Objective: To compare a phosphate buffered solution with normal saline as tonometric fluid in intramucosal PCO2 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: PCO2 was 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 PCO2 measurements per patient were evaluated, yielding a total of 48 measurements with each tonometric solution. Intrainstrumental comparison of the PCO2 determinations demonstrated an increase of 12.3 +/- 9.9% for ABL 2, 3.10 +/- 12.9% for Ciba Corning 288, and 101.2 +/- 31.5% for StatProfile 9 Plus with the phosphate buffered solution. The PCO2 values were decreased by the following amounts when the three instruments were compared, using the saline method: 14.2 +/- 8.2% (Ciba Corning 288 vs. ABL 2); 40.7 +/- 9.9% (StatProfile 9 Plus vs. ABL 2); and 30.9 +/- 9.35% (StatProfile 9 Plus vs. Ciba Corning 288). The difference in PCO2 determination, resulting from the different instrument designs, were significant between the three blood gas analyzers (p<.001). In addition, the variance of the intramucosal PCO2 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 PCO2 values in saline and phosphate buffered solution were r2=.85 for ABL 2, r2=.81 for Ciba Corning 288, and r2=.74 for StatProfile 9 Plus. When all 48 PCO2 values were analyzed, the interinstrumental coefficients of determination within a method for saline (and for phosphate buffered solution in parenthesis) were:r2=.83 (.92) between ABL 2 and Ciba Corning 288, r2=.72 (.92) between ABL 2 and StatProfile 9 Plus, and r2=.81 (.98) between Ciba Corning 288 and StatProfile 9 Plus.
Conclusions: A considerable instrumental bias in PCO2 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.