Distribution of Inspired Gas in Lungs

Abstract

Our knowledge of intrapulmonary gas distribution is still incomplete. Every model of distribution of inspired air in the lungs is open to more or less severe criticism. A series element may be present at exceptionally high breathing rates, but regional ventilation differences probably account for most of the inequalities observed under physiological circumstances. Regional ventilation differences may be caused by a number of factors, unequal volume expansion and differences in time constants of lung regions being the most commonly accepted ones. Single expiration curves of inert gas dilution give qualitative evidence of the presence or absence of significant non-uniformity of distribution. Multiple breath clearance curves may be analyzed in terms of regional ventilation differences or in terms of over-all ventilatory efficiency. Normal values of indices of non-uniformity with the nitrogen washout method have been established and these have shown little overlapping between normal and emphysematous subjects. Despite some sources of error the nitrogen clearance curve gives a good quantitative approach to nonumiform distribution of inspired air. This method is not only well suited for physiological investigations, but also for the study of effects of drugs, allergens and other agents which diminish the rate of renewal of alveolar gas. It seems likely that the distribution of ventilation and blood flow within the lungs are even more dynamic processes than is apparent from the experimental results. The hydrostatic pressure effects in pulmonary circulation appear to be compensated, to some extent, by changes in gas distribution, but this compensation is insufficient to maintain uniform V/Q (alveolar ventilation/pulmonary capillary blood flow) in the erect position. The higher V/Q ratios in the upper lobes result in higher alveolar oxygen tensions, which may bear some relation to the predominantly apical localization of phthisis. The higher ventilation rate of the lower lobes, on the other hand, would suggest that these lobes would be more exposed to inhaled pathogenic agents such as silica dust and carcinogens. In 2145 cases of bronchial carcinoma, however, 55.4% occurred in the upper lobes, 29.4% in the lower lobes and the remaining in the middle lobe and in the main bronchi, which suggests that the distribution of inhaled carcinogens in the lungs is not the most important factor in the pathogenesis of bronchial carcinoma.