Does impaired diffusion for oxygen exist in diseased lungs?

The historical development of ideas concerning impairment of diffusion in the lungs (alveolar-capillary block) is described. The major publications by Riley and Cournand, and others, which showed that impairment of diffusion exists in some diseased lungs were based on careful measurements of the arterial oxygen tension or saturation of patients. These papers described work carried out in the State of New York. Other workers claim to have shown that impairment of diffusion does not exist in diseased lungs: the impairment of oxygen transfer in alveolar-capillary block is said to be due to uneven ventilation-perfusion ratios. The publications which adopt this point of view are mostly based on measurements of the concentration of inert gases in the arterial blood. By chance, the workers who found that diffusion is not impaired in diseased lungs were all in the State of California. It is pointed out here that the California workers have failed to recognize that diffusion, like ventilation and perfusion, is distributed, i.e. varies greatly in different alveoli. This led them to ignore the fact that in some alveoli in diseased lungs, diffusion is so greatly impaired that some inert gases do not reach equilibrium at the end of the pulmonary capillary: the tension of inert gas in end-capillary blood is different from the tension of the same gas in alveolar gas. This invalidates the basic assumption underlying the inert gas method.