Assessment of a new prototype hydrogel CO2 sensor; comparison with air tonometry

Objective

Gastrointestinal ischemia is always accompanied by an increased luminal CO₂. Currently, air tonometry is used to measure luminal CO₂. To improve the response time a new sensor was developed, enabling continuous CO₂ measurement. It consists of a pH-sensitive hydrogel which swells and shrinks in response to luminal CO₂, which is measured by the pressure sensor. We evaluated the potential clinical value of the sensor during an in vitro and in vivo study.

Methods

The response time to immediate, and stepwise change in pCO₂ was determined between 5 and 15 kPa, as well as temperature sensitivity between 25 and 40 °C at two pCO₂ 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 CO₂-peak.

Results

The in vitro response time to CO₂ 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 °C at constant pCO₂ 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 CO₂ peak.

Conclusion

The hydrogel carbon dioxide sensor enabled fast and accurate pCO₂ 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.