Regional oxygen delivery in oxygen supply-dependent states

Assessment of the adequacy of systemic O₂ delivery (DO₂) is central in the evaluation of critically ill patients, but estimates of systemic DO₂ do not assess the effectiveness of regional DO₂ to all vascular beds whose functions may require different degrees of blood flow depending on their metabolic and functional demands. The oxygen supply-consumption curve includes a supply-in-dependent portion, which represents the reserve capacity of the body to maintain oxygen consumption (VO₂) despite inadequate increases in DO₂, and a supply-dependent portion, which represents the physiologic adaptation that occurs once DO₂ is unable to meet the metabolic demands of the body. Experiments in dogs revealed that when systemic DO₂ was progressively reduced, blood flow was maintained in the vital organs (heart and brain) and redistributed away from the kidneys and liver, enhancing the ability of the whole organism to use oxygen efficiently. Disease states and iatrogenic conditions that alter this vasoregulatory process may directly impair organ system function.