The pressure-flow relationships of the intrathoracic airway in man.

Measurement of the pressure-flow relationships of the human respiratory tract is basic to the study of ventilatory mechanics. The relationship of respiratory air flow to the total pressure drop along the respiratory tree (alveolus to mouth) has been extensively studied and been shown to depend on lung inflation (1-3) and the type and degree of airway disease (4). Recently the total pressure drop has been partitioned into that occurring along the lower or intrathoracic airways (alveolus to trachea) and that along the upper or extrathoracic airways (trachea to mouth) (5, 6). The resistance of the upper airway with mouth breathing was found 1) to comprise a significant portion of the total airway resistance in normal subjects, 2) to vary from subject to subject as well as in a given subject, 3) to depend on the type of breathing maneuver employed, and 4) to vary inversely with lung inflation to a slight degree (6). Only limited consideration has been given to the pressure-flow behavior of the lower airways (7). Yet in the evaluation of normal and abnormal ventilatory mechanics one is primarily interested in the behavior of the intrathoracic airways. It is the purpose of this study to analyze the pressure-flow characteristics of the lower airway in normal and emphysematous subjects and to relate the results to the usual methods of evaluating airway resistance in man. THEORY If one neglects the effect of gravity on the gas, the pressures along a stream of gas flowing through a nonuniform conduit system, such as the bronchial tree, may be analyzed by considering two properties of the flow, its frictional properties and its inertial properties. At any moment there are two superimposed pressure drops along the system related to these properties. A frictional pressure drop will exist related to the physical properties of the gas and to the flow geometry of the airways in a rather complicated manner. An inertial pressure drop will also exist related to the density of the gas and to the acceleration of the flow. The acceleration of the flow is of two varieties, local and convective. Local acceleration is the rate of change of velocity of the gas particles that occurs with time. Convective acceleration is the rate of change of velocity of the particles that occurs with the distance traveled by the gas particles, such as would occur with converging or diverging flow boundaries. In both cases the …