Effect of uneven ventilation on pulmonary diffusing capacity

¹ Pulmonary Function Laboratory, Department of Medicine, Wayne State University; and Detroit Receiving Hospital, Detroit, Michigan

In four normal subjects measurements of steady state diffusing capacity for CO (DL_CO) by an alveolar sample technique and of the degree of uneven ventilation by nitrogen washout were made simultaneously followed by measurement of DL_CO by rebreathing. A methacholine-histamine aerosol was then given. After this aerosol uneven ventilation worsened and steady state DL_CO fell, while rebreathing DL_CO did not change. Relief of uneven ventilation by isoproterenol was followed by increase of steady state DL_CO in two subjects. Alterations in rate and depth of breathing or changes in functional residual capacity do not explain the changes in steady state DL_CO which are attributed to increase in uneven ventilation as predicted by the theory of CO absorption. Theory also predicts a lack of effect of uneven ventilation on the rebreathing DL_CO, but these results must be accepted with caution because deep breathing may reverse the effects of the methacholine-histamine aerosol and O₂ consumption during rebreathing rises after this aerosol.