Personalized Rebreathing Device for Hypercapnia Administration

Cerebrovascular reactivity (CVR) is the ability of cerebral vessels to dilate or constrict in response to vasoactive challenges. CVR has been shown to be an important biomarker for diagnosing and monitoring neurological disorders. Assessment of CVR commonly requires a hypercapnic challenge, which is the elevation of the partial arterial carbon dioxide (CO₂) level or end-tidal CO₂ level (EtCO₂). Current methods used to induce hypercapnia consist of breath holding, fractional inspired CO₂, or sequential gas delivery. Limitations of the existing systems are that they are either not portable, require manual operations, take a long time to set-up, or do not induce an accurate and precise hypercapnic stimulus. Therefore, there is a need to develop a personalized, portable, and automated gas delivery system that induces hypercapnia in a controlled manner. The main objective of this thesis is to design a controller module for a portable and personalized device for hypercapnia administration which utilizes room air and patient's own expired air as gas mixtures. Our specific aim is to characterize changes in CO₂ concentration during full and partial rebreathing and model a system that enables partial rebreathing. We modelled the system by using a valve that mixes the patient's expired CO₂, previously filled in a 5 L reservoir bag, and room air. The valve has a 20mm opening for room air which is controlled by a piston attached to a linear actuator. Previous CVR studies have reported a 5% CO₂ stimulus to induce an average increase of 10mmHg in EtCO₂, hence, our aim is to induce a 10 mmHg increase in the EtCO₂ level of the patient by 5% CO₂ concentration in the valve. A sensor measures the CO₂ concentration administered to the patient, which is used as a feedback to control the output within the desired limit (5% ± 0.2). The CO₂ concentration in the reservoir bag during rebreathing increased linearly, allowing to determine CO₂ production of the subject across time. Changes in the CO₂ concentration with distance was measured between 0mm and 20 mm and significant changes in CO₂ were observed between 10-20mm. This finding is critical as it indicates that the room air opening must be controlled from 10 to 20 mm to allow for partial rebreathing. The valve can change the CO₂ concentration percentage by a maximum of 2.8%, revealing that the valve dimensions should be redesigned to accommodate for greater changes in CO₂ concentration. These findings and measures will help to develop a look-up table for design requirements in the clinical setting, i.e., required distance vs. change in CO₂ as well as to set PID controller parameters.