Determination of Pulmonary Diffusing Capacity From Carbon Monoxide Equilibration Curves

¹ From the Departments of Medicine and Surgery, University of Chicago, Chicago, Illinois

Assuming an ideal lung, equations are derived for calculation of the pulmonary diffusing capacity from data obtained during equilibration of alveolar gas with a constant mixture of carbon monoxide. During washin or washout of that gas, changes in end-tidal carbon monoxide tension are due to a summation of ventilation and diffusion effects. The rate of change of carbon monoxide tension due to ventilation alone may be determined by simultaneous study of an inert gas such as nitrogen; the magnitude of the additional changes determines the rate at which carbon monoxide diffuses from the lung. The diffusing capacity is related to this rate of diffusion by a simple expression involving only the lung volume and Pb. In the period in which carbon monoxide is washed out of an ‘ideal lung,’ the disappearance of carbon monoxide from end-tidal gas follows an exponential function. In an actual lung, deviations from an exponential washout may be due to varying diffusion in different lung regions. By means of the derived equations, diffusing capacities were calculated from equilibration data in 14 normal subjects. Within the limitations of the method, varying diffusion in different lung areas was not demonstrated.