Comparison of continuous transcutaneous and continuous intravascular PO2 measurement in fetal sheep

Continuously monitored carotid arterial PO2 (PaO2) and transcutaneous PO2 (tcPO2) from the scalp of fetuses from ewes not in labor have been compared under epidural anesthesia at different levels of fetal oxygenation. A reasonably good correlation existed between the two readings at normal and high PO2 values. In sharp contrast, at low PaO2 levels there was a considerable bias between the two. Since this bias was variable, the failure of tcPO2 to reflect PaO2 accurately at low PaO2 renders the use of the tcPO2 electrode questionable in the very situation for which it was advocated, namely, the prediction of fetal compromise.