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tening of the expiratory curve, but it can be distinguished at times by a lack of reproducibility of successive curves. The report of the technician administering the test is perhaps the most helpful factor in distinguishing poor effort from disease, in many cases.

Another frequent error in interpretation of expiratory curves is the presence of a knee (see Figure 2-3) high up on the curve, close to total lung capacity (TLC). This can be corrected by repeating the study while hyperextending the neck. The explanation is thought to be due to stiffening of the trachea and movement of the equal pressure point. In some younger patients, a "shoulder" can be seen further down the expiratory curve, where flows that were being maintained during the early part of the curve decrease suddenly and more steeply. In both these variants, however, a peak is visible. In healthy subjects, where these variants are usually seen, the peak flow rate is in the normal range, whereas with true tracheal disease, the rate is reduced.

The inspiratory portion, in contradistinction to the expiratory portion of the FVL, is entirely effort dependent. Whereas the effort independent portion of the FVL is a function of elastic recoil of the lung as the driving force, the inspiratory portion is entirely driven by the force generated by the inspiratory muscles. Interpretation of this portion of the loop can be very difficult because many subjects without upper airway obstruction have difficulty in making a sustained forceful inhalation and technicians may not compel full efforts. For example, variable extrathoracic airway obstruction may be overdiagnosed, based solely on interpretation of the inspiratory portions of the loop.

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