Expiratory Resistive Loading in Patients with Severe Chronic Air-Flow Limitation: An Evaluation of Ventilatory Mechanics and Compensatory Response

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Abstract
In order to estimate the extent of dynamic compression in patients with COPD who were flow-limited at rest, we measured tidal expiratory flows before and after application of small expiratory resistive loads (ERL). We sought also to evaluate the compensatory strategies available to such patients during ERL by measuring steady-state ventilatory resposnes. Nine patients with severe COPD (FEV1 .+-. SE, 27 .+-. 3% predicted) completed the study. Mean tidal flow-volume plots representing all breaths analyzed during 4 min of ERL (resistance, 8 cm H2O/L/s) and unloaded control (4 min) were compared at isoabsolute volume in each individual subject. In 6 subjects, ERL resulted in appreciable reduction of expiratory flows throughout the tidal volume (VT) when compared with volume-matched flows during control. In the remaining subjects, expiratory flows during loading and control coincided during part of the VT. In the group as a whole at 50, 30, and 10% of VT during ERL, when mouth pressure was increased by 3, 2.5, and 2 cm H2O, respectively, flow rates were significantly lower than volume-matched flows during control (.DELTA..ovrhdot.V, = 0.10, 0.09 and 0.06 L/s, respectively). Minute ventilation was reduced significantly by ERL, but only small insignificant changes in breathing pattern parameters occurred. End-expiratory volume increased by 0.1 L .+-. 0.02 (p < 0.005). We conclude that (1) the majority of patients with chronic air-flow limitation do not sustain significant dynamic compression at rest, and (2) loading response to ERL in patients with COPD are attenuated when compared with those in normal subjects.