The purpose of this study was to assess whether expiratory flow limitation (FL), as measured by applying negative pressure at the mouth during tidal expiration, is a better predictor of dyspnea than routine spirometry measurements. The study population consisted of 117 ambulatory patients with COPD. Dyspnea was assessed according to the ATS-DLD respiratory Questionnaire. Expiratory flow limitation was measured in supine and sitting positions, and expressed as a percentage of the expired control tidal volume affected by flow limitation (FL, % VT). Using Spearman's rank correlation (rs), we found that the correlation of dyspnea scale with FL was stronger (rs > 0.5) than with FVC (rs < -0.3) or FEV1 (rs < -0.4) in both positions. In a multiple regression analysis FL remained the best predictor of dyspnea scale even after adjustment for FEV1 (% pred). Finally, FL was almost as sensitive as FEV1 (% pred) but much more specific in assessing the severity of dyspnea scale. These findings suggest that expiratory flow limitation as measured by the negative expiratory pressure technique may be more useful in the evaluation of dyspnea in patients with COPD than spirometry measurements.

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Flow limitation and dyspnoea in healthy supine subjects during methacholine challenge. J. S AE ulc, C.A. Volta, Y. Ploysongsang, L. Eltayara, R. Olivenstein, J. Milic-Emili. #ERS Journals Ltd 1999 ABSTRACT: The purpose of this study was to assess whether during standard methacholine (Mch) challenge (concentration up to 128 mg . mL -1 ) healthy supine subjects a) develop tidal expiratory flow limitation (FL) and hyperinflation, and b) whether the onset of tidal FL is associated with dyspnoea.Eight healthy subjects were studied. Dyspnoea was assessed using the Borg scale, FL by the negative expiratory pressure (NEP) method and hyperinflation in terms of decrease in inspiratory capacity (IC).Seven patients became flow limited at Mch doses ranging 4±64 mg . mL -1 , with FL encompassing 34±84% of the control tidal volume. In six of them the onset of tidal FL was associated with little or no dyspnoea and a modest degree of hyperinflation (DIC <-0.4 L). In one subject, however, onset of FL was associated with a substantial reduction in IC (0.58 L) and moderately severe dyspnoea. In all of these seven subjects FL was transiently reversed after an IC manoeuvre.In conclusion, the results show that a) most healthy subjects may develop flow limitation and hyperinflation during methacholine challenge in supine position, and b) at onset of flow limitation there is little or no dyspnoea, suggesting that onset of dynamic airway compression per se does not elicit significant dyspnoea. Significant dyspnoea probably only occurs with marked dynamic hyperinflation. Eur Respir J 1999; 14: 1326±1331. According to PELLEGRINO et al. [1], administration of inhaled methacholine (Mch) to seated asthmatic patients readily induces tidal expiratory flow limitation (FL), which represents the starting trigger for the functional residual capacity (FRC) to increase in order to allow breathing at higher flows. PELLEGRINO et al.[1] also showed that Mch administration to seated healthy subjects is seldom associated with FL and increased FRC. Since in the supine position the responsiveness to bronchial challenge is enhanced [2], it is possible that Mch challenge in this position may also lead to tidal expiratory FL (i.e. inability to further increase flow (V ') over the tidal volume (VT) range by increasing transpulmonary pressure) in healthy subjects. The supine position is more susceptible to expiratory FL than the sitting posture mainly because in this position the FRC is low, with a concomitant decrease in expiratory V ' reserve over the resting VT range [3,4]. Dyspnoea is often experienced during bronchial challenge [5]. According to O'DONNELL et al.[6] flowlimiting dynamic airway compression may per se elicit dyspnoea as a result of afferent activity from the distorted and collapsed airways. According to this hypothesis, the onset of expiratory FL should be associated with dyspnoea.Conventionally, expiratory FL has been assessed by comparison of tidal with maximal expiratory (V '-volume (V)) curves: FL is present in subjects in whom, at compara...