Carbon monoxide uptake and elimination in fetal and maternal sheep

We studied the relation of carboxyhemoglobin concentration in the fetus, [HbCOf], to that of the pregnant mother, [HbCOm], and inspired CO concentrations in unanesthetized sheep. We exposed ewes with catheters chronically implanted in maternal and fetal blood vessels to varying inspired CO concentrations. At 30 parts per million (ppm) CO, [HbCOm] increased from base-line levels of about 1.1% to about 4.6% over a course of 8-10 h. [HbCOf] increased more slowly from base-line values of 1.8% to steady-state values of 7.4% by 36-48 h. At 50 ppm, steady-state [HbCOm] = 7.2% and [HbCOf] = 11.3%. At 100 ppm, steady-state [HbCOm] = 12.2%, [HbCOf] = 19.8%. The increase in [HbCOm] resembled a simple exponential process with a half-time of 1.5 h. The time for [HbCOf] to reach half its final value was 5 h. The decay curve for CO elimination showed similar reactions, with fetal washout occurring slower than in the mother. Elevated [HbCOf] significantly decreased the PO2 values in the fetal descending aorta and inferior vena cava. These decreases in PO2 values can cause hypoxia and fetal death.