Cardiac Arrest is among the leading causes of death, with survival still well under 50% and the majority of the survivors suffering from moderate to severe neurologic deficits. The human, social and economic costs are staggering. During resuscitation, damage is mitigated if chest compressions and other medical care are optimal, allowing some blood to reach the brain and some oxygen to reach the cells. Once the heart starts beating again, which is called return of spontaneous circulation, brain perfusion is reestablished, but usually not to normal. The now damaged brain is very fragile, can be sensitive to any changes in blood pressure or metabolic abnormalities, and swelling might set in. Hypoperfusion can persist, without the clinician's knowledge. All of these events further damage the brain and diminish the odds that the patient will regain a normal life. Therefore, the hours following return to spontaneous circulation are critical to the patient's future recovery, and constitute a window of opportunity to maximize the brain ability to heal. In order to optimize resuscitative efforts and post-arrest management, clinicians must know what is actually happening with the most vital organ, the brain. The problem is that it is very difficult to do in a comatose patient. The available technologies only reveal indirect evidence of brain suffering, like the swelling on CT-scans, but not to continuously evaluate at the bedside if the brain actually receives enough blood. The FDA recently approved a device named the c-flow, made by ORNIM. This device looks at red blood cells in the brain and the speed at which they move to evaluate an index of cerebral perfusion. It does so with sensors put on the patient's forehead, which emit and detect ultrasounds and infrared light. This index can inform the clinician about the amount of blood flow the brain receives, and it can be put in place very quickly, even during resuscitative efforts, and without any danger for the patient. The study looks at how well the information obtained with the c-flow matches the one obtained from other indirect indices and, more importantly, how well it predicts patient outcome. The investigators wish to establish threshold values of this index of perfusion that predict a good recovery so that this information may be used to optimize patient's neurological outcome in the near future.