The effect of relative hypoxemia on the pattern of breathing movements in fetal lambs

To determine whether there is a causal relationship between transient, modest hypoxemia and fetal breathing movements (FBM), we first raised fetal descending aortic PO2 (PaO2) from 17.3 +/- 0.8 (SE) to 24.9 +/- 1.8 mm Hg by administering 50% O2 in N2 to a plastic bag over the ewe's head for 26.5 +/- 3.8 min, then changed to 21% O2 in N2. Fetal PaO2 returned to control levels over the next 5 min. In 4 of 20 experiments on 11 fetal lambs (121-142 days gestation), FBM were absent when the change in maternal inspired O2 concentration was made, and did not occur in the following 5 min. In 16 trials, FBM were present for 2.0-24.1 min prior to the change to 21% O2. In 4 of these, FBM ceased within 2.1 +/- 0.7 min of the change, i.e., before PaO2 had fallen to control levels. In the remaining 12, FBM continued for 4.7-15.9 (mean = 8.6 +/- 1.0) min after the change to 21% O2, and the amplitude of FBM increased significantly (P less than 0.01) from 5.4 +/- 0.7 to 9.5 +/- 1.4 mm Hg over the first 4 min. Thereafter amplitude declined to the end of the breathing episodes. It is concluded that transient, modest reductions in vascular PO2 during episodes of FBM are a stimulus for FBM. However, it appears that transient hypoxemia cannot stimulate FBM from apnea.