Carbon dioxide gas is being pumped into the chest cavity of heart surgery patients in a large UK trial to see if it can prevent brain damage caused by trapped air bubbles. Brain injury—ranging from subtle cognitive decline to full stroke—strikes roughly half of all patients over 50 who undergo open-heart valve surgery. Surgeons already flood the chest with carbon dioxide to displace air, but no one knows for sure whether this actually protects the brain. The trial will randomise 704 patients to receive either carbon dioxide or medical air during their operation, then scan their brains for new lesions using MRI between five and ten days after surgery. If carbon dioxide insufflation proves effective, it would offer a cheap, simple, and widely available intervention to reduce the burden of brain injury in cardiac surgery—a problem that affects tens of thousands of patients each year in the UK alone. A mechanistic sub-study in 100 patients will also track gas bubbles in the brain’s blood vessels using ultrasound, helping to explain *how* the gas might work. The results could change standard practice in operating theatres worldwide.
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Background: Brain injury, in the form of stroke, delirium or cognitive dysfunction, is common after cardiac surgery. It is assumed to be triggered by the release of trapped air bubbles (microemboli) into the blood steam, which form when the heart is opened and then closed. Carbon dioxide insufflation (CDI) is widely used to displace air bubbles during surgery, but there is little evidence that it prevents brain injury. Aim: To evaluate the efficacy and safety of CDI compared with medical air insufflation in patients undergoing open heart valve surgery. Study design: Multicentre, placebo-controlled RCT with blinding of participants and all clinical staff except the perfusionist responsible delivering the study intervention, and an internal pilot to determine the feasibility of recruitment and adherence to the protocol. Population: Adults aged = 50 years undergoing left side heart valve repair or replacement surgery via a partial or full sternotomy using central aortic perfusion cannulae Exclusion criteria: contraindication to MRI or CDI Interventions: CDI: CO2 insufflation (5.0 L/min); Placebo: Medical air insufflation (5.0 L/min) Outcome measures: Primary: acute ischemic brain injury based on new brain lesions identified with standardised diffusion weighted imaging (DWI) between 5 and 10 days postoperatively or clinical evidence. Secondary: number and volume of DWI brain lesions; impairment caused by brain injury (NIH stroke scale assessed preoperatively, days 1, 3, and 7 and 3m); delirium (3-minute diagnostic interview for Confusion Assessment Method, assessed preoperatively and days 1, 3, and 7); functional status (modified Rankin scale; Barthel index; geriatric depression scale); neurocognitive function (verbal and visual memory, executive functioning, visuospatial or constructional praxis, attention, and information processing speed); quality of life (SF-12) all assessed preoperatively and at 3m, composite of all-cause mortality, stroke, or acute kidney injury within 30 days; survival to 3m; serious adverse events. Sample Size: 704 participants (352 per group) which will provide 90% power to detect a 25% relative reduction in the incidence of brain injury, assuming a 48% rate in the placebo group with 5% statistical significance (2-sided). Mechanistic sub-study: In 100 patients from the Bristol centre, we will investigate the association between gaseous cerebral microemboli detected using transcranial doppler ultrasound (TCD) and new brain lesions identified with DWI and whether CDI reduces microembolic load. Cerebral blood flow measured with TCD and tissue oxygen index of the frontal lobes measured using near-infrared spectroscopy will also be analysed. Project timetable including recruitment rate: The study will aim to recruit an average 3-4 patients/centre/month. Study duration 41m: 8m set-up; 24m recruitment (7m pilot and 17m main trial); 3m follow-up on all participants; 6m analysis and report. Team expertise: The project team includes cardiac surgeons, methodologists, anaesthetists and neuroimaging experts. The study has been presented to the Bristol Cardiovascular patient advisory group, who helped with the design. Dissemination: The research will be published in peer-reviewed journals, presented at national and international conferences and will be disseminated among members of national and international cardiothoracic societies. Our patient advisory group will advise on dissemination to relevant patient groups
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