Airway Compression and Flow Limitation
The failure of increasing effort to further augment flow over
the lower two thirds of the VC results from dynamic compression of the airways.
This has been described in a model of flow limitation called the equal
pressure point (EPP) concept. The pressure head, which moves air from
the alveoli to the mouth, is provided by the alveolar pressure
(PALV). At any given lung volume, alveolar pressure
is the sum of lung elastic recoil pressure (PL)
and pleural pressure (PPL).
At any lung volume when there is no flow, such as at end inspiration ( Fig.
26-9A
), PPL is subatmospheric and counterbalances
PL. The sum (PALV)
is zero, as are
Figure 26-8
Forced expiratory flow-volume curves inscribed during
maximum maneuvers performed by beginning at total lung capacity (TLC) and at 2 and
4 L below TLC. RV, residual volume.
pressures at the mouth and through the remaining airways. During forced expiration
(see Fig. 26-9B
), PPL
rises above atmospheric pressure (i.e., becomes positive), and the increased PALV
again is the sum of PL and PPL.
Pressure is dissipated along the airway to overcome resistance to flow and finally
reaches zero at the mouth. At some point along the airway, the intraluminal pressure
falls to a level that equals the surrounding PPL.
This site is the EPP. Toward the mouth (i.e., downstream), the lateral pressure
within the airway lumen is less than the compressing PPL,
and the airways tend to collapse. After maximum flow is reached, further increases
in PPL from extra effort do not affect airflow in
the upstream segment (i.e., from EPP toward the alveoli), because the driving pressure
along this portion of the airway is equal to PL.
Therein lies the origin of the term effort independent.
The increased PPL produces more compression of the
downstream airways (i.e., from EPP to mouth). Increasing effort produces more and
more airway compression but fails to increase flow.
The principal value of this dynamic airway compression is in the
production of an effective cough. Even though maximum flows may not be reached,
the compressed downstream airway develops an increased linear velocity of airflow,
which maximizes the removal of secretions along airway walls. At intermediate lung
volumes, EPPs lie in segmental bronchi, but they may move farther upstream toward
the alveoli at lower lung volumes. The dynamic compression involves primarily the
lobar and mainstem bronchi and the intrathoracic trachea. Coughing is therefore
most effective in removing material from these relatively large airways.
