The effect of bronchodilation on symptom time course during exercise in COPD

Inhaled bronchodilator drugs improve exercise performance in COPD by reducing operating lung volumes. Whether this treatment modifies the rate at which symptoms intensify relative to increasing lung volume is less clear. Such a change is plausible given the improvement in frequency dependence of respiratory system resistance recently reported with nebulised beta-agonists (Dellacà et al ERJ 2009). We studied 16 patients (11 men, mean FEV1 1.21 L, mean inspiratory capacity (IC) 77% pred) who omitted their usual inhaled bronchodilators for 12-24 hours as appropriate. Patients performed a constant workload cycle ergometry test at 70% VO2 max during which symptoms of dyspnoea, leg fatigue and IC were recorded every 1 minute. Tests were done 15 minutes after 5 mg nebulised salbutamol or placebo in a double blind randomized fashion, the alternative therapy being given at a standardized time on another day with a further exercise test subsequently. Salbutamol increased FEV1 (mean change 214 ml), IC (mean change 278 ml) and exercise time (mean change 1.7 minutes). The rate of change of symptoms per minute during the test was unchanged mean (sd) IC Vs time saline = -0.06 (0.1) and bronchodilator = -0.08 (0.1); leg fatigue Vs time saline = 0.8(0.49 and bronchodilator = 0.7 (0.3), dyspnoea Vs time = 0.6 (0.2) and bronchodilator = 0.7 (0.7) p=NS. The rate of rise of dyspnoea per unit fall in IC was unaffected (pre -10 (15) vs post -4.4 (5) dys/IC slope r2 0.56 and 0.46 respectively p=NS). Plotting the slope of dyspnoea Vs IC relationship is closely related and post is dependent on pre. These data support the view that the primary beneficial effect of bronchodilator dugs on exercise performance is to reduce operating volume and create more volume into which dynamic hyperinflation can occur.