Fraunhofer Institute for Physical Measurement Techniques IPM
Carbon dioxide measurements through the skin more accurate than breath analysis
Continuous monitoring of respiration (oxygenation and ventilation) is a routine component of medical emergency care. This typically involves the non-invasive measurement of carbon dioxide (CO2) levels in the exhaled air. Direct conclusions can be made about the ventilation process, perfusion and metabolism based on this concentration level. Modern ventilators measure and monitor the carbon dioxide (CO2) content in exhaled air via gas detectors integrated into the face mask.
However, the respiratory function of intensive-care patients is generally monitored by sensor electrodes placed on the skin which measure transcutaneous CO2 (TcPCO2) at the skin’s surface. A heating element integrated in the sensor locally warms up the skin to increase perfusion to the tissue. This enables CO2 to diffuse first through the skin and then through a gas-permeable membrane in the gas sensor. Transcutaneous carbon dioxide measurements are more accurate since – in contrast to measuring CO2 in exhaled air – factors such as age, body temperature or pulmonary diseases do not influence TcPCO2 readings. Transcutaneous arterial blood gas tests are therefore used, for instance, in patients who suffer from the widespread chronic obstructive lung disease (COPD), but they are also useful in COVID-19 patients, whose lungs are severely compromised in many cases.
Measurements on the surface of the skin can be taken without the need for a breathing mask. This offers another advantage in particular with regard to sleeping patients and patients with respiratory disorders stemming from a disease as well as in neonatology. Continuous arterial blood gas testing must take place in order to appropriately administer oxygen as necessary.
Low-maintenance photoacoustic blood gas sensors for measuring TcPCO2
Transcutaneous sensors currently available on the market must be calibrated every 12 hours in a complex process involving a calibration gas canister. The reason for this is the intrinsic drift involved. As part of the MINSK in-house Fraunhofer project, Fraunhofer IPM is developing a highly precise miniaturized photoacoustic CO2 sensor with long calibration intervals which is intended to replace conventional electrochemical CO2 sensors.