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 Pa(o₂) levels there was a considerable bias between the two. Since this bias was variable, the failure of tcPo₂ to reflect Pa(o₂) accurately at low Pa(o₂) renders the use of the tcPo₂ electrode questionable in the very situation for which it was advocated, namely, the prediction of fetal compromise.