Flow-volume curve in expiratory airway obstruction
|In expiratory airway obstruction we can
think of 5 scenarios:
||Generalized increase in airways
Airway narrowing occurs in most intrathoracic
airways, such as in asthma, COPD and cystic fibrosis.
The airway wall is thickened, and secretions may contribute
to obstruction. For the same flow a pressure drop equal
to Palv - Pbr = PL,el occurs
nearer the alveoli than in healthy subjects. The check
valve in more compliant airways leads to greater than
normal flow limitation. In principle at each lung volume
expiratory flow is smaller than in a healthy subject.
As the flow limiting segment does not acquire its final
configuration instantly (see animation on previous page)
expiratory peak flow may be relatively unaffected.
The illustration shows the flow-volume curve of a patient with asthma prior to and after bronchodilatation, the curve during tidal resting breathing after bronchodilatation, and the curve expected in a healthy subject.
||Flaccid lung, such as in ‘pure’
Due to diminished lung elastic recoil
the flow limitation is established in more compliant
airways (see 1). In addition, however, peripheral airways
have a smaller diameter and may be tortuous due to loss
of lung elastic recoil.
This contributes to the diminished expiratory flow.
||Flaccid lung and generalized airway obstruction
A combination of 1 and 2 in COPD with pulmonary emphysema. The forced vital capacity manoeuvre is projected over the loop obtained during quiet breathing, showing that in this subject each spontaneous expiration is subject to the same flow limitation as during an FVC manoeuvre; the ventilatory reserves are nil. During exercise incomplete exhalations due to limited expiratory time will lead to progressive hyperinflation, which negatively affects the function of the diaphragm.
||Fixed extensive obstruction of major intrathoracic
example, a tumor obstructing the intrathoracic trachea
may increase airway resistance sufficiently to prevent
the subject from generating flow sufficiently large to
give rise to a flow limiting segment. In the end, unless
the pressure drop along airways equals minimally PL,el,
an equal pressure point does not occur and a compressed
flow limiting segment is not established. However, at
low lung volumes the combination of low PL,el and higher peripheral airways resistance do lead to establishment
of a flow limiting segment, and possibly to flow limitation
on account of progressive airway closure.
||Fixed severe obstruction of extrathoracic
||Flow limitation without a dynamically
compressed airway segment may occur in the case of a
large obstructing lesion in the extrathoracic trachea
or the larynx. If the obstruction is so extensive that
this by itself limits the flow that can be generated,
the resulting flow may not suffice to create a pressure
drop along intrathoracic airways equal to PL,el.
As explained sub 4 dynamic airway compression does not
occur in the case an ‘equal pressure point’
does not occur within the thorax. The net flow may vary
somewhat with expiratory effort.
Tidal flow-volume curve
Inspiratory flow-volume curve