Diseases of the respiratory system; interpretations of tests of function.

Abstract

The lungs are perfectly designed to fulfill their function of ensuring full oxygenation of the blood and they achieve this by possession of the follow­ ing properties: (a) The distensibility and elasticity of the lungs enables a large volume of gas to be moved in and out of the alveoli at a rapid rate and with minimal effort. (b) Gas drawn into the lungs is distributed almost evenly to all alveoli and the blood perfusing the lungs goes equally to all parts. (c) The alveolar membrane is so fine that gas diffusion through it is very rapid. (d) The very large alveolar surface (ca. 55 sq. m.) and the relatively small volume of blood contained within the alveolar capillaries (ca. 60 cc.) ensures the saturation with oxygen of blood passing through the lungs. (e) The pulmonary vascular bed can accommodate a considerable increase of blood flow withou t a corresponding increase in pulmonary arterial pressure. It might be thou ght that such detailed knowledge of the attributes of the normal lung would have led to the development of tests precise enough to measure quantitively all the various disturbances of function that occur in disease. This is not the case. One of the reasons for this state of affairs is the difficulty in assessing the normality of one function in the presence of impairment of others. Another is that the methods of investigation are often technically difficult to perform and until the introduction of rapid physical methods of gas analysis, the tests of most value were often extremely time consuming. Pulmonary function tests are concerned with the first three of the attri­ butes of normal lung function detailed above, and may be conveniently considered under the following headings:(a) the mechanical efficiency of breathing and the subdivisions of lung volume, (b) measurements of the uni­ formity of gas distribution within the lungs, (c) measurements of the effi­ ciency of haemo-respiratory exchange. In this review, an attempt is made to assess the clinical value of some of these tests, although in our opinion their main valu e is in research. In general,