This study will validate the sensitivity of a purine biosensor (SMARTChip) in detecting acute cerebral ischaemia in a group of patients undergoing elective carotid endarterectomy. The study will also investigate whether blood purine levels are indicative of future risk of stroke by comparing pre-operative blood purine levels in carotid endarterectomy patients (high risk group) and blood purine levels in a group of age and sex-matched controls.

Blood purine levels have been shown to be associated with ischaemic brain conditions. Studies have consistently shown that levels of purines in the blood rise rapidly when oxygen flow to the brain is reduced (hypoxia) and returns to the pre-hypoxic level within 30 minutes upon restoring the oxygen supply. There is therefore the potential for blood purine levels to be used to diagnose strokes more quickly. The investigators have undertaken a programme of work to develop, test and refine a purine biosensor, called SMARTChip. This is a prospective observational study to test the sensitivity of SMARTChip in detecting fluctuations in blood purine levels throughout various stages of carotid endarterectomy. Carotid endarterectomy patients have been chosen for this study because the procedure provides a controlled normoxic and hypoxic brain environment without compromising patient safety and has clear and systematic technical steps which will ensure reliable and robust data. The investigators will test the pre-, peri- and post-operative blood purine levels in 40 patients undergoing carotid endarterectomy at University Hospitals Coventry and Warwickshire NHS Trust, University Hospitals of North Midlands NHS Trust and University Hospitals Birmingham NHS Foundation Trust.

In this study the investigators will also test the hypothesis that purine levels may be predictive of future stroke risk. In a previous study carried out by the trial team patients undergoing carotid endarterectomy, a group that is by definition at increased risk of stroke, were found to have significantly higher purine levels than would be expected in a healthy population. Therefore a case-control study, including 80 patients recruited from the day surgery schedules at the three hospitals as controls and the 40 CEA patients recruited for the observational study as cases, will be undertaken alongside the observational study to assess whether purine levels differ systematically in CEA patients and controls.

Inclusion Criteria:

CEA Patients:

• Aged 18 years and over
• Scheduled for carotid endarterectomy

Controls

• Aged 18 years and over
• Scheduled for day surgery

Dynamic controls

• Aged 18 years and over
• Scheduled for open aortic aneurysm repair, critical ischaemic leg surgery (this includes: Femoral endarterectomy, Axillo-Femoral Bypass Graft, Femoral distal bypass, Fem-Fem crossover graft, Axillary bifemoral graft, Aortobifemoral graft, and Femoral popliteal bypass), endovascular aneurysm repair, Kidney transplants, or Free flap surgery.

Local sampling

• Aged 18 and over
• Scheduled for elective lower limb vascular revascularisation, elective lower limb orthopaedic surgery with a tourniquet or diagnosed with acute compartment syndrome requiring a fasciotomy

Exclusion Criteria:

CEA patients:

- Unable/unwilling to provide informed consent

Controls:

• Unable/unwilling to provide informed consent
• History of cerebral ischaemia
• History of cancer
• History of cardiovascular disease
• Unable/unwilling to provide a fasting blood sample

Dynamic controls

• Unable/unwilling to provide informed consent
• History of cerebral ischaemia
• History of cancer (except for Free flap surgery patients)

Local sampling

• Unable/unwilling to provide informed consent
• History of cerebral ischaemia
• History of cancer
• History of cardiovascular disease
• Unable/unwilling to provide a blood sample

40 male or female patients, aged 18 years or over, scheduled for carotid endarterectomy and 80 age and sex-matched control patients scheduled for day surgery. 50 dynamic controls. 25 Local sampling patients