Continuous measurement of scalp tissue oxygen tension and carotid arterial oxygen tension in the fetal lamb.

Scalp tissue PO2, carotid arterial PO2 fetal heart rate were continuously measured in the anaesthetized fetal lamb in utero while variations in oxygen supply were brought about. In some experiments the transcutaneously measured fetal scalp PO2 was recorded in addition. Scalp tissue PO2 was measured using specially designed miniature needle-type oxygen electrode, incorporated in an easily applicable spiral scalp electrode as commonly used for fetal heart rate monitoring. The measurements showed that fetal carotid arterial hypoxaemia is always nearly immediately followed by fetal scalp tissue hypoxia, and that the recovery of scalp tissue PO2 after a hypoxic period has a remarkably varying time course. Fetal heart rate usually decreased during hypoxia, but in some instances it did not change or even increased, demonstrating that heart rate is not always a reliable indicator of fetal hypoxia. PO2 values obtained with the transcutaneous method were higher than those with the needle electrode, because of the effect of the heating system of the transcutaneous electrode on tissue blood flow and haemoglobin oxygen affinity. It would seem that during hypoxaemia the decrease in scalp tissue PO2 is poossibly the combined result of the fall in arterial PO2 and a concomitant decrease in blood flow through the skin.