Reply: The Underestimated Effect Of Normobaric Hyperoxia On Cerebral Blood Flow And Its Relationship To Neuroprotection

Sir,Neurons in the ischaemic penumbra are at risk from ischaemia-induced constrained oxygen delivery. Therefore, increasing arterial oxygen content by normobaric oxygen therapy (NBO) has been thought a logical strategy, and as expected further shown to increase markedly and rapidly the very low tissue oxygen pressure that prevails in the ischaemic penumbra (Shin et al. , 2007; Baskerville et al. , 2011). However, due to controversial results in rodent stroke models, there is still no clear consensus on the benefits of NBO. By demonstrating in a recent issue of Brain that NBO prevents behavioural deficits and almost suppresses neuronal loss in a rat model of transient ischaemic attack, the work of Ejaz et al. (2016) brings renewed interest as for the benefits of NBO in brain ischaemia.One caveat when critically assessing the effects and mechanisms by which NBO, alone or in combination with tissue plasminogen activator (tPA), provides benefits on brain ischaemia is that little attention is paid to the facilitating action of NBO on cerebral blood flow (Shin et al. , 2007; Baskerville et al. , 2011; David et al. , 2012) and to the use of transient rather than permanent, and thromboembolic rather than mechanical models of ischaemic stroke. Doubtlessly, this is due to the fact that the ability of NBO to improve the very low tissue oxygen pressure that prevails in the ischaemic penumbra is generally believed to occur through passive-mediated oxygen transport (i.e. oxygen diffusion).However, contrary to this straightforward reasoning, and to hyperbaric oxygen therapy, NBO has been shown to fail reducing cell injury in acute brain slices exposed to oxygen and glucose deprivation …