Relationships between respiratory and airway resistances and activity-related dyspnea in patients with chronic obstructive pulmonary disease

BACKGROUND The aims of the study were: (1) to compare numerical parameters of specific airway resistance (total, sRaw(tot), effective, sRaw(eff) and at 0.5 L · s(-1), sRaw(0.5)) and indices obtained from the forced oscillation technique (FOT: resistance extrapolated at 0 Hz [Rrs(0 Hz)], mean resistance [Rrs(mean)], and resistance/frequency slope [Rrs(slope)]) and (2) to assess their relationships with dyspnea in chronic obstructive pulmonary disease (COPD). METHODS A specific statistical approach, principal component analysis that also allows graphic representation of all correlations between functional parameters was used. A total of 108 patients (mean ± SD age: 65 ± 9 years, 31 women; GOLD stages: I, 14; II, 47; III, 39 and IV, 8) underwent spirometry, body plethysmography, FOT, and Medical Research Council (MRC) scale assessments. RESULTS Principal component analysis determined that the functional parameters were described by three independent dimensions (airway caliber, lung volumes and their combination, specific resistance) and that resistance parameters of the two techniques were not equivalent, obviously. Correlative analyses further showed that Raw(tot) and Raw(eff) (and their specific resistances) can be considered as equivalent and correlated with indices that are considered to explore peripheral airways (residual volume (RV), RV/ total lung capacity (TLC), Rrs(slope)), while Rrs(mean) and Raw(0.5) explored more central airways. Only specific resistances taking into account the specific resistance loop area (sRaw(tot) and sRaw(eff)) and Rrs(slope) were statistically linked to dyspnea. CONCLUSION Parameters obtained from both body plethysmography and FOT can explore peripheral airways, and some of these parameters (sRaw(tot), sRaw(eff,) and Rrs(slope)) are linked to activity-related dyspnea in moderate to severe COPD patients.