The effect of increased lung volume on the expiratory rate of rise of alveolar carbon dioxide tension in normal man.

Abstract

The rate at which alveolar CO2 partial pressure (PaCO2) rises during expiration was measured in 7 healthy medical students. PaCO2 rate of rise (.DELTA.PaCO2/.DELTA.t) was measured by a method utilizing constant expiratory flow rates in individual breaths in 2 subjects, and was calculated from airway PCO2 and expiratory tidal volume in the remaining 5 subjects. Steady-state runs were recorded at 2 or more metabolic rates with the subject making no special effort to control mean lung volume. This was done to establish the relationship between .DELTA.PaCO2/.DELTA.t and the rate of CO2 production (.ovrhdot.VCO2) at normal lung volume in individual subjects. Steady-state runs were also recorded at high lung volume. In each subject, .DELTA.PaCO2/.DELTA.t was less than would have been obtained at normal lung volume. Inversion of a hypothetical relation between .DELTA.PaCO2/.DELTA.t, .ovrhdot.VCO2 and average lung volume (VLa; DuBois, Britt and Fenn, 1952) yielded calculated values of VLa for both the normal and the high-lung-volume states. Lung gas volume was measured in a whole body plethysmograph, (box volume) both for the normal and high lung volume states, in each subject. Mean VLa and box volume estimates showed only moderately good agreement; the estimated differences between normal and high lung volume obtained by the 2 methods were virtually identical. Apparently, the expiratory PaCO2 rate of rise is determined, in the steady state, partly by the rate of CO2 production (a directly proportional relationship) and partly by the mean lung volume (an inversely proportional relationship).