Pulmonary oxygen transfer during IPPV in man

The efficiency of pulmonary oxygen transfer, measured as the difference between pulmonary end-capillary and arterial oxygen content, was assessed at several different inspired oxygen concentrations in 20 patients during intermittent positive pressure ventilation (IPPV) with oxygen/nitrogen mixtures. In only three of the patients did the patterns of response correspond with those predicted from iso-shunt and ventilation/perfusion mismatch models of lung function, in which it is assumed that oxygen exerts only a passive physical effect within the lungs. It is suggested that the pattern of response obtained during IPPV in the remaining 17 patients is explicable on the basis of an active physiological effect of oxygen such that an increasing alveolar oxygen partial pressure is associated with a progressive increase in the proportion of the pulmonary blood flow which bypasses ventilated alveoli. This "hyperoxic shunt", which is readily reversible, may be the result of redistribution of either ventilation or blood flow within the lung.