In the UK, around 21,000 people undergo open heart surgery each year to repair or replace a heart valve. Up to 12,500 of these people will experience some degree of brain injury as a result of their surgery.

The severity of the brain injury can vary. Most people have mild symptoms, such as confusion and memory problems, and many are unaware that they have had a brain injury in the first place. Unfortunately, in some people, a brain injury can be serious. Up to six out of 100 people will have a stroke after the operation.

As the average age of the surgical population continues to rise, brain injury from open heart surgery can put an increased burden on healthcare providers with longer hospital stays, slower recovery, poorer quality of life, and even chronic or chronic injuries causing progressive dementia.

Patient undergoing an MRI scan

Patient undergoing an MRI scan

Experts believe that a brain injury occurs because tiny air bubbles enter the bloodstream when the heart is opened during surgery. These tiny air bubbles travel to the brain and block small blood vessels, preventing blood from flowing to that area and causing damage.

It is believed that blowing carbon dioxide over the heart during surgery (carbon dioxide insufflation) reduces the amount of air bubbles entering the bloodstream. Carbon dioxide is heavier and more soluble than air, so it should prevent air from entering the surgical wound during surgery and quickly dissolving when it enters the bloodstream.

However, at this time there is no definitive guidance that would recommend the use of carbon dioxide insufflation. Some heart surgery centers in the UK routinely use it while others don’t use it at all.

Researchers in Bristol are currently conducting a study to see if carbon dioxide insufflation is effective in reducing brain injury. The CO2 Study is a multicentre, placebo-controlled, blinded, randomized, controlled study by the Bristol Trials Center to investigate whether carbon dioxide insufflation in patients undergoing heart valve surgery protects against brain injury by comparing it to a placebo. medical air, in addition to standard venting techniques.

CO2 study logo

CO2 study logo

This study will use a new and sensitive imaging technique to identify brain injuries known as diffusion-weighted imaging (DW MRI). In contrast to standard MRI or CT imaging, DW-MRI detects brain injuries (lesions) within minutes of the occurrence of the injury, so there is no need to scan before surgery.

Because the technique is so sensitive, it can identify very small lesions that would not be visible with conventional imaging. Many of these small lesions go away over time. Therefore, scans should be performed within a week of surgery for optimal identification.

Neurocognitive function tests can also detect the effects of brain injury following open heart surgery. However, these tests are influenced by external factors such as pain relievers and may not reflect the actual brain injury.

We hope to recruit 704 participants 50 and older who are undergoing scheduled heart valve surgery from across the UK to participate in the study.

Doctor working on computer showing MRI brain scans

Doctor working on computer showing MRI brain scans

© Gorodenkoff /

Participants will ideally undergo brain DW-MRI between 2 and 7 days after surgery. The images are reviewed by a clinical neuroradiologist at the Wellcome Center for Integrative Imaging, Oxford University, who reports the age, number, and size of the lesions present.

We will compare the number and size of lesions seen in the pictures between patients who have had their heart blown with carbon dioxide or air during surgery to see if there is any difference between the groups. Study participants will undergo neurocognitive tests before surgery and then 3 days and 3 months after surgery.

While we are opening up to recruitment after the COVID-19 pandemic, we continue to support the centers that are already participating, but we also welcome all centers interested in participating. If you have any questions about the study or would like to participate as a recruiting center, please contact

This project (17/145/40) is funded by the Efficacy and Mechanism Evaluation (EME) program, a Medical Research Council (MRC), and a partnership with the National Institute for Health Research (NIHR). The views expressed in this publication are those of the authors and not necessarily those of the MRC, NIHR, or the Department of Health and Social Affairs.


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