Cerebral oxygen vasoreactivity and cerebral tissue oxygen reactivity † †AJJ is supported by an unrestricted neurosciences intensive care research grant from Codman. LAS is supported by grants from the Margarete und Walter Lichtenstein-Stiftung (Basel, Switzerland), a Myron B. Laver Grant (Department of Anaesthesia, University of Basel, Switzerland) and the Swiss National Science Foundation, and is recipient of an Overseas Research Student Award (Committee of Vice-Chancellors and Principals of the Universities of the United Kingdom).