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Vital capacity (

FIGURE 9 Flow-volume loops. Maximal and half-maximal efforts produce similar expiratory flows at low lung volumes, indicating flow limitation, but inspiratory flow is proportional to effort at most lung volumes. (After Mead and Agostoni, Chap. 14 in Fenn and Rahn, eds., Handbook of Physiology, Section 3, Respiration. Bethesda, MD: American Physiological Society, 1964.)

inspiratory and expiratory flows are similar, and both occur near the midpoint of the volume change above FRC. This contrasts sharply with the outer curves in Fig. 9 for maximal ventilatory effort. Maximum inspi-ratory flow occurs over a wide range of lung volumes between RV and TLC, but maximum expiratory flow occurs only near TLC and it decreases progressively with lung volume. Examining the curves for a halfmaximal ventilatory effort reveals further differences between inspiration and expiration. Inspiratory flow appears to increase in proportion to inspiratory effort, but expiratory flow is independent of effort at lung volumes less than about half of TLC.

Extensive measurements show that this envelope of the expiratory flow-volume curve cannot be exceeded, and expiratory flow at these lung volumes is referred to as effort independent. Dynamic compression of airways is the physiologic mechanism determining maximum expiratory flow rate at low lung volume, and it depends on airway transmural pressure (Ptm). Factors affecting airway Ptm during expiration include (1) expiratory effort, which raises Ppl outside the airway; (2) lung volume and compliance, which affect pressures inside and outside the airways by elastic recoil, as described in Fig. 6; and (3) airway resistance, which causes a pressure gradient inside the airways from the alveoli and the mouth. Figure 10 illustrates how these factors interact to collapse the airways at certain lung volumes, independent of further increases in expiratory efforts.

Figure 10A shows the conditions before inspiration with no air flow. A positive 5 cm H2O Ptm gradient distends the alveoli and the entire length of the airways. Inside pressure equals atmospheric pressure (0 cm H2O) from the alveoli to the mouth with no flow. Outside pressure equals the subatmospheric Ppl, caused by the lung's tendency to collapse, and the tendency of the chest wall to expand, at FRC (see Fig. 6). In Fig. 10B, the respiratory muscles expand the thoracic cavity and decrease the Ppl. This Ppl decrease is transmitted to the alveoli, so flow occurs down a pressure gradient within the airways, from 0 at the mouth to —2 cm H2O in the alveoli. Notice that this gradient of pressure inside the airways that is driving flow during inspiration can actually increase the positive Ptm and distend the airways at some point.

The effects during expiration are very different. At the end of inspiration (Fig. 10C), the alveoli and airways are distended to a larger volume by a larger Ptm difference. The Ptm is the same at all points along the airways in this condition with no flow. A maximum expiratory effort is initiated (Fig. 10D) by contraction of expiratory muscle, which increases Ppl. PA increases and expiratory flow occurs down the pressure gradient inside the airways toward the mouth. However, the decrement in airway pressure along the airways means that outside (intrapleural) pressure can exceed inside (airway) pressure at some point and cause a negative Ptm, which collapses the airways. This is called dynamic compression, and the point of airway collapse is called the equal pressure point.

Dynamic compression occurs at low lung volumes because the elastic recoil of the lung contributes less to

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