Uneven distribution of pulmonary ventilation and perfusion may be studied by continuous recording of the fractional concentration of gases such as He, C0₂, and O₂ during expiration. On the basis of a simple lung model including one balloon and one tube, which may be locally enlarged to a second compartment of a series model with good mixing in each compartment, an exponential function is derived for the exponential part of the expiratory curve. The main parameter, the time constant (T), may be expressed in terms of the minimum radius of curvature (R_min) of the exponential phase. In the simplest case, R_min = 2.598 T. By measuring R_min in helium expiratory curves of normals it is proved that they have no series ventilation in their lungs, if they are in recumbent position at rest.For cases more complex than this model, a study is made of the concentration fronts moving parallel from the alveolar entrances to the mouth. The theory suggests R_min as a diagnostic criterion for the evaluation of centrilobular emphysema.

It may be possible to describe and diagnose 4 lung diseases of the group CNSLD by laboratory data only, by computer, apart and separated from physician’s diagnosis. Recording of concentration curves of expired gases yield 4 lung parameter values: He_1•5, MHe, RHe, RCO₂. These, together with 8 other parameter values: FEVC, FEV₁ % FEVC, M₁ (= FEV₁%FIV₁%), MVV₃₀/ FIVC, FRC % TLC, RV % TLC, DLCO/body surface area, reversibility of FEV₁, and 3 administrative data: age, season and sex, form 15 ‘symptoms’, used together with ‘a priori probability’ to compute diagnosis of (A) asthma bronchiale, (B) bronchitis asthmatica, (C) bronchitis chronica and (E) emphysema pulmonum, from 703 patients in one clinic. The numerical values of the symptoms are grouped in symptom classes. By using chi-square test, Wilcoxon test, and T (=Δ/SE/Δ) test with and without classes, each disease (A, B, C or E) significantly differed in at least 7, 8, 8 and 10 symptoms, respectively, (on the average 12) from the other and in at least 11 symptoms from normal condition. Rank lists for discriminative properties of the symptoms show the importance of the gas concentration curve parameters RCO₂, RHe, MHe and He_1•5 and also of the parameters: reversibility and M₁.