The influence of circulatory factors on arterial oxygenation during anæsthesia in man*

FIGURE 1 Diagrammatic representation of an alveolar unit with its shunt component.The symbols used are explained in the text.through which desaturated blood might pass and thus avoid contact with the alveolar gas exchange surface.The lungs as an intact organ may be considered to consist of a large number of such units, but it is advantageous to use the simple 'black-box' approach described by Nunn* in order to explain the basic concepts of pulmonary gas exchange.If there were no such shunt pathway, all the mixed venous blood would come into contact with the gas exchange surface of the alveoli.If normal capillary transit time and an adequate Po 2 gradient between the alveolar gas and the mixed venous blood may be assumed, equilibrium will have been attained between the gas and blood phases by the time the blood has reached the end of the pulmonary capillary.Consequently, the arterial blood would have the same Po 2 as that of the alveolar gas.Such a hypothetical situation does not exist in practice, however, since the anatomical distribution of the cardiovascular system is such that a small proportion of the venous return is not directed to the right side of the heart, but is diverted either into the pulmonary veins or the left-sided chambers of the heart.Since this blood is desaturated, this arrangement behaves as a right-to-left extrapulmonary shunt.In normal man, this has been demonstrated to consist of a variable proportion of the blood flow through the bronchial 3 and thebesian circulations 4 and probably accounts for about 2% of the total cardiac output.There may also be a variable amount of intrapulmonary shunting which is small in normal young adults5, but which appears to become magnified in diseased or damaged lungs, and with advancing age.