Advances in Pulmonary Laboratory Testing: NEP
Although an effect similar to NEP could be obtained by asking subjects to simply augment expiration slightly during a transient breath, the NEP technique has the advantage of detecting expiratory flow limitation in patient groups that cannot easily respond to instruction. In addition’ it is possible that breathing mechanics immediately preceding the expiration may be altered slightly when individuals are asked to prepare to augment a subsequent expiratory effort.
In subjects spontaneously breathing at rest’ the application of NEP at the onset of expiration has sometimes resulted in a drop in flow below the flow rate generated during the preceding tidal expira-tion’ presumably due to reflexive partial or total upper airway narrowing. A subsequent study’ however’ suggested that reflex-mediated changes in upper airway diameter (genioglossus) tended to occur near the end of expiration rather than at the initiation’ except in a few subjects. Another potential limitation to the NEP technique is the inability to quantify the degree of expiratory flow limitation. Early expiration cannot be easily assessed due to the spike artifact caused from NEP. However’ this artifact is quite brief’ and’ typically’ flow limitation occurs predominantly over the later portion of expiration. birth control pills
Thus’ in most subjects the percentage of the tidal breath that is limited can be quantified’ except perhaps in subjects in whom expiratory time is significantly reduced. The NEP technique alone does not provide insight on other markers of VE constraint’ such as the change in EELV or an index of elastic load. It also does not provide insight into inspiratory flow constraints’ which may occur in patients with muscle weakness or fatigue or in those with significant hyperinflation. Unless the NEP technique is performed with the assessment of the tidal FV loops’ flow limitation is defined as “all or none” rather than in terms of the graded degrees of limitation defined with the tidal loops. An all-or-none assessment of expiratory flow limitation tells little about the degree of VE constraint’ especially since subjects may augment VE despite flow limitation by dynamic hyperinflation or further encroachment on the IRV. Thus’ for the assessment of ventilatory constraint’ further studies are necessary to determine which indexes (eg’ the presence of flow limitation’ degree of flow limitation’ change in IC’ or some other index) will represent the most sensitive markers of VE limitation. A combination of the NEP technique along with traditional estimates of breathing reserve and use of the extFVL may provide the greatest amount of information on VE constraints imposed by the lung and chest wall.