The key to metabolic management during cardiopulmonary bypass (CPB) is the maintenance of adequate blood flow and oxygen delivery to the body's tissues. Utilizing the CPB machine, the perfusionist provides the optimum conditions necessary for operations on the heart, lungs or major vessels, while supporting the patient's physiological and metabolic needs. The perfusionist calculates a CPB blood flow utilizing the patient's body surface area (BSA) and cardiac index (CI). Metabolic acidosis during CPB is almost always the result of hypoperfusion leading to oxygen delivery inadequate to meet metabolic demands for aerobic respiration. Oxygen consumption is thus a major determinant of CPB flow requirements. Deep hypothermic circulatory arrest (DHCA) is used to dramatically lower the body's metabolic demand while protecting organs, particularly the brain, during a period in which perfusion is suspended. Some in-line devices provide a continuous calculation of oxygen consumption based on pump flow and the arterio-venous oxygen differential.