All recruited patients will in any case have probes inserted in theatre for brain tissue glucose monitoring as a part of their standard clinical care. Probes will be allowed to stabilize following insertion as is standard clinical practice. A peripheral venous catheter will be inserted by the study team to allow introduction of the intravascular microdialysis probe. This will be attached to a regular infusion pump set at 3ml/hr and normal saline, to produce the microdialysate. The efflux of microdialysate will be connected to the biosensor, and once passed over the biosensor the subsequent dialysate will be destroyed. A risk assessment will be made on the basis of the patient's coagulation status and concurrent venous thromboembolism prophylaxis, and if necessary, low dose low molecular weight heparin will be included in the saline infusion to ameliorate risk of local thrombosis in the device. This has a local effect only and does not produce systemic anticoagulation.