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Flow-volume curve in expiratory airway obstruction

In expiratory airway obstruction we can think of 5 scenarios:
1 AsthmaGeneralized increase in airways resistance
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.
 
2 Flaccid lung, such as in ‘pure’ pulmonary emphysema
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.
3 Flow-volume curve in severe airways obstructionFlaccid 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.
 
4 Flow-volume curve and fixed extensive intrathoracic airway obstructionFixed extensive obstruction of major intrathoracic airway
For 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.
 
5 Fixed severe obstruction of extrathoracic airway
  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.

See also:
Tidal flow-volume curve
Inspiratory flow-volume curve

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