Spontaneous pre-existing hypoxia does not affect brain damage after global cerebral ischaemia in late-gestation fetal sheep.

There is considerable evidence that a mild, non-injurious insult can protect (precondition) against a subsequent injurious insult. Typically, protection is seen when the gap between insults is several days to a week. However, the effect of mild but persistent hypoxia is unknown. In this study we examined the hypothesis that mild pre-existing hypoxia (PaO2<17 mm Hg) would reduce neural injury in chronically instrumented late-gestation (0.85 gestation) fetal sheep exposed to 30 min of global cerebral ischaemia induced by bilateral carotid artery occlusion (normoxia: n=9 vs. pre-existing hypoxia: n=9) or normoxia plus sham ischaemia (sham controls: n=9). Histopathology was assessed after 7 days of recovery. Fetuses with pre-existing hypoxia had lower PaO2 values (16.1±0.6 vs. 26.0±1.1 mm Hg) and were lighter at post-mortem (4,033±412 vs. 5,261±238 g) compared to normoxic fetuses. Cerebral ischaemia was associated with secondary cortical oedema and seizures, reduced final EEG power, loss of sleep state cycling, and significant loss of neurons and oligodendrocytes, with no significant effect of pre-existing hypoxia. Pre-existing hypoxia was associated with a significantly attenuated rise in mean arterial pressure between 18 and 36 h and slower resolution of cortical oedema between 96 and 150 h after ischaemia. These data suggest that chronic hypoxia is not associated with a significant preconditioning effect.