Which blood gas caused the brain damage?

In order to analyse the effects of various physiological parameters including blood gas data on the cerebral oxygenation, we developed a model to enable us to estimate PtO2 (tissue PO2) from these physiological parameters. This model was applied successfully to a set of blood gas data of a patient who had brain damage after cardiac surgery. Contrary to the original impression, we concluded that a very low PaCO2 of 13 mmHg towards the end of CPB was the most likely cause of his damage.