September 2021

CO2 study recruitment due to start soon

Study Manager: Rachel Todd

Brain dysfunction and injury are common complications after open heart surgery. For most patients, symptoms are mild and include difficulties with thinking, attention and memory. However, sometimes it can be more serious; around 2-6% of patients experience a stroke, and there is evidence that brain injury may increase the risk of developing dementia.

It is believed that brain injury occurs when tiny air bubbles (microemboli) are released into the blood stream during surgery, which travel to the brain and block blood vessels.

Gently blowing carbon dioxide (insufflation) into the area being operated on is thought to reduce air microemboli forming, as carbon dioxide is heavier than air and so stops air from entering the wound. There is a lack of evidence to demonstrate that carbon dioxide insufflation should be standard practice, and currently use of this technique varies between hospitals.

The CO2 study will recruit 704 participants aged ≥ 50 years undergoing planned heart valve surgery. Participants will be randomised to receive either carbon dioxide or medical air (placebo) insufflation during surgery to evaluate the effectiveness and safety of carbon dioxide insufflation.

Neurocognitive and functional assessments will take place before surgery, and again at three days and three months post-surgery. Between 2-10 days after surgery, all participants will have an MRI scan to look for any evidence of brain injury.

The primary outcome of the study is acute ischemic brain injury within 10 days post-surgery, based on either new brain lesions identified by MRI scan or clinical evidence of permanent brain injury.

The first participating hospital (Bristol Heart Institute) received study training on 23rd August, and the study team hope to start recruiting participants in September/October 2021.

If you have any questions about the study or are interested in taking part as a recruiting centre, please contact us at

You can read more about the study at

Funder acknowledgement/HTA reference:  EME17/145/40

Twitter handle: @CO2study