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

The rate at which alveolar PCO2 (PA, CO2) rises during expiration has been measured in seven healthy medical students. PA, CO2 rate of rise [delta PA, CO2/delta t] was measured by a method utilizing constant expiratory flow rates in individual breaths in two subjects, and was calculated from airway PCO2 and expiratory tidal volume in the remaining five subjects. Steady-state runs were recorded at two 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 PA, CO2/delta t and the rate of CO2 production (VCO2) at normal lung volume in individual subjects. Steady-state runs were also recorded at high lung volume. In each subject delta PA, CO2/delta t was less than would have been obtained at normal lung volume. Inversion of a hypothetical relation between delta PA, CO2/delta t, VCO2 and average lung volume (VLa; DuBois, Britt & 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, whereas the estimated differences between normal and high lung volume obtained by the two methods were virtually identical. These experiments suggest that the expiratory PA, CO2 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).