Blood flow to organs: parameters for function and survival in critical illness.

Tissues are usually considered as being supply-dependent (e.g., heart and brain) and supply-independent (e.g., splanchnic area, kidneys, skin, and resting muscle) for oxygen delivery. When cardiovascular function is compromised, circulatory compensations are aimed at maintaining supply-dependent tissues. In the long term, this leads to the possibility of an inadequate blood flow to supply independent tissues. The perfusion maintenance of all organs requires adequate cardiac output, blood volume, and arterial BP. When BP and cardiac output fail, regional perfusion diminishes. Although the human system tolerates anemia well, optimum Hct levels are probably between 30% to 40%. Inadequate perfusion can be supplemented partially by increasing the FIO2 on a temporary basis. Hyperoxic arterial oxygen tensions may cause maldistribution of blood flow within organs.