Figure 25-1.

Model of expiratory flow limitation.

Top, The static relationships of pleural pressure (Ppl), alveolar pressure (Palv), and intraluminal airway pressure (Paw), and airway dimensions at a fixed lung volume. Middle and bottom, Conditions at the onset of maximal flow and with increased expiratory effort, respectively. Dotted lines show static airway dimensions for comparison with the dynamic state. All three panels show pressures (cm H₂O) at the same lung volume: 60% of total lung capacity where lung elastic recoil pressure is +16 cm H₂O and equals the transpulmonary pressure (Pl) (Pl = Palv − Ppl). Top, When conditions are static, Palv is zero (i.e., atmospheric) and flow () at the mouth is zero. Middle, The subject makes a forced expiratory effort at the same lung volume. Now is 6.5 L/sec driven by Palv of +36 cm H₂O. Because of the resistances down the airways from alveolus to mouth, the Paw decreases to the point where Paw = Ppl (+20 cm H₂O, which is called the equal pressure point [EPP] because Ppl = Paw). Between the alveolus and the EPP, the airways are not compressed, but distal to the EPP there is compression and airway narrowing, because Ppl exceeds the pressure within the airways. For this lung volume, 6.5 L/sec is the maximal flow possible (see discussion of bottom panel, next). Bottom, The subject makes a forced expiratory effort starting at the same volume as in the top and middle panels (Pl = Palv − Ppl = +16). In this instance, the expiratory effort is markedly increased, reflected by the increased Ppl (+50 cm H₂O) and Palv (+66 cm H₂O). However, the flow generated is still only 6.5 L/sec because the increased effort succeeds only in compressing the airways more, dissipating the increased driving pressure across the increased resistance offered by the more narrowed airways; thus flow is maximum for this particular lung volume.

(Modified from Rodarte JR: Respiratory mechanics. In Basics of RD, New York, 1976, American Thoracic Society.)