Respiratory monitoring and other clinical applications of measurements of carbon dioxide in expiratory air

Research Group:



The variation in transit time between an ultrasonic transmitter and receiver can be a measure of molecular weight of gases inside a specific volume. This method is used for carbon dioxide measurement for demand-controlled ventilation. The measurement principle is cheaper than those that are based on spectra photometry or the absorption of infrared light. Ultrasonic devices do not require as large air volume sample as these other methods. The topic of this study was if this measurement principle could be useful for breath monitoring, preferably on patients in postoperative clinics. This was first verified by a provocation study on healthy test subjects. This study was made with a simple prototype and was performed by personal on the postoperative clinic at the central hospital in Västerås. The result was that the method works acceptable, but needs to be refined. To improve the method, the sensor has been further developed. The receiver has been replaced by a reflector and instead of measuring the transit time, the transmitters electrical impedance is measured. This has made the sensor even cheaper and simpler to use. A parallel project is working on miniatyrisation of the sensor. A literature study has been made to verify which parameters that are important to monitor for postoperative patients, which methods makes this most reliable, and how to adapt to the clinical situation. The literature study emphasize that detection of carbon dioxide in expiratory air is the most reliable method to verify respiratory rate, apnoea. Measuring of carbon dioxide in expiratory air is also the best way for monitoring that the bloodgases are in acceptable levels. The literature study also gives an idea about how to collect the expiratory air. The best suggestion so far is a divided canula giving oxygen in one of the nostrils and measure carbon dioxide in the other one. This solution is limited caused to the facts that air from the mouth cannot be measured and that the position of the canula can disturb the patient or worse affect the breath.

Ylva Bäcklund, Professor

Room: U3-129
Phone: +46-21-101302