Sites and Mechanisms of Decreased Air Flow in Disease
Abnormal expiratory flow rates may be seen in many disease states
and may result from alterations in any of the
Figure 26-9
A model depicting the equal pressure point (EPP) concept
of expiratory flow limitation. PALV, alveolar pressure;
PAW, intraluminal airway pressure; PPL,
pleural pressure; PM, mouth pressure.
three major determinants of flow (i.e., PEmax, PL,
and Raw), as seen in Table 26-3
.
For example, patients with neuromuscular disease who may exhibit decreased expiratory
flows include those with myasthenia gravis, muscular dystrophy, Guillain-Barr%e syndrome,
and spinal cord transection. Decreased ability to generate expiratory effort is
the principal cause of low expiratory flows in these patients, who seldom exhibit
increases in Raw or decreases in PL. Other categories
of restrictive disease, such as musculoskeletal deformities (e.g., kyphoscoliosis,
ankylosing spondylitis, interstitial lung disease), are often associated with near-normal
muscle strength. In these situations, expiratory flows may be slightly increased
because of an increased PL associated with
TABLE 26-3 -- Mechanisms for decreased expiratory flow rates
|
Physiologic Variables |
|
Disease |
PEmax |
Raw |
PL |
|
Neuromuscular weakness |
↓ |
N |
N |
|
Emphysema |
N |
N |
↓ |
|
Asthma, bronchitis |
N |
↑ |
N |
|
Peripheral airway disease |
N |
N |
N |
|
N, normal; PEmax, maximum
static expiratory pressure; Raw, airway resistance; PL,
lung elastic recoil pressure; ↑, increased; ↓, decreased. |
reductions in lung volume. Reduced lung volumes associated with long-term neuromuscular
disease also may be associated with increases in PL,
which may result in more normal expiratory flow rates.
The classic example of decreased expiratory flow associated with
decreased lung recoil (PL) is emphysema. In this
disease, expiratory muscle strength is usually adequate, and lung distention tends
to increase airway size, such that Raw is also usually normal. Conversely, in patients
with bronchitis and asthma, airway narrowing is prominent. In these two variants
of obstructive lung disease, the major factor reducing flow is increased Raw. Early
changes in these obstructive lung diseases may be confined to the smaller peripheral
airways. Narrowing in these airways may reduce expiratory flows at middle and low
lung volumes, but it does not appreciably affect measurements in Raw or the other
determinants of airflow.
