Equatorial compression of the fetal head is known to occur during labor; its effect on trancutaneous scalp Po2(Ptco2) was investigated in acute experiments in four fetal lambs. Fetal Ptco2, and oxygen tension (Pao2) and flow in a carotid artery were continuously measured. Equatorial head compression was obtained by inflating a cuff around the fetal head. A strong relationship was shown to exist between Ptco2and Pao2values in the normoxemic and hypoxemic fetus. Stepwise compression of the fetal head resulted in a Ptco2of zero at cuff pressures of 40 to 50 mmHg. Laborlike fetal head compression with an intensity of 50 to 70 mmHg and a duration of one minute resulted in a 30 to 60% fall in Ptco2. Fetal Pao2remained stable in all experiments. The present findings indicate that values of Ptco2during fetal head compression do not reflect fetal arterial oxygen tension. This questions the reliability of transcutaneous Po2monitoring as a method of intrapartum surveillance.
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