Despite advances in post-resuscitation care of patients with cardiac arrest (CA), the majority of survivors who are treated after restoration of spontaneous circulation (ROSC) will have sequelae of hypoxic-ischemic brain injury ranging from mild cognitive impairment to a vegetative state. Early prognostication in comatose patients after ROSC remains challenging. Recent recommendations suggest carrying out clinical and paraclinical tests during the first 72 h after ROSC, to predict a poor neurological outcome with a specificity greater than 95% (no pupillary and corneal reflexes, bilaterally absent N20 somatosensory evoked potential wave, status myoclonus, highly malignant electroencephalography including suppressed background ± periodic discharges or burst-suppression, neuron-specific enolase (NSE) \> 60 µg/L, a diffuse and extensive anoxic injury on brain CT/MRI), but with a low sensitivity due to frequent confounding factors. The heart rate variability (HRV) is a simple and non-invasive technique for assessing the autonomic nervous system function. In patients with a recent myocardial infarction, reduced HRV is associated with an increased risk for malignant arrhythmias or death. In neurology, reduced HRV is associated with a poor outcome in severe brain injury patients and allows to predict early neurological deterioration and recurrent ischemic stroke after acute ischemic stroke. A reduced HRV could be a sensitive, specific and early indicator of diffuse anoxic brain injury after CA. This multicenter prospective cohort study assesses the added value of early HRV (within 24h of ICU admission) for neuroprognostication after cardiac arrest.