PURPOSE: It is currently unknown how ventilatory constraints, dynamic hyperinflation and dyspnea evolve as airway obstruction progressively worsens. Accordingly, the purpose of this study was to examine the interactions between breathing pattern, dynamic hyperinflation and the time course of change in dyspnea during constant work rate (CWR) cycle exercise in COPD patients with varying levels of airway obstruction.
METHODS: Data from two previously published replicate randomized, double-blind, placebo-controlled clinical trials were retrospectively analyzed. Patients (n=427) were divided into disease severity quartiles based on %predicted forced expiratory volume in 1 second (FEV1). Spirometry, plethysmography derived lung volumes and physiological and perceptual responses to CWR cycle exercise at 75% of peak incremental work rate were compared.
RESULTS: Patients were well matched for age, body size and COPD duration across the quartiles. Decreases in FEV1 from quartiles 1 to 4 (means of 62, 49, 39 and 27%predicted) was associated with progressive increases in FRC (144, 151, 164 and 185%predicted) and decreases in both inspiratory capacity (IC) (86, 81, 69 and 60%predicted) and CWR endurance time (9.7, 9.3, 8.2 and 7.3min). Peak ventilation (VE) and tidal volume (VT) decreased and breathing frequency and dyspnea increased during exercise as FEV1 quartile worsened. Mechanical constraints, as defined by a plateau in VT, occurred at a progressively lower VE (p<0.0005), similar percentage of peak VE (82-86%) and similar VT/IC ratio (73-77%). Dyspnea intensity at this plateau was similar across quartiles (3.1-3.7 Borg units) but accelerated steeply to intolerable levels thereafter.
CONCLUSIONS: Progressive deterioration in pulmonary function with advancing COPD was associated with an increasingly shallow, rapid breathing pattern and worsening dyspnea at lower levels of ventilation during exercise. Regardless of disease severity, dyspnea increased rapidly to intolerable levels when VT reached ~75% of IC.
CLINICAL IMPLICATIONS: Measurement of IC is currently underutilized but provides important clinically relevant information in patients with COPD. Therapies that increase IC should favourably alter ventilation, breathing pattern and dyspnea during exercise.
DISCLOSURE: The following authors have nothing to disclose: Jordan Guenette, Katherine Webb, Francois Maltais, Denis O'Donnell
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