Relating measuring signals from PO2 electrodes to tissue PO2: a theoretical study.

Abstract

Organ surface PO2 measurement by oxygen sensitive electrodes has proved to be an efficient tool for monitoring changes in tissue oxygenation status. A parameter giving more direct information is the PO2 distribution within tissue cells which, however, can only be assessed by more invasive methods. The present study addresses the problem of relating electrode measured surface PO2 to muscle cell PO2. To that end, the magnitude of tissue volumes the PO2 in which electrodes are sensitive to, has been reassessed. It turned out that the measuring signal of current membrane covered PO2 electrodes is only sensitive to the PO2 within the muscle surface and not any deeper, thus rendering the measured PO2 an area weighted average over the surface PO2 rather than a spatial average. The consequences of this finding are illustrated in an example of a maximally working muscle. Under the assumption that there are capillaries located on the muscle surface or immediately beneath it, the PO2 predicted to be measured by a surface electrode is almost four times the average muscle fiber PO2. However, PO2 values actually measured by surface PO2 electrodes are even higher calling for further explanation, in which superficial blood vessels and surface fascia may play an important role. Quantitative information on vascular morphology near muscle surfaces is needed in order to more definitely determine the importance of mechanisms discussed, for experimental results.