The ductus arteriosus, pre- and post-ductal oxygen saturation measurements in fetal lambs.

In the fetus, the arterial oxygen saturation (SaO2) in the ascending aorta is higher than in the descending aorta. We questioned whether this difference over the ductus arteriosus (delta SaO2) would change during hypoxaemia. Therefore, six chronically instrumented fetal lambs (119-126 days of gestation) were studied, by changing the inspired oxygen (FIO2) via a tracheal tube to the ewe. The SaO2 was measured intermittently every 15 min with blood samples obtained from the ascending and descending aorta, and continuously with 2 pulse oximeters at both sides of the ductus arteriosus. delta SaO2 was at a level of 3.4-5.3% and had a tendency to decrease at preductal SaO2 levels of 10-20% and at pH levels below 7.25. The precision of the pulse oximeters, expressed as standard deviation of the differences between sample SaO2 and pulse oximeter SaO2, was around 5.0% for the individual calibration curves. This precision was not enough to show details of the course of delta SaO2 between the blood samples. Our results show that there is no change in delta SaO2 across the ductus arteriosus.