occur during expiratory flow limitation at two different lung volumes. The airways and alveoli are surrounded by the pleural space and the chest wall. The airways are shown as tapered tubes because the total or collective airway cross-sectional area decreases from the alveoli to the trachea. At the start of exhalation but before any gas flow occurs, the pressure inside the alveolus (PA) is zero (no airflow) and pleural pressure (in this example) is −30 cm H2O. Transpulmonary pressure is thus +30 cm H2O (PL = PA − Ppl). Because there is no flow, the pressure inside the airways is zero and the pressure across the airways (Pta, transairway pressure) is +30 cm H2O [Pta = Pairway − Ppl = 0 − (−30 cm H2O)]. This positive transpulmonary and transairway pressure holds the alveoli and airways open.
When an active exhalation begins and the expiratory muscles contract, pleural pressure rises to +60 cm H2O (in this example). Alveolar pressure also rises, in part because of the increase in pleural pressure (+60 cm H2O) and in part because of the elastic recoil pressure of the lung at that lung volume (which in this case is 30 cm H2O). Alveolar pressure is the sum of pleural pressure and elastic recoil pressure (i.e., PA = Pel + Ppl = 30 cm H2O + 60 cm H2O = 90 cm H2O in this example). This is the driving pressure for expiratory gas flow. Because alveolar pressure exceeds atmospheric pressure, gas begins to flow from the alveolus to the mouth when the glottis opens. As gas flows out of the alveoli, the transmural pressure across the airways decreases (i.e., the pressure head for expiratory gas flow dissipates). This occurs for three reasons: (1) there is a resistive pressure drop caused by the frictional pressure loss associated with flow (expiratory airflow resistance); (2) as the cross-sectional area of the airways decreases toward the trachea, gas velocity increases and this acceleration of gas flow further decreases the pressure; and (3) as lung volume decreases, the elastic recoil pressure decreases.
Thus as air moves out of the lung, the driving pressure for expiratory gas flow decreases. In addition, the mechanical tethering that holds the airways open at high lung volumes diminishes as lung volume decreases. There is a point between the alveoli and the mouth at which the pressure inside the airways equals the pressure that surrounds the airways. This point is called the equal pressure point. Airways toward the mouth but still inside the chest wall become compressed because the pressure outside is greater than the pressure inside (dynamic airway compression). As a consequence the transairway pressure now becomes negative [Pta = Paw − Ppl = 58 − (+60) =−2 cm H2O] just beyond the equal pressure point. No amount of effort will increase the flow further because the higher pleural pressure tends to collapse the airway at the equal pressure point, just as it also tends to increase the gradient for expiratory gas flow. Under these conditions, airflow is independent of the total driving pressure. Hence the expiratory flow is effort independent and flow limited. It is also why airway resistance is greater during exhalation than during inspiration. In the absence of lung disease, the equal pressure point occurs in airways that contain cartilage, and thus they resist collapse. The equal pressure point, however, is not static. As lung volume decreases and as elastic recoil pressure decreases, the equal pressure point moves closer to the alveoli.
One additional measurement of dynamic lung mechanics should be mentioned, and this is the measurement of dynamic compliance. A dynamic pressure-volume curve can be created by having an individual breathe over a normal lung volume range (usually from FRC to FRC +1 L). The
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