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Background: In intensive care observational studies, hypercapnia after cardiac arrest (CA) is independently associated with improved neurological outcome. However, the safety and feasibility of delivering targeted therapeutic mild hypercapnia (TTMH) for such patients is untested.Methods: In a phase II safety and feasibility multi-centre, randomised controlled trial, we allocated ICU patients after CA to 24h of targeted normocapnia (TN) (PaCO2 35-45mmHg) or TTMH (PaCO2 50-55mmHg). The primary outcome was serum neuron specific enolase (NSE) and S100b protein concentrations over the first 72h assessed in the first 50 patients surviving to day three. Secondary end-points included global measure of function assessment at six months and mortality for all patients.Results: We enrolled 86 patients. Their median age was 61 years (58, 64 years) and 66 (79%) were male. Of these, 50 patients (58%) survived to day three for full biomarker assessment. NSE concentrations increased in the TTMH group (p=0.02) and TN group (p=0.005) over time, with the increase being significantly more pronounced in the TN group (p(interaction)=0.04). S100b concentrations decreased over time in the TTMH group (p<0.001) but not in the TN group (p=0.68). However, the S100b change over time did not differ between the groups (p(interaction)=0.23). At six months, 23 (59%) TTMH patients had good functional recovery compared with 18 (46%) TN patients. Hospital mortality occurred in 11 (26%) TTMH patients and 15 (37%) TN patients (p=0.31).Conclusions: In CA patients admitted to the ICU, TTMH was feasible, appeared safe and attenuated the release of NSE compared with TN. These findings justify further investigation of this novel treatment. [ABSTRACT FROM AUTHOR]