SALINE PCO(2) IS AN IMPORTANT SOURCE OF ERROR IN THE ASSESSMENT OF GASTRIC INTRAMUCOSAL PH

Objective: To determine whether the measurement error of saline Pco(2) , using blood gas analyzers, is relevant for the interpretation and cl inical use of the gastric intramucosal pH measurement. Design: A compa rison of four different blood gas analyzers (ABL-520, Ciba Coming, IL- 1302, and Nova), using tonometered saline as the reference. Setting: C linical laboratory of a university hospital intensive care unit. Inter ventions: None. Measurements and Main Results: The bias and the precis ion of each blood gas analyzer was determined for measurements of Pco( 2) in saline samples. These samples had been balanced to Pco(2) levels of 30, 45, and 68 torr (4, 6, and 9 kPa, respectively). In addition, the effect of buffering the saline was evaluated. The bias of the Pco( 2) measurement increased (p < .001) at the higher Pco(2) levels. The b ias ranged from -5.2 to -25.9 torr (-0.69 to 3.45 kPa) at a Pco(2) of 45 torr (6 kPa) and from -5.2 to -33.1 torr (-0.69 to -4.41 kPa) at a Pco(2) of 68 torr (9 kPa), and there was a significant (p < .001) anal yzer-Pco(2) level interaction. The type of the analyzer also influence d the bias (p < .001). The Nova analyzer underestimated the Pco(2) by 505 to 60%. The other analyzers underestimated the Pco(2) by 5% to 19% . The use of the buffer reduced the bias of all analyzers (p < .001). Based on the precision of the saline Pco(2) measurement, a difference in gastric intramucosal pH of 0.06 pH units can be reliably detected a t a Pco(2) of 45 torr (6 kPa) by all analyzers, with the exception of the Nova analyzer. Conclusions: Measurement of saline Pco(2) is an imp ortant source of error in the assessment of gastric intramucosal pH, a nd the error depends on both the analyzer used and the actual Pco(2) l evel. Direct comparison of pH values obtained by different analyzers i s not valid. Changes in gastric intramucosal pH of 0.06 pH units can b e detected by most analyzers in the clinically relevant Pco(2) level.