Patients successfully resuscitated from sudden cardiac arrest are often comatose, having suffered a period of low blood flow and oxygen delivery to the brain. They are also at risk of suffering further brain injury during the immediate period after resuscitation, in which the brain's normal regulatory functions are impaired. To diagnose and treat secondary brain injury in comatose patients after cardiac arrest, doctors use a variety of neurological monitoring techniques. One of these methods involves measuring the oxygen saturation of blood going into and out of the brain to determine whether the brain is receiving and utilizing oxygen in an optimal manner. The oxygen saturation of blood exiting the brain is called the jugular venous oxygen saturation (SjO2). It is measured by inserting a catheter into the jugular vein in the neck and sampling blood as it exits the skull. The blood sample is sent to the hospital laboratory and the oxygen saturation is measured on a blood gas machine. This method of SjO2 measurement has limitations, particularly that blood must be taken out of the patient and sent to the lab for analysis, which can only be done feasibly every few hours. Special catheters exist that can measure the oxygen saturation of blood passing by the tip of the catheter inside the patient on a second-by-second basis, without needing to withdraw blood and send it to the laboratory. With such rapidly available data, doctors may be able to better diagnose and treat brain oxygen abnormalities in post cardiac arrest patients. In this study, the investigators plan to determine the accuracy of an existing, Food and Drug Administration (FDA)-cleared catheter capable of continuous, indwelling measurement of venous blood oxygen saturation for SjO2 monitoring in comatose patients early after cardiac arrest. The SjO2 measurements from the study catheter will be compared with standard SjO2 measurements made by withdrawing blood and analyzing it in the laboratory to determine if the new catheter is accurate. The investigators will also collect blood samples using the study catheter to measure levels of specific proteins that indicate damage to brain tissue. The study will enroll 25 participants admitted to the intensive care unit at one hospital cared for by a group of doctors that specialize in the neurological care of patients after cardiac arrest. The investigators hypothesize that the study catheter will accurately measure SjO2 compared to the standard laboratory method.