Skin temperature influence on transcutaneous carbon dioxide (CO2) conductivity and skin blood flow in healthy human subjects at the arm and wrist

Abstract

Objective: present transcutaneous carbon dioxide (CO₂)-tcpCO₂-monitors suffer from limitations which hamper their widespread use, and call for a new tcpCO₂ measurement technique. However, the progress in this area is hindered by the lack of knowledge in transcutaneous CO₂ diffusion. To address this knowledge gap, this study focuses on investigating the influence of skin temperature on two key skin properties: CO₂ 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 CO₂ 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 cm³·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 tcpCO₂ 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 tcpCO₂ correlates well with the arterial paCO₂ even at such low skin temperatures. These two conclusions further strengthen the viability of non-heated tcpCO₂ sensors, thereby paving the way for the development of wearable transcutaneous capnometers.

Keywords: blood flow; carbon dioxide; diffusion; exhalation rate; hyperaemia; ptCO2; tcpCO2; transcutaneous.