Page 129 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

106 BIOMECHANICS OF THE HUMAN BODY

shown are for increasing levels of effort during expiration. The maximal effort curve sets a portion
common to all of the curves, the effort-independent region. Expiratory flow limitation is another
example of the choked flow phenomenon discussed earlier in the context of blood flow. Similar flow
versus driving pressure curves shown in Fig. 4.11 can be extracted from Fig. 4.13 by choosing flows
at the same lung volume and measuring the pressure drop at that instant, forming the isovolume
pressure-flow curve. Since airway properties and E vary along the network, the most susceptible
place for choked flow seems to be the proximal airways. For example, we expect gas speeds prior to
choke to be largest at generation n = 3 where the total cross-sectional area of the network is
minimal; see Table 4.1. So criticality is likely near that generation. An interesting feature during
choked flow in airways is the production of flutter oscillations, which are heard as wheezing breath
sounds, 13,15,16 so prevalent in asthma and emphysema patients whose maximal flow rates are
significantly reduced, in part because E and U are reduced.
c

4.6.2 Airway Closure and Reopening
Most of the dynamics during expiration, so far, have been concerned with the larger airways. Toward
the very end of expiration, smaller airways can close off as a result of plug formation from the liquid
25
lining, a capillary instability, 11,21 or from the capillary forces pulling shut the collapsible airway, or
from some combination of these mechanisms. Surfactants in the airways help to keep them open by
both static and dynamic means. The lung volume at which airway closure occurs is called the closing
volume, and in young healthy adults it is ~10 percent of VC as measured from a nitrogen washout
test. It increases with aging and with some small airway diseases. Reopening of closed airways was
mentioned earlier as affecting the shape of the P-V curve in early inspiration as the airways are
recruited. When the liquid plugs break and the airway snaps open, a crackle sound, or cascade of
sounds from multiple airways, can be generated and heard with a stethoscope. 1,12 Diseases that lead
to increased intra-airway liquid, such as congestive heart failure, are followed clinically by the extent
of lung crackles, as are certain fibrotic conditions that affect airway walls and alveolar tissues.

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