Validation of air as an equilibration medium in gastric tonometry: an in vitro evaluation of two techniques for measuring air PCO2.

This laboratory-based bench study was undertaken to evaluate the accuracy and equilibration characteristics of air and saline respectively as CO2 equilibrating media in the silicone balloon of a gastric tonometer and to compare two methods of measuring air PCO2. Two gastric tonometers were suspended in a bath containing 0.9% saline maintained at 37 degrees C. Certified calibration gases at three different CO2 concentrations were bubbled into the bath. When the bath PCO2 measurement was stable the tonometers were primed with 5 ml of air and 2.5 ml normal saline respectively and allowed to equilibrate for 30 and 90 minutes. Following equilibration, samples were aspirated and analysed in duplicate in a blood gas analyser. Bias and precision were calculated from the measured and expected PCO2 values. A consistent negative bias (21-23%) was seen with air at all three CO2 concentrations at 30 and 90 minutes with a coefficient of variation between 2.7 and 3.3%. Imprecise data were obtained with saline at different levels of CO2. A similar experimental set-up was used to compare air PCO2 measurement by a blood gas analyser and an infra-red analyser (Tonocap). Similar bias was obtained with the blood gas analyser with respect to air PCO2 measurement as in experiment 1. The infra-red analyser measurement was highly precise with negligible bias. Air appears to be a better CO2 equilibration medium during bench testing of tonometry producing a systematic negative offset and requiring a uniform correction factor of 1.25. This correction factor is independent of equilibration time and equilibrating CO2 concentration. The use of the infra-red analyser eliminates any bias in the measurement of air PCO2 and obviates the need for a correction factor.