Decline of resting inspiratory capacity in COPD: the impact on breathing pattern, dyspnea, and ventilatory capacity during exercise.

BACKGROUND: To better understand the interrelationships among disease severity, inspiratory capacity (IC), breathing pattern, and dyspnea, we studied responses to symptom-limited cycle exercise in a large cohort with COPD.

METHODS: Analysis was conducted on data from two previously published replicate clinical trials in 427 hyperinflated patients with COPD. Patients were divided into disease severity quartiles based on FEV(1) % predicted. Spirometry, plethysmographic lung volumes, and physiologic and perceptual responses to constant work rate (CWR) cycle exercise at 75% of the peak incremental work rate were compared.

RESULTS: Age, body size, and COPD duration were similar across quartiles. As the FEV(1) quartile worsened (mean, 62%, 49%, 39%, and 27% predicted), functional residual capacity increased (144%, 151%, 164%, and 185% predicted), IC decreased (86%, 81%, 69%, and 60% predicted), and peak incremental cycle work rate decreased (66%, 55%, 50%, and 44% predicted); CWR endurance time was 9.7, 9.3, 8.2, and 7.3 min, respectively. During CWR exercise, as FEV(1) quartile worsened, peak minute ventilation ($$\dot{\mathrm{V}}$$e) and tidal volume (Vt) decreased, whereas an inflection or plateau of the Vt response occurred at a progressively lower $$\dot{\mathrm{V}}$$e (P < .0005), similar percentage of peak $$\dot{\mathrm{V}}$$e (82%-86%), and similar Vt/IC ratio (73%-77%). Dyspnea intensity at this inflection point was also similar across quartiles (3.1-3.7 Borg units) but accelerated steeply to intolerable levels thereafter.

CONCLUSION: Progressive reduction of the resting IC with increasing disease severity was associated with the appearance of critical constraints on Vt expansion and a sharp increase in dyspnea to intolerable levels at a progressively lower ventilation during exercise.