ABC of oxygen Oxygen transport—2. Tissue hypoxia

Delivery to organs and tissues The mechanisms controlling oxygen distribution are incompletely understood but involve a series of convective and diffusive processes. Convective oxygen transport refers to the bulk movement of oxygen in air or blood and depends on active, energy consuming processes generating flow in the tracheobronchial tree and circulation. Diffusive transport refers to the passive movement of oxygen down its concentration gradient across tissue barriers, including the alveolar-capillary membrane, and across the extracellular matrix between the tissue capillaries and individual cells to mitochondria. The amount of diffusive oxygen movement depends on the oxygen tension gradient and the diffusion distance. In many critically ill patients tissue hypoxia is due to disordered regional distribution of blood flow both between and within organs. Regional and microcirculatory distribution of cardiac output is determined by a complex interaction of endothelial, receptor, neural, metabolic, and pharmacological factors working on small resistance arterioles and precapillary sphincters. In critical illness, particularly sepsis, hypotension and loss of normal autoregulation cause shunting and tissue hypoxia in some organs despite high global oxygen delivery and mixed venous saturation. The perfusion pressure is an important determinant of regional perfusion, but adrenaline or noradrenaline given to maintain systemic blood pressure may reduce regional distribution, particularly to the renal and splanchnic capillary beds. Drugs are often used to try to improve regional tissue perfusion. Dopamine has been widely used in the belief that it improves renal blood flow, but it probably increases overall cardiac output rather than regional distribution. Dopexamine, which has no alpha effects, may selectively increase renal and splanchnic perfusion. During critical illness tissue hypoxia is often caused by capillary microthrombosis after endothelial damage and neutrophil activation rather than by arterial hypoxaemia. Manipulation of the coagulation system using low molecular weight heparins may reduce microthrombosis. Thrombosis or embolism in a large artery can also produce organ or tissue hypoxia despite adequate global oxygen delivery. Surgical or pharmacological removal of the obstruction is needed before irreversible damage occurs. Ideally, individual tissue oxygenation needs to be measured directly to assess and manage organ hypoxia correctly.