Hypoxia and stroke

Sulter et al. reported that 63% of stroke patients had at least one episode of hypoxia, defined as a saturation of less than 96% for more than 5 minutes [3]. In a recent local study we found that the mean oxygen saturation in awake stroke patients was 94.5%, which was about 1% lower than in controls. It might be argued that a 1% difference is not clinically relevant, even if statistically significant. However, the S-Shape of the oxygen dissociation curve means that small falls in arterial oxygen pressure do not result in any changes in saturation. Thus any drop in saturation signifies that arterial oxygen pressure has already fallen considerably. This does not necessarily apply to single bedside assessments of oxygen saturation, since most oximeters have a 2% margin of error for each measurement. But even here a persistent change in saturation, which represents multiple individual assessments, is reliable since random errors even out over repeated measurements.