Inclusion Criteria1. Aged 16 and over
2. New episode of acquired brain injury, of any form to include haemorrhage, trauma, infarction
3. Their condition necessitates admission to and care on neurocritical care
4. Their condition necessitates intracerebral microdialysis probe insertion
5. Their condition necessitates arterial line insertion/central venous access and regular/venous blood gas sampling.
Exclusion Criteria1. Refusal or withdrawal of consultee agreement and any known patient wishes precluding inclusion.
3. Peripheral venous cannulation clinically contraindicated.
4. Patients already enrolled in three or more other research studies.