Validation of Air as an Equilibration Medium in Gastric Tonometry: An in Vitro Evaluation of Two Techniques for Measuring Air Pco₂

Abstract

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°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.

Monitoring Tissue Oxygenation during Resuscitation of Major Burns

“…41 However, we have confirmed in an earlier bench study that the magnitude of the error involved with saline PCO 2 measurements in the ABL 620 blood gas analyzer is small. 54 Also, saline continues to be widely used, and the data generated with salinebased tonometry have been shown to predict outcome in certain groups of critically ill patients. 20,22,42,55 It could be argued that the use of higher resuscitation volumes in this study may have the potential to create more tissue edema and contribute partly to the pattern of results obtained.…”

Effect of temperature on gastric intramucosal P CO 2 measurement by saline and air tonometry

“…Our results imply that the saline method may not give an accurate Pi CO 2 value when the body temperature changes. The accuracy of tonometer measurements varied with the Pi CO 2 level itself [6]. The limitation of the present study was that we examined only one level of Pb CO 2 (40 mm Hg).…”

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“…Some of these problems may be avoided with air tonometry, in which surrounding intraluminal gastric carbon dioxide is allowed to rapidly equilibrate with the air-filled tonometer balloon PCO 2 [4,6]. Various types have been proposed, depending on the mode of recirculation of air in the catheter between measurement intervals, and one is commercially available (Tonocap; Datex, Helsinki, Finland) [1,5,6].…”

“…Various types have been proposed, depending on the mode of recirculation of air in the catheter between measurement intervals, and one is commercially available (Tonocap; Datex, Helsinki, Finland) [1,5,6]. This automated semicontinuous technique (involving an ex vivo infrared sensor) may both over- and underestimate saline PCO 2 tonometry, depending, among other reasons, on correction factors for incomplete equilibration times and the bias of the blood gas machine used for the latter [1,3,4,6,7]. …”

“…Other authors have determined gastric air or juice PCO 2 using blood gas machines, with the presumption that mucosal PCO 2 is in equilibrium with gastric air and juice PCO 2 at steady state; this thereby circumvents the need for equilibration with saline in a balloon tonometer [1,6,8]. `Balloonless' gastric air PCO 2 tonometry has been compared in dogs with saline balloon PCO 2 tonometry by Salzman et al [8], who found that these methods were equivalent under controlled circumstances after full acid secretion suppression with H 2 -blockade.…”