Clinical Pulmonary Physiology

During the past ten years a number of techniques and theoretical concepts have been developed for the analysis of various aspects of pulmonary function in health and in disease. An understanding of the principles of modern respiratory physiology enables the physician to provide the proper amount of artificial ventilation to the apneic subject and to assess the nature and the extent of the functional impairment of the lung in patients with pulmonary disease. This paper outlines certain principles of respiratory physiology, the theoretical basis for some tests of pulmonary function, and some ways in which pulmonary function may be impaired in patients with pulmonary disease. Table 1 is a convenient classification of pulmonary functions. The first function of the lungs is to provide an optimum gaseous environment for the exchange of carbon dioxide and of oxygen between the blood and the air in the alveoli. This ventilatory function is dependent upon the integrity of the thoracic and diaphragmatic muscles and upon the patency of the airways. Ventilation must be adequate to maintain the proper concentration of oxygen nd of carbon dioxide in the alveoli. Next, oxygen must diffuse from the alveoli into the blood and carbon dioxide from the blood into the alveoli. This diffusion function is dependent upon the size of alveolar-capillary surface area available for gas exchange and upon the thinness of the barrier between the alveolar space and the red blood cells. Finally, the blood flowing through the lung must be diverted, to the alveoli which are ventilated and not to non-ventilated areas or through arteriovenous communications. One or all of these three functions may be disturbed in disease. Assessment of ventilatory function and of the total ventilation required by the apneic subject requires an understanding of the factors which determine the composition of the air in the alveoli. Carbon dioxide is produced in the tissues and diffuses from the venous blood into the alveoli, and oxygen is taken up by the blood in the alveolar capillaries and carried to the tissues where it is consumed. The concentration of oxygen and of carbon dioxide in the alveolar gas is dependent upon the metabolic rate and upon the volume of air brought in and out of the alveoli. Since, as will be explained later,