Airway obstruction and chronic exertional dyspnoea in patients with persistent bronchial asthma.

In patients with COPD, flow limitation (FL) predicts chronic exertional dyspnoea (CED) better than routine spirometry. Whether, and to what extent, FL and CED are overlapping quantities in chronic asthma has not yet been defined. Forty consecutive clinically stable asthmatic patients without smoking history or cardiopulmonary disorders, were studied. In each subject respiratory function, including static and dynamic pulmonary volumes, was evaluated; maximal (MEFV) and partial (PEFV) expiratory V'-V curves and isovolumic partial to maximal flow ratio (M/P). FL was assessed in a seated patient by comparing tidal and PEFV curves; FL was detected when tidal flows were superimposed or exceeded those obtained during PEFV curves, and was expressed as a percentage of the expired control tidal volume (V(T)) affected by flow limitation (FL% VT). Dyspnoea was assessed by both MRC scale and Baseline Dyspnoea Index (BDI) focal score. Half of the patients were found to have FL. They were older, more dyspnoeic and more obstructed (P<0.03 - P<0.000005) than the non-FL group. FEV1, vital capacity (VC), age, body mass index, FL and M/P ratio were all related to dyspnoea scores. FL was significantly related to FEV1 (r = - 0.59). Multiple regression analysis showed that FEV1 (P=0.003, r2= 15-3% and P = 0.004, r2= 20.3%) and age (P = 0.0006, r2 = 26.8% and P = 0.016, r2 = 11%) independently predicted a part of the variance of MRC (P = 0.0001, r2 = 42.1%) and BDI (P = 0.0008, r2 = 31.3%), respectively. With dyspnoea scale being the gold standard, diagnostic accuracy (sensitivity and specificity) by ROC (receiver operating characteristics) analysis was similar for FEV1 and FL. The results indicate that FL may be present in this subset of asthmatics. CED may not be easily explained by abnormalities of routine spirometry or FL, the largest part of the CED variance remained unexplained. Thus, routine spirometry, FL and CED in patients with bronchial asthma are only partially overlapping quantities which need to be assessed separately.