Factors influencing oxygen availability.

This review gives an account of some of the factors affecting the transport of oxygen from the lungs until its final discharge in the tissues. The sequence of physiological and metabolic processes to be described may be considered collectively as the 'coarse adjust-ment' of oxygen transport. The 'fine adjustment' will be discussed in the succeeding chapter which surveys the local factors influencing distribution of oxygen from the microcirculation. Three terms which will doubtless be used more than once during this Symposium are worthy of definition. 'Oxygen availability' consists of two main components: blood flow and oxygen content (which is the product of haemoglobin concentration and oxygen saturation). By means of the Fick equation it can be shown that 'oxygen consumption' equates with the product of blood flow and arteriovenous difference for oxygen. Finally, 'oxygen debt' is the term used to describe the ability of certain tissues, notably muscle, to survive for short periods in the absence of oxygen. This factor may be expressed quantitatively as the product of the reduction in oxygen consumption during a period of hypoxia or ischaemia and its duration. Oxygen transport The oxygen transport mechanism is devised in such a way as to combine a large reserve of oxygen together with the capacity for rapid oxygen exchange both in the lungs and in the tissues. The progression of transfer processes has been likened to a cascade (Flenley, 1967) with the partial pressure of oxygen remaining high until the oxygen passes from arterial to capillary blood. The oxygen gradient between capillary blood and the tissues determines the oxygen tension of the mitochondrion which, in turn, determines substrate utilization. Since capillary blood oxygen tension is difficult to measure in clinical practice, mixed venous oxygen tension (or saturation) is often used as a measure of the overall adequacy of oxygen et al, 1975). Recently, apparatus has been developed which allows continuous measurement and display of mixed venous oxygen saturation in 'at-risk' or critically ill patients (Polanyi, 1974). If arterial oxygen tension is maintained stable, mixed venous oxygen saturation varies directly with cardiac output and inversely with oxygen consumption. In certain clinical situations, eg, after cardiac surgery, oxygen consumption remains largely unaltered (Raison et al, 1970) and under these circumstances mixed venous oxygen saturation is an excellent guide to changes in cardiac output. In other clinical situations, the value of mixed venous oxygen saturation may be less striking since a …