Radiological diagnosis of nonuniform pulmonary ventilation.

Abstract

Laboratory methods of determining many aspects of pulmonary function have become remarkably accurate and sophisticated in the past ten years. Application of these new tests, however, has been limited by the fact that many of them require expensive apparatus not readily available, skilled technicians, and considerable time and effort. In a previous paper (1) we described the qualitative information about disordered ventilatory capacity which one might obtain by appropriate roentgen methods which are easily available. In the present paper the author proposes to discuss another aspect of disturbed pulmonary function in which ventilation is uneven in relation to capillary blood flow and to show the radiologic evidence which suggests that this disturbance is present. The efficient performance of gas exchange (O2 and CO2), which is the primary function of the lungs, requires a nice matching between the amount of air circulating through alveoli and the quantity of blood passing along the alveolar walls. In the normal lung, the air-blood volume relationships are constant for any particular lung area although these constants vary somewhat depending on location within the chest (2). One might consider, however, the normal average situation to be one in which alveolar ventilation equaled 2 liters∕min.∕lung, and capillary blood flow 2.5 liters∕min.∕lung, resulting in a ratio of 0.8 (Fig. 1). When injury or disease affects a localized area of lung, either ventilation or blood flow may be impaired selectively (Figs. 2 and 3). This may be accompanied by a compensatory decrease in the reciprocal function, which, however, seldom produces a new balance with a normal ratio of 0.8. Thus, even though total lung volumes may be adequate, mismatching of ventilation and blood flow results in the hypoxemia that accompanies pulmonary insufficiency and failure in many different clinical conditions (3). Many years ago, Golden (4) showed that the pattern of the vascular markings in the lungs, as seen by fluoroscopy and films, reflected accurately the symmetrical changes which occur in the tracheobronchial tree during respiration in the normal person. Since the blood vessels parallel the branching of the airways, irregularity and asymmetry of the vascular shadows in any area of the lung mean that the distribution of air in that region is uneven. Therefore, close inspection of roentgenograms of the chest made in various projections may lead to a diagnosis of nonuniform ventilation, localized or generalized (Figs. 4 and 5). This evaluation of qualitative pulmonary dysfunction can be quite accurate when compared with laboratory tests of lung function (Fig. 6). Discussion Nonuniform pulmonary ventilation may lead to alveolar hypoventilation and result in chronic hypoxemia.