Transcutaneous scalp P_O2 measurements in the fetal sheep: The influence of external pressure on the electrode or on the skin

Measurements of continuous transcutaneous P_O2 (tcP_O2) have been performed on the scalps of six fetuses from ewes not in labor, while they were under epidural anesthesia. The effects of artifacts such as pressure upon the electrode and pressure on a circle around the electrode (tonsure effect) have been studied. It appears that tcP_O2 measurements are markedly influenced by pressure as well as tonsure. As a result, an artificially low tcP_O2 reading caused by pressure or tonsure cannot be distinguished from a low value caused by a real decreased intravascular P_O2. The "relative heating power" necessary to keep the temperature of the electrode at 44° C is influenced by a number of variables and hence does not solely reflect capillary flow beneath the electrode. Fetal tcP_O2 monitoring is unlikely to function as a suitable method of antepartum monitoring throughout delivery in the same manner as the cardiotocograph. The value of dynamic tests at the start of labor such as testing the response to administration of medicaments or gas mixtures needs to be evaluated.