Measurements of the partial pressure of oxygen in the carotid body of fetal sheep and newborn lambs.

The partial pressure of oxygen in chemoreceptors tissue of the carotid body was measured in parallel with arterial oxygen tension in eleven sheep fetuses exteriorized from 9 ewes anaesthetized with pentobarbitone, in 6 of these fetuses in the transitional period following occlusion of the umbilical cord and the start of artificial pulmonary ventilation and in 6 naturally born lambs, 10-36 h old. In the fetal carotid body, O2 gradients were very small and, in the majority of tests, the arterial-to-tissue O2 difference was small or negligible. During the transitional period, this difference increased as arterial PO2 rose and in the newborn lamb, this difference was as large as that reported in the adult carotid body. These results indicate that in the fetal carotid body, the levels of blood flow and metabolism are likely to be homogeneous and that the ratio of O2 supply and O2 consumption is high, that in the transitional period and in the newborn, either O2 supply falls or O2 consumption increases or both, and that these changes are both O2-dependent and persist although arterial PO2 in the newborn is reduced to or close to fetal levels. The possible mechanisms are discussed.