Abstract
Objective: present transcutaneous carbon dioxide (CO(2))-tcpCO(2)-monitors suffer from limitations which hamper their widespread use, and call for a new tcpCO(2) measurement technique. However, the progress in this area is hindered by the lack of knowledge in transcutaneous CO(2) diffusion. To address this knowledge gap, this study focuses on investigating the influence of skin temperature on two key skin properties: CO(2) permeability and skin blood flow. Methods: a monocentric prospective exploratory study including 40 healthy adults was undertaken. Each subject experienced a 90 min visit split into five 18 min sessions at different skin temperatures-Non-Heated (NH), 35, 38, 41, and 44°C. At each temperature, custom sensors measured transcutaneous CO(2) conductivity and exhalation rate at the arm and wrist, while Laser Doppler Flowmetry (LDF) assessed skin blood flow at the arm. Results: the three studied metrics sharply increased with rising skin temperature. Mean values increased from the NH situation up to 44°C from 4.03 up to 8.88 and from 2.94 up to 8.11 m·s(-1) for skin conductivity, and from 80.4 up to 177.5 and from 58.7 up to 162.3 cm(3)·m(-2)·h(-1) for exhalation rate at the arm and wrist, respectively. Likewise, skin blood flow increased elevenfold for the same temperature increase. Of note, all metrics already augmented significantly in the 35-38°C skin temperature range, which may be reached without active heating-i.e. only using a warm clothing. Conclusion: these results are extremely encouraging for the development of next-generation tcpCO(2) sensors. Indeed, the moderate increase (× 2) in skin conductivity from NH to 44°C tends to indicate that heating the skin is not critical from a response time point of view, i.e. little to no skin heating would only result in a doubled sensor response time in the worst case, compared to a maximal heating at 44°C. Crucially, a skin temperature within the 35-38°C range already sharply increases the skin blood flow, suggesting that tcpCO(2) correlates well with the arterial paCO(2) even at such low skin temperatures. These two conclusions further strengthen the viability of non-heated tcpCO(2) sensors, thereby paving the way for the development of wearable transcutaneous capnometers.