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Carbon Dioxide Insufflation and Brain Protection During Open Heart Surgery. A Randomized Controlled Trial. CO-Two trial.

Chief Investigator



Funding Stream

Grant Ref


Dr Ben Gibbison 

University of Bristol

01/10/2019 - 28/02/2023 (41 months)

NIHR Efficacy and Mechanism Evaluation (EME)




Brain injury is a major complication after heart surgery. It affects about 6 in every 10 people who undergo open heart surgery. The injury can be severe, for example stroke, although this is rare. In most people the injury is milder, manifesting as problems with memory and thinking. In some patients, these problems can persist and may increase the risk of developing dementia. Surgeons believe that the brain injury results from microscopic air bubbles that enter the bloodstream when the heart is opened up during surgery. These air bubbles are carried to the brain where they can get stuck in the small blood vessels causing damage. One way that surgeons think they can reduce the number of air bubbles is by gently blowing the gas carbon dioxide into the area they are operating on. Carbon dioxide dissolves in blood much more easily than air, so it can displace air bubbles. We want to conduct a study known as a randomised controlled trial, in which we will randomly divide patients who are undergoing surgery to repair or replace one or more of their heart valves into two groups; one group will receive carbon dioxide gas blowing into the heart and the other group will receive medical air (placebo). The medical air has no effect on the amount of air entering the bloodstream. Neither surgeons nor participants will know which type of gas is being used; only the person operating the cylinder will know. Everything else about the operation will be exactly the same. Participants will then have a very sensitive brain scan 7-10 days after their surgery (magnetic resonance imaging, or MRI). This is safe and should take no more than 30 minutes. Participants will also complete questionnaires to assess brain function and quality of life before their operation and 3 months after the operation. We will also collect information about how the operation went, and any complications that the patients experience during and after the surgery, for example, strokes and kidney damage. We hope to recruit 704 patients from 8 cardiac surgery centres into the trial. The trial is expected to take about 3 and half years to complete. We also want to do a small sub-study within the trial to determine whether carbon dioxide does lead to fewer air bubbles and whether fewer air bubbles means less damage to the brain. We will ask some patients taking part (about 100) to have an extra test during their surgery. They will have an ultrasound scan of the main artery going to their head during their operation. The test is safe and will not delay the operation. The scan will allow the number of air bubbles to be counted using a computer. This information can be added to the brain scan to see if the patients with fewer bubbles have fewer areas of damage in their brains. We have cardiac surgeons, an anaesthetist, brain MRI experts, and experts in clinical trials on the team. We have also consulted with our patient advisory group (patients who have undergone heart surgery) and described our study to them. Patients were reassured that the brain scans are safe and that they happen during the hospital stay. They also did not mind performing memory tests but indicated that those before the operation should be done at pre-operative assessment rather than when they come in for surgery, to avoid extra burden on patients who are worried about a major operation.

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