The effects of changes of maternal Pa_o2 and Pa_co2 on the fetal Pa_o2 and Pa_co2 - in vivo study

Umbilical Pa_O2 and Pa_CO2 were continuously monitored in vivo in acute fetal lamb preparations with a semipermeable membrane connected to a mass spectrometer. The response time of this system (0 to 90% of final value) was 36 sec. In seven pregnant sheep (128-135 days gestation) the maternal inspired mixture was abruptly changed and the following changes in fetal Pa_O2, and Pa_CO2 were observed: (I) 100% O₂; to room air: Pa_O2 decreased from 21.5 ± O.8 (mean ± SEM) to 14 ± 1.1 mm Hg at a rate of 1.63 ± O.33 mm Hg/min. Following return to 100% O₂ the Pa_O2 returned to 21 ± 1.1 mm Hg at a rate of 2.44 ± O.4 mm Hg/min. (2) 100% O₂ to 12% O₂/10% CO₂: after 6 min the PaO₂ fell from 19.3 ± 1.3 to 6.3 ± O.3 mm Hg at a rate of 4.65 mm Hg/min and the Pa_CO2 rose from 37 ± 8 to 70 ± 5 mm Hg. At 100% O₂ the Pa_O2 returned to 19 ± 1.0 mm Hg at a rate of 11.76 ± O.086 mm Hg, the Pa_CO2 to 39 ± 7 mm Hg. (3) 100% O; to 90% O₂/10% CO₂. The Pa_O2, and Pa_CO2 increased by 4.7 and 22 mm Hg, respectively. The changes of fetal Pa_O2 and Pa_CO2 occurred after 1 minute of changing the maternal inspired mixture except in the transition from 12% O₂/10% CO₂; to 100% 0; (34 ± 12 sec). Following the reinstitution of 100% the fetal Pa_O2 and Pa_CO2 returned to their previous values within 4 and 16 min. respectively.