Theoretical basis of the transcutaneous blood gas measurements

The measurements of transcutaneous PO2 relies on maximum dilation of the local vasculature in the upper dermis. In maximum hyperemia which is only possible with normal circulation, tissue PO2 mirrors the PaO2, since it could be shown that under this condition the PO2 becomes independent of smaller flow changes (flow independency), but there remains always a PO2 difference between PaO2 and PtcO2. Maximum vasodilation could only be achieved by heating up the skin surface to a temperature of about 43°C. By the application of heat, the O2 dissociation curve of blood is displaced so that the PO2 increases above the PaO2 at body temperature. This heat induced PO2 increase can be used to overcome the just mentioned remaining PO2 decrease. However, the PO2 increase by temperature is larger; thus, additionally, an O2 consuming method was applied to obtain a PtcO2 which is close to the PaO2 at body temperature. As a rule, this compensation works well with newborn infants, fairly well with normal adults, but since there are large variations of anatomical and physiological data of the skin, such a compensation can never hold for all conditions.The elements which determine the relation between PaO2 and PtcO2 are highly nonlinear, but they cancel each other in a certain degree, so that up to about 150 torr (20 kPa) a quasilinear relationship exists. This holds only for a large hyperemia (flow independency). Thus, attempts to measure PaO2 should only be made, if the state of flow independency is simultaneously measured. PtcO2 measurements without knowing the state of flow should not be made. Unfortunately, the commercially available electrodes, as far as the author knows, do not fulfill this presupposition in a proper way. Only if the measurement of the state of flow is improved in the future, this method can be safely applied and used. It is not supposed to replace the single blood gas analysis but its strength is that it allows a quantitative continuous monitoring if properly used.