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Evaluation of PCO2 measurements using a gastric air tonometer located in non-mixing solution

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Many studies reported that measurements of gastric PCO2 with the air tonometer method were more precise than those obtained with saline tonometer. Gastric tonometers, however, were usually validated in vitro in a solution bubbled with CO2. In this study, we evaluated the accuracy and equilibration characteristics of PCO2 in the balloon of a gastric air tonometer located in saline solution either with or without bubbling of CO2.


A gastric tonometer was immersed in a 0.9% saline solution maintained at 37ºC, in which certified calibration gases at three different CO2 concentrations (5%,9%, and 12%) were bubbled. When solution was saturated with CO2 gas, PCO2 in the balloon was measured every 10 min for 30 min by an infra-red analyzer with or without CO2 bubbling. Bias and precision were calculated from the measured and expected PCO2 values.


Under CO2 bubbling, relatively small bias (-4%~ 9%) was seen at all three CO2 concentrations. On the contrary, large negative bias (26%–39%) was observed without bubbling.


It has been demonstrated that quick equilibration of PCO2 between inside and outside of the tonometer balloon needs the mixing of sample solution. This result strongly suggests that measurements of PCO2 with the air tonometer would underestimate gastric PCO2 when it is located in the gastric juice with little mixing.

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Sato, Y., Inoue, H., Ikegami, K. et al. Evaluation of PCO2 measurements using a gastric air tonometer located in non-mixing solution. Crit Care 4 (Suppl 1), P151 (2000).

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