The presence of obstruction to airflow in the trachea can be evaluated by inspection of the FVL. Traditionally, this has been divided into fixed obstruction and mobile obstruction, and the latter subdivided into extrathoracic or intrathoracic obstruction. The term mobile is related to fluctuation of the diameter of the affected region under the influence of intraluminal or extraluminal pressure. In turn, these pressures are dependent on whether flow is occurring during inspiration or expiration.
Fixed Upper Airway Obstruction
In fixed upper airway obstruction, both the inspiratory and expiratory loops demonstrate a plateau (see Figure 2-1B). Miller and Hyatt carried out experiments, in which forced exhalation and inspiration was performed through a series of progressively narrower tubes (Figure 2-4).1,2 These studies demonstrated that flattening of both inspiratory and expiratory curves occurred to an increasing degree with narrowing of the orifices. The features on the expiratory curve are first visible at about a 1-cm tracheal diameter, implying that there is a lack of sensitivity with mild narrowing of the trachea. However, once flow limitation begins, the reduction in flow rate is very rapid, with the peak flow rate falling from about 90% predicted at a 1-cm tracheal diameter to 25% at a 5-mm diameter. Both the length of the plateau and the degree of peak flow reduction are proportional to the degree of obstruction.
Miller and Hyatt also evaluated the sensitivity of other standard pulmonary function tests in the diagnosis of tracheal obstruction (Figure 2-5).1,2 The peak expiratory flow rate was the most sensitive test, followed by the maximum voluntary ventilation. The forced expiratory volume in the first second (FEV1) does not show a recognizable fall outside of the normal range until at an approximately 6-mm tracheal diameter. The peak inspiratory flow rate is the most sensitive test for detecting inspiratory flow limitations. The ratio of maximum inspiratory flow (MIF) to maximum expiratory flow (MEF) at 50% vital capacity (MIF50/MEF50) remains about 1.0, since both parts of the FVL are altered to about the same degree.
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