Surgery places a person’s body under physiological stress: the magnitude of the stress response is proportional to the severity of the surgical trauma. The surgical stress response increases an individual’s O2 consumption (V̇O2). Patients who are less physiologically fit will be less able to increase their O2 delivery and may therefore be unable to match the increase in V̇O2. Respiratory and cardiovascular co-morbidities place a major limitation on the cardiovascular response to major surgery, but physical deconditioning also plays a significant role.