Every year, efforts are made to resuscitate about 30,000 people when their hearts stop outside of the hospital environment ('out-of-hospital cardiac arrest'). Early damage to the brain due to 'oxygen starvation' (seemingly paradoxically) gets worse when blood flow is restored. Of the 6,350 survivors admitted to intensive care units, 46% die from brain damage, and half of those who survive suffer long-term brain damage. Apart from avoiding a high temperature, nothing has been found which can protect the brain or improve outcome.

'Ketones' are chemicals naturally produced in the body from fat during starvation. They act as an energy source, but also as regulators of metabolism, and appear to protect cells from damage when oxygen supplies are scarce, or when blood flow is restored. The investigators want to see whether a ketone drink will protect the brain after out-of-hospital cardiac arrest.

The investigators will study 10 cardiac arrest patients, and participants will be given the ketone drink via a feeding tube (which is routinely passed into the stomach in such cases). The investigators shall check that the drink is absorbed, and measure the ketone levels in the blood. The investigators will also measure important aspects of blood chemistry (including pH and blood sugar) and collect data on brain (electrical recordings called 'EEG' and 'SSEP') and heart function (ultrasound scans or 'echocardiographs') - both of which it is hoped might improve - in order to demonstrate that this is possible if it is to be included in a subsequent large trial. The study will be scrutinised by world experts in the field, who have also helped design the study.

If this pilot study is a success, the investigators will apply to a major grant body to fund an appropriately-powered randomised controlled trial to determine whether ketones improve neurological outcome and survival in these patients. Results will also allow similar studies to be planned in heart attack, stroke and traumatic brain injury.

Inclusion Criteria:

• Aged 18 years or above
• Male or female
• Comatose survivors of out-of-hospital cardiac arrest, defined as return of spontaneous circulation for at least 5 minutes with a score of <8 on the Glasgow Coma Scale
• For admission to intensive care unit for full, active treatment

Exclusion Criteria:

• Primary neurological or intracranial cause of cardiac arrest
• In-hospital cardiac arrest
• Inclusion in another trial at time of recruitment
• Over 4 hours from time of return of spontaneous circulation to enrollment