ADVANTAGE OF BUFFERED SOLUTIONS OR AUTOMATED CAPNOMETRY IN AIR-FILLEDBALLOONS FOR USE IN GASTRIC TONOMETRY

Objective: To test accuracy, reproducibility and time constants of pCO (2) measurement with the tonometric technique, using different media f or filling the silastic balloon (saline, phosphate buffer, citrate buf fer, air) and employing different analyzer devices (ABL3, ABL330, Nova Stat 5, automated capnometry). Design: Comparative laboratory study o f different tonometric techniques, measuring test solutions with known pCO(2) values due to pre-equilibration with three different pCO(2) co ncentrations. Setting: Clinical laboratory of a university hospital in tensive care unit. Measurements and results: The use of saline, as sug gested for routine tonometry, led to negative bias values throughout, i. e. underestimation of pCO(2) values, the extent of which depended o n the blood gas analyzer device employed. Registration of the equilibr ation kinetics showed that full equilibration demanded 90 min regardle ss of the environmental pCO(2) level. Replacing saline by buffered ele ctrolyte solutions resulted in a significant improvement of bias, but did not change the kinetics of pCO(2) equilibration. The employment of air-filled balloons, combined with automated capnometry, led to very low bias values, approaching zero, for all pCO(2) levels, along with e xcellent precision. Time constants of equilibration were dramatically reduced, with full equilibration being achieved within 12.5 min. Concl usions: Buffered electrolyte solutions are preferable to saline for ac hieving reliable pCO(2) measurements in gastric tonometry. Air-filled balloons, combined with automated capnometry, present excellent accura cy and reproducibility together with short equilibration times, thus o ffering ''on-line'' monitoring of even rapid changes in environmental pCO(2).