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RESEARCH PRODUCT

Relating Measuring Signals From P O 2 Electrodes to Tissue P O 2: A Theoretical Study

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Organ surface P O 2 measurement by oxygen sensitive electrodes has proved to be an efficient tool for monitoring changes in tissue oxygenation status in a number of experimental and clinical situations. A parameter giving more direct information is the P O 2 distribution within tissue cells which, however, can only be assessed by more invasive methods. To date, a quantitative relation between P O 2 electrode measurements and P O 2 in tissue cells has not been established. Part of this problem lies in the fact that any surface electrode P O 2 measurement is not confined to tissue cells but rather represents some average over P O 2 values in a certain volume containing not only tissue cells but also blood vessels and connective tissue. Such a catchment volume of a typical P O 2 surface electrode of 15 µm diameter is thought to be a half sphere of about 25 µm diameter [7], the mean P O 2 in which corresponds to the measuring signal. The present study addresses the problem of relating surface P O 2 measurements to intracellular P O 2. To that end, the notion of electrode catchment volume is critically evaluated. Its magnitude and errors in electrode P O 2 measurement are quantified for a frequently used electrode type. The results are then used to calculate the P O 2 which will be measured by the surface electrode in a tissue exhibiting the heterogeneous P O 2 distributions typical of working muscle.