A phase III multi-centre randomised controlled trial to evaluate the ability of slightly higher than normal carbon dioxide levels, compared to normal carbon dioxide levels, to reduce brain damage in resuscitated cardiac arrest patients admitted to the intensive care unit.

INTERVENTION: Patients allocated to the targeted therapeutic mild hypercapnia protocol will be sedated as per unit gudelines and at the clinical discretion of the treating intensive care doctors to achieve moderate to deep sedation (a target Richmond Agitation Scale Score of –4). Arterial blood gases and end‐tidal carbon dioxide levels will be measured at baseline and then used to guide respiratory rate adjustments of minute ventilation to remain within the target PaCO2 range of 50‐55 mmHg for 24 hours following randomisation.. Arterial blood gases will be repeated every 4 hours for 24 hours following randomisation or if end‐tidal carbon dioxide values change >5 mmHg. Adjustments to be made to the minute ventilation in order to stay within the target PaCO2 range will be made at the clinical discretion of the treating intensive care doctors. Strategies to monitor adherence to the target PaCO2 range will be via respiratory rate monitoring that displays a continuous record of PaCo2 values and as recorded on the participant's physiological observation chart while in the intensive care unit. CONDITION: Out‐of‐hospital cardiac arrest PRIMARY OUTCOME: The proportion of patients with a favourable neurological outcome as assessed by the Glasgow Outcomes Scale Extended (GOSE) method. The GOSE uses an 8‐level scale from death (1) to upper good recovery (8) and can be administered by proxy or direct patient interview.. A GOSE outcome is deemed to be favourable if a patient scores greater or equal to 5. SECONDARY OUTCOME: Cognitive recovery as assessed using the Montreal Cognitive Assessment (MoCA‐blind) questionnaire. Functional recovery as assessed by the Cerebral Performance Category scale Functional recovery as measured by the modified Rankin scale Mortality status at 6 months Mortality status at hospital discharge Mortality status at intensive care unit (ICU) discharge Quality Adjusted Life Years (QALYs) Quality of life assessed by the EuroQol5D5L scale INCLUSION CRITERIA: Adult (greater than or equal to 18 years of age) Out‐of‐hospital cardiac arrest of a presumed cardiac or unknown cause Sustained return of spontaneous circulation ‐ defined as 20 minutes with signs of circulation without the need for chest compressions Unconscious (FOUR‐score motor response of less than 4, not able to obey veral commands after sustained return of spontaenous circulation Eligible for intensive care without restrictions or limitations Within less than 180 minutes of return of spontaneous circulation