Objective: To test accuracy, reproducibility and time constants of pCO2 measurement with the tonometric technique, using different media for filling the silastic balloon (saline, phosphate buffer, citrate buffer, air) and employing different analyzer devices (ABL3, ABL330, Nova Stat 5, automated capnometry).
Design: Comparative laboratory study of different tonometric techniques, measuring test solutions with known pCO2 values due to pre-equilibration with three different pCO2 concentrations.
Setting: Clinical laboratory of a university hospital intensive care unit.
Measurements and results: The use of saline, as suggested for routine tonometry, led to negative bias values throughout, i.e. underestimation of pCO2 values, the extent of which depended on the blood gas analyzer device employed. Registration of the equilibration kinetics showed that full equilibration demanded 90 min regardless of the environmental pCO2 level. Replacing saline by buffered electrolyte solutions resulted in a significant improvement of bias, but did not change the kinetics of pCO2 equilibration. The employment of air-filled balloons, combined with automated capnometry, led to very low bias values, approaching zero, for all pCO2 levels, along with excellent precision. Time constants of equilibration were dramatically reduced, with full equilibration being achieved within 12.5 min.
Conclusions: Buffered electrolyte solutions are preferable to saline for achieving reliable pCO2 measurements in gastric tonometry. Air-filled balloons, combined with automated capnometry, present excellent accuracy and reproducibility together with short equilibration times, thus offering "on-line" monitoring of even rapid changes in environmental pCO2.