Lung diffusing capacity: cyclically and continuously ventilated closed systems

¹ Department of Epidemiology, Medical Department, and Central Laboratory, Blegdamshospitalet, Copenhagen, Denmark

In previous communications, distribution of ventilation and lung diffusing capacity (Dl) were calculated from changes occurring in N₂ and CO concentrations when a subject, whose lungs contained atmospheric air at the beginning of the experiment, respired in a spirometer containing oxygen and a little CO. In the calculations, the system was assumed to be continuously ventilated and without dead space. The present study examines which errors may arise from these simplifications. Equations for cyclic ventilation are derived. Definite values for dead space and for volumes, alveolar ventilation, and Dl of two nonuniformly ventilated lung regions are assumed. Thereupon it is computed how the concentrations of N₂ and CO will change, if a subject with the assumed values respires in a spirometer of a given volume. From these changes in concentration, Dl and the distribution of ventilation are calculated using equations for continuous ventilation. The differences between the assumed and the calculated values of Dl are small.

Submitted on February 18, 1964