• Rinze W F ter Steege, Sebastiaan Herber, Wouter Olthuis, Piet Bergveld, Albert van den Berg, and Jeroen J Kolkman.
• Gastroenterology, Medical Spectrum Twente Hospital, Ariensplein 1, PO Box 50000, Enschede, 7500 KA, Netherlands. rinzetersteege@hotmail.com
• J Clin Monit Comput. 2007 Apr 1; 21 (2): 83-90.

ObjectiveGastrointestinal ischemia is always accompanied by an increased luminal CO(2). Currently, air tonometry is used to measure luminal CO(2). To improve the response time a new sensor was developed, enabling continuous CO(2) measurement. It consists of a pH-sensitive hydrogel which swells and shrinks in response to luminal CO(2), which is measured by the pressure sensor. We evaluated the potential clinical value of the sensor during an in vitro and in vivo study.MethodsThe response time to immediate, and stepwise change in pCO(2) was determined between 5 and 15 kPa, as well as temperature sensitivity between 25 and 40 degrees C at two pCO(2) levels. Three sensors were compared to air tonometry (Tonocap) in healthy volunteers using a stepwise incremental exercise test, followed by a period of hyperventilation and an artificial CO(2)-peak.ResultsThe in vitro response time to CO(2) increase and decrease was mean 5.9 and 6.6 min. The bias, precision and reproducibility were +5%, 3% and 2%, resp. Increase of 1 degrees C at constant pCO(2) decreased sensor signal by 8%. In vivo tests: The relation with the Tonocap was poor during the exercise test. The response time of the sensor was 3 min during hyperventilation and the CO(2) peak.ConclusionThe hydrogel carbon dioxide sensor enabled fast and accurate pCO(2) measurement in a controlled environment but is very temperature dependent. The current prototype hydrogel sensor is still too unstable for clinical use, and should therefore be improved.

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