Advances in Pulmonary Laboratory Testing: Clinical Application
Although not yet generalizable to the larger population, smaller studies have suggested that in patients with asthma and mild-to-moderate emphysema, and in healthy older subjects (subjects with mild-to-moder-ate obstructive airway changes) EELV will dynamically increase during exercise to varying degrees as expiratory flow limitation develops to take advantage of the flow available at the higher lung volumes. The increase in EELV in turn causes greater encroachment on the IRV (the EILV/TLC ratio, which increases elastic load) and potential encroachment by the inspiratory tidal loop on the inspiratory FV curve. In patients with more significant obstructive disease, the magnitude of the rise in EELV, as assessed with serial IC maneuvers, correlates more strongly with exercise endurance and dyspnea than do traditional measurements of FEV1. Thus, in some populations, the change in the EELV at rest or during exercise may represent the most sensitive marker of ventilatory constraint. Patients with interstitial lung disease do not appear to demonstrate significant increases in EELV during exercise, while patients with chronic heart failure and subjects with significant obesity may actually breathe with reduced EELVs despite significant expiratory flow limitation and a normal Ve/MVV relationship. canadian health&care mall
Limitations to the widespread use of the extFVL in the clinical setting include difficulty in defining the true MFVL, effort dependence of the IC maneuvers, and constancy of TLC. Small errors in either of these measurements or assumptions could lead to erroneous conclusions. Partial MFVLs or maneuvers of MFVLs at different levels may be necessary to better define the maximal expiratory flow boundary in some patient populations. Additional studies are necessary to determine the following: (1) the optimal MFVL for comparison to the extFVL; (2) the ability of patients to perform multiple IC maneuvers; (3) which indexes are most clinically useful in describing the degree of VE constraint (eg, the percentage of Vt flow limited, increase in EELV, or increase in EILV/TLC ratio); (4) the relationship of various indexes of constraint to measurements of dyspnea and exercise intolerance; and (5) the degree of constraint that begins to influence exercise tolerance (eg, exercise capacity, perception of dyspnea, and cardiovascular function).