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.