The difference between fetal transcutaneous pO2 and arterial pO2 during labour

Sub partu measurements of the transcutaneous pO2 (tcpO2), the relative local skin perfusion, the fetal heart rate, and the amniotic fluid pressure were carried out with commercial equipment on 40 pregnant women at term. In 19 cases (47.5%), the tcpO2 could be monitored until the fetal head was delivered. The relationship between the tcpO2 and the arterialised capillary scalp blood was investigated in 20 patients on 42 micro-blood samples. Changes of the fetal heart rate pattern were analysed by means of a cardiotocogram scoring system with regard to its influence on the tcpO2 values. Two main groups of tcpO2 measurements could be differentiated. In 9 of 19 cases the tcpO2 remained stable over a prolonged period and decreased only during the final phase of birth to values between 6 and 18 mm Hg. In 10 of 19 cases the tcpO2 values began to decline to low levels in the course of labour as early as 90 min ante partum and reached values of 0-3 mm Hg at birth. In 9 of these 10 cases a tcpO2 'zero line' could be monitored in the last 2-90 min prior to birth. The tcpO2 did not correlate to either the pO2 of the arterialised fetal scalp blood or to the pO2 of the umbilical artery blood. Even with a tcpO2 of approximately 0 mm Hg, an arterial pO2 of 14.5-22.5 mm Hg in the scalp blood and of 10-20 mm Hg in the umbilical artery blood could be determined. In most of the cases a difference between the tcpO2 and the arterial pO2 developed.