By rearranging the terms of the pulmonary shunt equation the physiological factors affecting alveolar-arterial oxygen tension difference can be examined. The effect of the inspired oxygen fraction and haemoglobin concentration are illustrated. It is demonstrated that there is an important cardiac output dependent term which has considerable effect. Therefore the alveolar-arterial oxygen tension difference cannot be used as a reliable measure of pulmonary function. This is illustrated with data from two case histories.
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