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1 Medicine, Queen's University, Kingston, Canada
2 Boehringer Ingelheim (Canada) Ltd., Burlingston, Canada
* To whom correspondence should be addressed. E-mail: odonnell{at}post.queensu.ca.
During constant work-rate exercise in COPD, dyspnea increases steeply once inspiratory reserve volume (IRV) falls to a critical level that prevents further expansion of tidal volume (VT). We studied the effects of this mechanical restriction on the quality and intensity of exertional dyspnea and examined the impact of an anticholinergic bronchodilator. Methods: In a randomized, double-blind, crossover study, 18 patients with COPD (FEV1= 40±3 %predicted; mean±SEM) inhaled tiotropium 18 µg or placebo once daily for 7-10 days each. Pulmonary function tests and symptom-limited cycle exercise at 75% of each patient's maximal work capacity were performed 2-hours after dosing. Dyspnea intensity (Borg scale), operating lung volumes, breathing pattern and esophageal pressure (Pes) were measured during exercise. Results: Dynamic hyperinflation reached its maximal value early in exercise and was associated with only mild increases in dyspnea intensity and the effort-displacement ratio, which was defined as the ratio between tidal swings of Pes (expressed relative to maximum inspiratory pressure) and VT (expressed relative to predicted vital capacity). After a minimal IRV of 0.5±0.1 L was reached, both dyspnea and the effort-displacement ratio rose steeply until an intolerable level was reached. Tiotropium did not alter dyspnea/IRV relationships, but the increase in resting and exercise inspiratory capacity was associated with an improved effort-displacement ratio throughout exercise. Conclusion: Once a critically low IRV was reached during exercise, dyspnea rose with the disparity between respiratory effort and the VT response. Changes in dyspnea intensity after tiotropium were positively correlated with changes in this index of neuromechanical coupling.
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