Advances in Pulmonary Laboratory Testing: Negative Expiratory Pressures
The use of the extFVL would be helpful in many patient groups in which dyspnea and exercise intolerance were noted and mechanical constraints to breathing are a possibility. In patients with normal lung function or with severe airway changes (eg, FEV1 < 30 to 40%), such an assessment is unlikely to yield significant additional insight, however, in patients with more moderate obstructive changes or to assess the effectiveness of treatment, further evaluation using the extFVL may be valuable. Figure 2 shows an example of the rest and extFVLs plotted within the MFVL in a physically active patient with moderate COPD. Interestingly, this patient had a normal exercise capacity for his age, however, he clearly could not increase VE further due to the degree of constraint (ie, increased EELV and significant expiratory flow limitation.)
It should also be emphasized that the degree of VE constraint is not only dependent on the area circumscribed by the MFVL and the breathing strategy, but also on the VE demand. Thus, a highly fit healthy adult may encroach to a similar degree on the MFVL as a significantly deconditioned patient with some obstructive airway disease due to a significantly higher Ve. natural inhalers for asthma
Assessment of Expiratory Flow Limitation Using Negative Expiratory Pressures
Expiratory flow limitation promotes a dynamic increase in EELV with a concomitant increase in inspiratory work, impairment of inspiratory muscle function, and adverse effects on hemodynamics. These changes, combined with the dynamic compression of airways, likely contribute to dyspnea in patients with obstructive airway disease. The NEP technique evolved out of a need for an accurate assessment of expiratory flow limitation. It was originally used to assess expiratory flow limitation during mechanical ventilation, however, it has more recently been applied to various patient populations during breathing at rest and during exercise.
Figure 2. Example of the FV response to exercise in a patient with moderate obstructive airway changes. EELV increases with only moderate exercise due to expiratory flow limitation. By peak exercise the patient experiences both expiratory and inspiratory flow constraint and approaches a high EILV/TLC ratio. Clearly, little room exists to increase flow or volume relative to the profile of healthy adult shown in Figure 1.