Oxygen transport2. Tissue hypoxia

Abstract

Tissues vary considerably in their sensitivity to hypoxia. Neurological cells tolerate hypoxia for only a few minutes whereas bladder smooth muscle may survive for several days without oxygen. This has important implications in the management of oxygen transport and monitoring of tissue hypoxia in critically ill patients. Regional oxygen transport and cellular use are often neglected but are important in maintaining tissue function and overall survival ### 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. Effect of oxygen tension gradient and diffusion distance on availability of oxygen to cells 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. Severe tissue hypoxia due to capillary microthrombosis in critically ill patient with meningococcal septicaemia …