Brain oxygen tension during hyperoxia in a swine model of cerebral ischaemia

Arterial hyperoxia improves oxygen tension measured into the cerebral tissue (ptiO2). The extent of this improvement in ameliorating O2 delivery to the cerebral tissue, when cerebral blood flow (CBF) is reduced, is still unclear. The present experiment was developed to investigate the effect of arterial hyperoxia at normal or reduced CBF (baseline, CBF = 50-60%, and CBF = 20-30% of the baseline). CBF reduction was achieved in 7 pigs by saline infusion in a lateral ventricle. PtiO2 was measured by Licox equipment. Arterovenous oxygen difference (AVDO2) was calculated as the difference between arterial oxygen content and superior sagittal sinus oxygen content. Hyperoxia was induced by increasing inspired oxygen fraction to 100%. PtiO2 moved respectively from 27.95(+/- 10.15) to 45.98 (+/- 15.31), from 14.77 (+/-3.58) to 30.71 (+/- 12.2), and from 3.45 (+/-2.89) to 11.1 (+/- 12.6) mmHg at normal CBF, after the first reduction and after the second reduction. O2 supply showed only a negligible increase. AVDO2 decreased during the phases of intact and moderate CBF impairment, while it did not change during the phase of severe CBF impairment. In conclusion: an increase of ptiO2 does not necessarily correspond to an improvement of brain oxygen delivery. The small increase in oxygen delivery due to hyperoxia may cause a slight improvement in the balance between O2 delivery and consumption during mild CBF reduction, but such improvement is negligible when severe CBF reduction occurs.