Among patients admitted after an out-of-hospital cardiac arrest (OHCA) in intensive care unit (ICU), almost two thirds of patients will develop in the first hours a post-cardiac arrest (CA) shock. This post-CA shock, combines cardiac and hemodynamic failure, generally resulting in multi-organ failure and early death in up to 35% of patients. Experimental data suggest that intravenous ascorbic acid (vitamin C) may attenuate inflammation and vascular injury related to sepsis or surgery. Preclinical and clinical studies also provide safety data of high dose intravenous vitamin C (\> 200mg/kg/day) with no significant adverse event reported and favorable impact on outcome. Experimental data also suggest beneficial effect of vitamin C in post-CA management with improvement of shock and multi-organ failure with potential benefit on neuroprotection and outcome. The study is a phase II multicenter prospective controlled open-label trial randomized in two parallel groups : * Expérimental group: Standard of care care for post-CA shock + Vitamin C (Vit-C) 200mg/kg/d IV (started as early as possible, no later than 1 h after randomization + thiamin (Vit B1) 200mg every 12 h during 3 days. * Control group: Standard of care care for post CA shock according international guidelines. Patient number to be enrolled : 234, Study duration :24 months and 28 days, Inclusion duration : 24 months, Patient participation : duration : 28 days