## Forced Expiration

### Abstract

1 Mechanism of Expiratory Flow Limitation
1.1 Wave‐Speed Limitation
1.2 Viscous Flow Limitation
1.3 Computational Model
2 Measurement and Analysis
2.1 Volume‐Time
2.2 Flow‐Volume
3 Relative Merits of Presentations
3.1 Volume‐Time
3.2 Flow‐Volume
4.1 Volume and Time History
4.2 Effects of Changing Gas Properties
4.3 Dysanaptic Growth
4.4 Axial Bronchial Tension
4.5 Thoracoabdominal Mechanics
 Figure 1. Left: flow‐volume plot for normal subject. values are plotted against their corresponding volume at A, B, and C and define the MEFV curve (solid line). Right: three isovolume pressure‐flow curves from same subject. Curves A, B, and C were measured at volumes of 0.8, 2.3, and 3.0 liters from total lung capacity (TLC), respectively. Transpulmonary pressure is the difference between pleural (estimated by an esophageal balloon) and mouth pressures. Figure 2. Representation of flow limitation at wave speed. A: typical curve of airway area (A) as a function of p, the difference between lateral airway pressure and pleural pressure. If dissipative pressure losses are neglected, p can be calculated by subtracting the Bernoulli terρ2/2A2 from alveolar pressure (PALV). Because PALV (or PA) is measured relative to pleural pressure, it is equivalent to static recoil pressure. The intersection of this curve with the airway pressure‐area curve determines pressure and airway area at a given flow. There is a maximal flow for which a point common to both curves exists. B: airway pressure‐area curves for generations 2, 3, and 4 and the Bernoulli curve for maximal flow. Flow limitation occurs in generation 3, but there is an additional pressure drop because of compression downstream of the flow‐limiting site in generation 2.From Wilson et al. 149 Figure 3. Viscous flow limitation. A: pressure‐area curve of a smaller airway. B: if the pressure gradient in the flow is described by the Poiseuille equation, then for a fixed pressure at the upstream end of the tube (p1), flow will depend on the pressure at the downstream end of the tube (p2).From Wilson et al. 149 Figure 4. Top: flow‐volume plot of a forced expired vital capacity maneuver from a normal subject. Bottom: derived volume‐time trace of the same breath. , maximum expiratory flow; TLC, total lung capacity; FEF, mean forced expiratory flow between two designated volume points in FVC; FEV, forced expiratory volume in time interval.Adapted from Hyatt 51 Figure 5. A: two consecutive forced vital capacity efforts by same subject with flow and volume measured at the mouth. B: same efforts with volume measured from body plethysmograph. Efforts are identical because gas compression has been corrected for.From Hyatt 52 Figure 6. Averaged flow‐volume curves for normal subject. Upper panels: initial standing (left) and supine (right) curves. Middle panels: comparison of standing (solid lines) and supine (dashed lines) average flow‐volume curves obtained initially (left) and 3 mo later (right). Lower panels: comparison of standing (solid lines) and supine (dashed lines) slope ratios (SR) versus volume (V) plots obtained initially (left) and 3 mo later (right). , flow.From Castile et al. 19 Figure 7. Characteristic flow‐volume loops produced by major airway lesions. Fixed lesion resulted from fracture of larynx. Variable extrathoracic lesion was due to bilateral vocal cord paralysis. Variable intrathoracic pattern was produced by malignancy of trachea at carina.From Hyatt and Black 53 Figure 8. A: maximal expiratory flow‐volume curve (dashed curve) obtained from series of expiratory efforts, none of which represented an acceptable flow‐volume curve effort. Sequence was effort a (initiated from full inspiration) followed by effort b and then effort c. B: lower two‐thirds of maximal expiratory flow‐volume curve can be reliably estimated from a series of coughs initiated from full inspiration. For clarity flow transients have been filtered out.From Hyatt and Black 53 Figure 9. Maximal expiratory flow‐volume curves from patient with asthma. Patient was asymptomatic during control period. Symptoms including wheezing were induced by inhaling an extract of ragweed. Resulting change in lung mechanics is readily identified from maximal expiratory flow‐volume curves.From Hyatt and Black 53