This study is designed to test the hypothesis that poor cerebral perfusion during critical illness is a risk factor for acute and long-term neurological dysfunction among survivors. We use near-infrared spectroscopy to measure brain tissue oxygenation as a non-invasive surrogate marker for cerebral perfusion. Acute neurological dysfunction is defined as the presence of delirium, which is assessed using the Confusion Assessment Method-Intensive Care Unit (CAM-ICU). Chronic neurological dysfunction is defined as having quantitative impairments on robotic testing (KINARM robot) and traditional neuropsychological screening (Repeatable Battery for the Assessment of Neuropsychological Status).