Alveolar capillary membane diffusing capacity and pulmonary capillary blood volume in normal adults

Objective: Measuring pulmonary diffusing capacity （D_L）, alveolar capillary membrane diffusing capacity （Dm） and pulmonary capillary blood volume （Vc） in normal adults; calculating the regression equation correlated to age, height and body weight; evaluating the effect of age on Dm and Vc; studying the relationship among D_L, Dm and Vc.Methods:Spirometry was performed in the sit position. Several measurements of diffusint capacity of the lung for carbon monoxide （D_L CO） were taken by single-breath method at increasing alveolar oxygen tension （P_A O₂）. The estimations of Dm and Vc were done by the method of Roughton and Forster.Results:（1）There was a significant negative corelation between age and D_LCO, Dm and Vc. The correlations between D_L and Dm, Vc as well as V_A and D_L, Dm, Vc are significantly positive.（2）D_L and Dm decreased after 40 and Vc decreased after 50. D_L/V_A Dm/V_A and Vc/V_A decrease after 60 years of age. There was no significant difference of Dm/Vc among groups. （3）Height and/or age were the main variables in stepwise multiple linear regression equations for D_L, Dm AND Vc in all groups.Conclusion:（1）There is a significant negative correlation between age and D_L, Dm and Vc.（2）D_L and Dm decrease after 40 and Vc decrease after 50. Age has not a significant influnce on Dm/Vc.（3）The declining in D_L may be due to the decrease of Dm and Vc. （4）The decrease of Dm may be related to the reduction of V_A with age. （5）Height and/or age are the main variables in regression equations for D_L, Dm and Vc.