The CO2 study: Carbon dioxide insufflation and brain protection during open-heart surgery

INTERVENTION: Participants will be randomised in a 1:1 ratio (stratified by type or procedure and centre) to receive either carbon dioxide insufflation (IMP) or medical air insufflation (placebo). Insufflation with the allocated intervention will be delivered at a flow rate of 5L/min from just before cannulation for cardiopulmonary bypass until 10 minutes after cardiopulmonary bypass. Participants will be followed up until they are discharged and again at three months. CONDITION: Brain injury following open‐heart valve surgery ; Surgery PRIMARY OUTCOME: Acute ischemic brain injury within 10 days post‐surgery based on new brain lesions identified with DW MRI or clinical evidence of permanent brain injury according to the updated definition of stroke for the 21st century SECONDARY OUTCOME: ; 1. Number and volume of DWI brain lesions measured using the DW MRI conducted for each participant between 2‐10 days post‐surgery; 2. Objective quantification of the impairment caused by new ischemic brain injury assessed using the National Institutes of Health Stroke Scale at pre‐surgery, day 3 and three months post‐surgery; 3. Delirium assessed using the 3‐minute diagnostic interview for Confusion Assessment Method at pre‐surgery, day 3 and three months post‐surgery; 4. Functional status assessed using the Barthel Index score at pre‐surgery and three months post‐surgery; 5. Neurocognitive function assessed at pre‐surgery and three months post‐surgery using the following tests:; 5.1. Addenbrooke’s Cognitive Examination III; 5.2. Trail making Tests A and B; 6. Quality of life assessed using the 12‐Item Short‐Form Health Survey (SF‐12) at pre‐surgery and three months post‐surgery; 7. Composite of all‐cause mortality, clinical stroke, or acute kidney injury within 30 days of surgery, by review of participants medical notes; 8. Serious adverse events (SAEs) to 3 months, by review of participants medical notes and discussion with the participant at the three month post‐surgery follow up visit; 9. Survival to 3 months by review of the participants medical notes and attendance at the three‐month post‐surgery follow up visit; INCLUSION CRITERIA: 1. Age = 50 years 2. Planned left side aortic or mitral valve surgical repair or replacement (with or without another procedure, e.g. coronary artery bypass graft) via a partial or full sternotomy using central aortic perfusion cannulae