Monitoring gastric mucosal carbon dioxide pressure using gas tonometry: In vitro and in vivo validation studies

Background: Saline gastric tonometry of carbon dioxide has been proposed as a means to assess the adequacy of splanchnic perfusion. However, this technique has several disadvantages, including the long time interval needed for gases to reach equilibrium in saline milieu. Thus the authors evaluated a system that uses a gas-filled instead of a saline-filled gastric balloon. Methods: In vitro, we simultaneously placed two tonometry catheters in an equilibration water bath maintained at a predetermined and constant pressure of carbon dioxide (P-co2). The first catheter's balloon was filled with air and the second with saline. The performance of gas tonometry was tested by comparing the P-co2 measurements of the bath obtained via gas tonometry (Pg(co2)) to the P-co2 measurements of direct bath samples (Pbath(co2)). These results were also compared with the P-co2 measurements obtained simultaneously by saline tonometry (Ps(co2)). The response time of gas versus saline tonometry was also studied. In vivo, the performance of gas tonometry was tested comparing the measurements of gastric intramucosal P-co2 obtained by gas tonometry (Pg(co2)) at different equilibration times with those obtained by saline tonometry (Ps(co2)) using an equilibration time of 30 min. Two nasogastric tonometry catheters were placed simultaneously in seven stable patients in the intensive care unit. The first balloon was filled with air and the second with saline. Results: In vitro, there was a close correlation between Pg(co2) and Pbath(co2), for each level of Pbath(co2), and for each different gas equilibration time. For an equilibration time of 10 min at a Pbath(co2) level of approximately 40 mmHg, the bias of the gas device defined as the mean of the differences between Pbath(co2) and Pg(co2), and its precision defined as the standard deviation of the bias, were -0.3 mmHg and 0.7 mmHg, respectively. Using the same definitions, the bias and precision of saline tonometry were 11.2 mmHg and 1.4 mmHg, respectively. If the equilibration time-dependent correction factor provided by the catheter manufacturer for saline tonometry was applied, the bias and precision were -6.9 mmHg and 2.9 mmHg, respectively. In vivo, using an equilibration time of 10 min for gas and 30 min for saline tonometry, there was a close correlation between the two techniques (r(2) = 0.986). A Bland and Altman analysis revealed a bias (+/- 2 SD) of 0.1 +/- 6.8 mmHg. The correlation between the two methods was not improved if we prolonged the equilibration time of the gas tonometer. Conclusions: Gas tonometry is comparable to saline tonometry for measuring gastric intramucosal P-co2. Because gas tonometry is easier to automate, it may offer advantages over saline tonometry.