Survival in the fetal rabbit exposed to intermittent hypoxia

To investigate the time course and mechanisms of fetal O2 deprivation, we exposed 93 near-term, pregnant rabbits and their fetuses in utero to intermittent hypoxia. We administered a gas mixture containing 3, 4, 5 or 6 per cent O2 in N2 to a rabbit for 0.5, 1, 2, or 3 minutes, alternately with a recovery period of air breathing of 0.5, 1, 2, or 3 minutes. A given cyclical pattern was continued for 2 hours and fetal survival recorded 1 hour thereafter. Fetal survival decreased significantly at lower O2 levels, longer durations of exposure, and shorter durations of recovery intervals between exposures. Chances for survival were significantly greater among fetuses located at either end of the uterine horn.