Cardiac arrest is a major health problem in New Zealand - a common and catastrophic event effecting around 4500 per annum. Of the 27% of people who experience a cardiac arrest and survive to hospital admission, only 29% survive and leave hospital. Preventing brain injury is a key priority as brain injury is one of the leading causes of death in the Intensive Care Unit following cardiac arrest. Having slightly higher levels of carbon dioxide in the bloodstream may restore normal brain blood flow after cardiac arrest and reduce brain damage, improving patient outcomes. We will study 1700 patients to determine whether slightly higher blood carbon dioxide levels improves neurological outcomes six months after patients are admitted to the ICU following a cardiac arrest.