Measurement of respiratory gas composition by a mass spectrometer lags behind the measurement of gas flow. To obtain specific gas volumes (e.g., the N2 volume) by multiplication and integration of concentration and flow, one has to synchronize flow and concentration signals using the delay time (TD) of the gas analyzer. During the N2 washout, however, gas composition changes and causes alterations of TD. This leads to errors of up to 17 and 70% in the measurement of pulmonary volume and series dead space, respectively, in an ideally mixing physical model of the lung. On the basis of Poiseuille's law and exact measurements of the characteristics of the capillary it is possible to adjust the synchronization, which improves the absolute accuracy considerably.

A study of the prevalence of asthma in school children in Birmingham which was first carried out in 1956-57 was repeated in 1968-69. There has been an increase in the prevalence of definitely diagnosed asthma from 1-8% to 2-3%, not including an even higher number of children (3-2%) with wheezing. A considerably higher prevalence in boys than in girls was again found both for definite asthma and for wheezing but the tendency to recovery in boys with definite asthma was slight whereas there was a marked recovery in cases of wheezing which almost certainly represented mild asthma.Negro children born in England had a similar prevalence to European children but children born outside England in the West Indies or in Asia had a significantly lower prevalence of asthma and of wheezing for reasons which are not fully understood but which might profitably be considered further. Asian children, however, appeared to retain their low prevalence of asthma even when born in England.

A model of the pulmonary airways was used to study three single-breath indices of gas mixing, dead space (VD), slope of the alveolar plateau, and alveolar mixing inefficiency (AMI). In the model, discrete elements of airway volume were represented by nodes. Using a finite difference technique the differential equation for simultaneous convection and diffusion was solved for the nodal network. Conducting airways and respiratory bronchioles were modeled symmetrically, but alveolar ducts asymmetrically, permitting interaction between convection and diffusion. VD, alveolar slope, and AMI increased with increasing flow. Similar trends were seen with inspired volume, although slope decreased at high inspired volumes with constant flow. VD was affected most by inspiratory flow and AMI and alveolar slope by expiratory time. VD fell approximately exponentially with time of breath holding. Eight different breathing patterns were compared. They had a small effect on alveolar slope and AMI and a greater effect on VD. The model shows how series and parallel inhomogeneity occur together and interact in asymmetrical systems: the old argument as to which is the more important should be abandoned.

An asymmetrical model of the human pulmonary acinus is described, in which elements of volume are represented by nodes joined by conductors permitting convective flow and molecular diffusion. The method of analysis permits simultaneous convection, diffusion, and dimensional change in any direction and requires only simple boundary conditions. Inspiration of O2 into a resident gas of 79% N2 followed by expiration was simulated at two flows. On expiration the slope of the alveolar plateau was 1.7%, and the alveolar N2 mixing efficiency was 97.0%. A symmetrical but otherwise similar model gave a slope of zero and a mixing efficiency of 99.9%. The patterns of gas concentration within the asymmetrical acinus during the respiratory cycle confirm and extend previous observations on the interactions between simultaneous convection and diffusion in asymmetrical structures (16, 21, 22). Even though these in combination within alveolar duct asymmetry can account for the slope of the alveolar plateau, they are insufficient to account for the failure of complete gas mixing found in normal subjects.

No abstract

No abstract

No abstract

No abstract