Advances in Pulmonary Laboratory Testing: Assessment of Expiratory Flow Limitation
When assessing flow limitation using the tidal breath method relative to the expiratory boundary of the MFVL, it remains controversial how to best define the maximal expiratory flows for comparison to the tidal FV loop. This is due to several factors, which have been shown to have varying influences on the size of the MFVL, as summarized in Table 3.
In addition, the placement of the tidal FV loop is dependent on the accuracy of the IC maneuvers. To illustrate these potential problems in defining maximal expiratory flows, some studies have observed that tidal flows exceed the MFVL during exercise. This suggests, therefore, that the point where the tidal flows meet the expiratory boundary of the MFVL represents “impending” flow limitation (the point at which pleural pressure begins to increase out of proportion to flow) rather than “true” flow limitation (the point at which flow plateaus relative to pleural pressure) in some patient populations.
In healthy subjects, the influences of dynamic compression, lung volume, and flow history on the expiratory boundary of the MFVL are likely to be small, however, they can become more substantial in patients with obstructive airway changes. Although these changes can generally be accounted for by varying expiratory effort when performing the MFVL maneuvers, by performing partial MFVL maneuvers, or by repeating maneuvers if airway caliber is suspected to change (eg, during or after exercise), these technical concerns can, for the most part, be eliminated using the NEP technique to assess expiratory flow limitation. natural asthma inhaler
The NEP technique was introduced for detecting expiratory flow limitation, which does not require the performance of forced expiratory efforts on the part of the patient, use of a body plethysmograph to correct for gas compression, or the performance of IC maneuvers. It consists of applying a negative pressure at the mouth during a tidal expiration and comparing the ensuing expiratory flow curve with that of the previous control expiration.
Table 3—Factors Influencing the Maximal Expiratory Flows During a Forced Maneuver
|Suggested Influence on the MFVL||Suggested Mechanism of Altering Maximal Flow|
|Excessive expiratory pleural pressure generation||Causes dynamic compression of airways|
|Lung volume history prior to performing the MFVL||Alters viscoelastic forces, airway resistance|
|Speed of inspiration preceding the MFVL maneuver||Affecting elastic recoil|
|Exercise-induced changes in airway function||Bronchoconstriction or bronchodilatation|