The study population will include all adult patients (≥ 18 years of age) with non-traumatic out of hospital cardiac arrest. Exclusions will include patients with advanced directives precluding resuscitation, traumatic cardiac arrest, and irreversible signs of death (e.g. rigor mortis). A matched control population will include hemodynamically stable patients who present to the ED with chest pain that is not of cardiac etiology (non-traumatic chief complaints). Blood draws in this cohort will be collected as per protocol and will end upon the termination of medical care or as per protocol - whichever is of the least duration.