurgical critical care unit of a university hospital.PatientsSix septic patients.InterventionsTwo 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 ResultsPCO2was determined with three blood gas analyzers (ABL 2 [Radiometer, Copenhagen, Denmark], Corning 288 [Ciba Corning Diagnostics GmbH, Neuss, Germany], and StatProfile 9 Plus [Nova Biomedical, Waltham, MA]). Eight parallel PCO2measurements per patient were evaluated, yielding a total of 48 measurements with each tonometric solution.Intrainstrumental comparison of the PCO sub 2 determinations demonstrated an increase of 12.3 plus minus 9.9% for ABL 2, 31.0 plus minus 12.9% for Ciba Corning 288, and 101.2 plus minus 31.5% for StatProfile 9 Plus with the phosphate buffered solution. The PCO2values were decreased by the following amounts when the three instruments were compared, using the saline method: 14.2 plus minus 8.2% (Ciba Corning 288 vs. ABL 2); 40.7 plus minus 9.9% (StatProfile 9 Plus vs. ABL 2); and 30.9 plus minus 9.35% (StatProfile 9 Plus vs. Ciba Corning 288).The difference in PCO sub 2 determination, resulting from the different instrument designs, were significant between the three blood gas analyzers (p less than .001). In addition, the variance of the intramucosal PCO2values was significant between blood gas analyzers (p less than .001) with normal saline as tonometric solution, but not with phosphate buffered solution.The coefficients of determination between PCO sub 2 values in saline and phosphate buffered solution were r sup 2 equals .85 for ABL 2, r2equals .81 for Ciba Corning 288, and r2equals .74 for StatProfile 9 Plus. When all 48 PCO2values were analyzed, the interinstrumental coefficients of determination within a method for saline (and for phosphate buffered solution in parentheses) were: r sup 2 equals .83 (.92) between ABL 2 and Ciba Corning 288, r2equals .72 (.92) between ABL 2 and StatProfile 9 Plus, and r2equals .81 (.98) between Ciba Corning 288 and StatProfile 9 Plus.ConclusionsA considerable instrumental bias in PCO2analysis 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.(Crit Care Med 1996; 24:512-516)