Influence of Temperature on Transcutaneous Blood Gas Diffusion: CAPNOS Project

From a clinical point of view, the arterial carbon dioxide (CO2) pressure - paCO2 - is a particularly interesting physiological data because it gives information on the proper functioning of the cardiorespiratory system. However, the current "gold standard" measurement of this paCO2 requires an arterial puncture, an operation that requires qualified personnel, a rapid analysis of the blood samples collected, and entails risks and discomfort for the patient. In fact, the transcutaneous partial pressure of CO2 - tcpCO2 - is often used as an indirect measure for paCO2, due to its good correlation with the latter. However, current tcpCO2 monitors are bulky, expensive (€10-20k), and possess significant drift resulting in the need to recalibrate the measurement electrode every 4-8 hours. Moreover, these monitors heat the skin to temperatures between 41 and 44°C which can lead to burns, especially when used on infants. Indeed, an alternative to current tcpCO2 monitors seems highly desirable. In particular, in the face of the rise of wearable electronics, a solution approaching wrist-worn pulse oximeters but for CO2 measurement would be an undeniable asset that cannot be achieved without an overhaul of the tcpCO2 measurement technology. It is in this global context that the present research is set; to develop a portable tcpCO2 sensor, two main avenues are to be explored. On the one hand, it is necessary to know the modalities of CO2 diffusion through the subcutaneous tissues and the skin towards an external medium (ambient air or sensor). On the other hand, it is necessary to develop a reliable technique to measure CO2 by means of a sensor placed against the skin and this with a minimal drift. The present research focuses exclusively on the first of these two tracks, i.e. on the phenomenon of CO2 diffusion through the skin. Indeed, the literature on this subject is old and incomplete. In particular, the diffusion rate of CO2 through the skin as a function of skin temperature is not known. However, this variation is of crucial interest for the dimensioning of a tcpCO2 sensor in terms of autonomy. Indeed, the CO2 diffusion rate through the skin has a direct influence on the response time of such a sensor. The present research is therefore purely exploratory, with the objective of acquiring new knowledge in physiology. It aims to fill the gaps in the literature on the variations of transcutaneous CO2 diffusion rate as a function of temperature, with the long-term objective of developing a new type of tcpCO2 sensor circumventing the constraints of current monitors. The aim is not to develop a new type of tcpCO2 sensor, but to characterize the diffusion rate of CO2 through the skin using a system developed specifically for this study. The measurement system used is an experimental device not intended to be marketed as a medical device. Indeed, it measures a CO2 flow rate and not a partial pressure - partial pressure which is, as a reminder, the quantity of clinical interest.