Subcutaneous oxygen tension in the fetal scalp during labour continuous monitoring with a needle electrode.

Continuous intrapartum PO2 recordings were made in 25 fetuses, using a needle electrode that measures PO2 in the subcutaneous (sc) tissue of the scalp or the breech. This small electrode is combined with a spiral ECG electrode and can be easily applied in early labour. One hour after application of the electrode in the early first stage, a mean fetal scPO2 of 29.4 +/- 7.5 mm Hg was found. In the course of labour, scPO2 gradually fell to a mean value of 22.2 +/- 5.6 mm Hg, recorded just before the onset of bearing down contractions, and stabilised in the second stage of labour. ScPO2 values of less than 20 mm Hg were not recorded for a longer period except in one case of severe fetal distress. The relation between scPO2 just before delivery and cord arterial PO2 revealed a coefficient of correlation of 0.85. In general, scPO2 appeared higher than tcPO2 as described in the literature. This is ascribed to the different technique of measurement: the needle electrode measures in the deeper layers of the scalp, whereas the tc electrode is supplied with oxygen form the more superficial capillaries, where blood flow is more liable to be affected by mechanical factors.