Detection of expiratory flow limitation during exercise in COPD patients

Abstract

The negative expiratory pressure (NEP) method was used to detect expiratory flow limitation at rest and at different exercise levels in 4 normal subjects and 14 patients with chronic obstructive pulmonary disease (COPD). This method does not require performance of forced expirations, nor does it require use of body plethysmography. It consists in applying negative pressure (-5 cmH2O) at the mouth during early expiration and comparing the flow-volume curve of the ensuing expiration with that of the preceding control breath. Subjects in whom application of NEP does not elicit an increase in flow during part or all of the tidal expiration are considered flow limited. The four normal subjects were not flow limited up to 90% of maximal exercise power output (Wmax). Five COPD patients were flow limited at rest, 9 were flow limited at one-third Wmax, and 12 were flow limited at two-thirds Wmax. Whereas in all patients who were flow limited at rest the maximal O2 uptake was below the normal limits, this was not the case in most of the other patients. In conclusion, NEP provides a rapid and reliable method to detect expiratory flow limitation at rest and during exercise.

Role of inspiratory capacity on exercise tolerance in COPD patients with and without tidal expiratory flow limitation at rest

“…In most previous studies, it was found that forced expiratory volume in one second (FEV1) was a poor predictor of exercise capacity [1±3]. Recently, however, it has been shown that indices related to dynamic hyperinflation, such as the inspiratory capacity (IC), are more closely related to exercise tolerance than FEV1 [4,5].Even at rest, patients with COPD often exhibit tidal expiratory flow limitation (FL) [6,7], promoting an increase in end-expiratory lung volume (EELV) due to dynamic hyperinflation with a concomitant decrease in inspiratory capacity and inspiratory reserve volume (IRV) [8,9]. During exercise, normal subjects increase the tidal volume (VT) at the expense of both the IRV and the expiratory reserve volume [8,9].…”

“…Assessment of FL was made using a simple noninvasive method that consists of applying a negative expiratory pressure (NEP) at the mouth during a tidal expiration and comparing the ensuing expiratory flow-volume curve with that of the previous control expiration [6]. The NEP technique has been extensively applied and validated, and has been used in stable COPD patients both at rest and during exercise [7,8,10]. …”

“…During exercise, normal subjects increase the tidal volume (VT) at the expense of both the IRV and the expiratory reserve volume [8,9]. In contrast, in flowlimited COPD patients, VT increases only at the expense of their reduced IRV and eventually it impinges into the flat portion of the static volume-pressure relationship of the respiratory system [8,9]. Thus, in flow-limited COPD patients the maximal VT (VT,max) achieved during exercise should depend on the magnitude of IC.…”

“…Even at rest, patients with COPD often exhibit tidal expiratory flow limitation (FL) [6,7], promoting an increase in end-expiratory lung volume (EELV) due to dynamic hyperinflation with a concomitant decrease in inspiratory capacity and inspiratory reserve volume (IRV) [8,9]. During exercise, normal subjects increase the tidal volume (VT) at the expense of both the IRV and the expiratory reserve volume [8,9].…”

“…Thus, in flow-limited COPD patients the maximal VT (VT,max) achieved during exercise should depend on the magnitude of IC. Since the exercise capacity is related to VT,max [8], a close association of maximal work rate (WRmax) and oxygen uptake during exercise (V'O 2 ,max) to IC should be expected in patients who exhibit tidal expiratory flow limitation already at rest. In contrast, in COPD patients without tidal FL (non-FL) at rest other factors should play a prominent role in determining exercise capacity.…”