Transcutaneous partial pressure sensor

To reduce the invasivity, numerous supplementary methods have been evaluated to determine their usefulness in replacing some invasive methods or obtaining additional data. Some of these tested methods involve transcutaneous sensors for PO2 and PCO2 partial pressures in the tissue (tcp02 and tcpC02) and transcutaneous measurements of oxygen saturation in peripheral vessels or invasive intravasal measurements of oxygen saturation. 

Transcutaneous measurement of partial pressures is based on the gas permeability of human skin. An electrochemical sensor is placed on the skin which is heated to increase arterial blood in superficial blood vessels [1]. 

The electrochemical measurement of PO2 by use of a polarographic Clark cell offers the advantage of designing small and compact sensors which show a linear response to oxygen partial pressure. Figure 23-4 presents a sectional view of a combined transcutaneous sensor for tcp02 and tcpC02. 

The value of the partial pressure measured at the skin surface depends in a complex way on blood partial pressure, constitution of the skin, local perfusion, metabolism in the associated tissue, cardiac output, and application temperature. An increased temperature of 43 °C raises the gas permeability and expands the capillary vessels of skin which are filled with more artial blood. The local hyperemia has the disadvantage of limiting the application time at a certain site. Assuming stable circulation conditions, transcutaneously measured values correlate with arterial partial pressure by a factor of 1.2 (neonates) to 1.0 (small children) [1]. The measured value for adults proved to be very unreliable. In the case of unstable conditions or shock with a reduction of peripheral blood flow, the transcutaneous value drops very early. Inconvenience in routine use is caused by long preparation times of the sensor, the need for periodic membrane changes, the long run-in time of freshly prepared sensors, the necessity for periodic calibrations and the slow response time to changes in partial pressure. 

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