Purpose of the Study Critical illness requiring life support affects over 150 000 people in Ontario every year. With aggressive support, the number of people who die from critical illness is decreasing. However, people who survive often have neurological problems. These neurological problems often include difficulties with memory, concentration, and attention. These problems are associated with poor quality of life among survivors of critical illness. The aim of this study is to identify the factors that contribute to these neurological deficits. Specifically, the investigators are testing whether changes in oxygen delivery to the brain during critical illness are related to both short- and long-term neurological complications.