Evaluation of respiratory muscles strength in stable asthma and chronic obstructive pulmonary disease

Abstract

Introduction: Chronic obstructive pulmonary disease (COPD) and asthma are chronic inflammatory respiratory disorders characterized by airflow limitation. Their main clinical characteristics are shortness of breath and chest tightness which are not always in correlation to airflow limitation, but in some cases are due to chronic hyperinflation. Chronic hyperinflation in some patients leads to impairment of respiratory muscle function. We aimed to compare maximal occlusive inspiratory and expiratory pressures (PImax and PEmax) between patients with stable asthma and COPD, determining their correlation with lung volumes in order to evaluate respiratory muscles function in stable forms of obstructive lung diseases. Materials and methods: Spirometry and body plethysmography were compared to respiratory muscle strength (PImax and PEmax) values after application of bronchodilator in 63 patients with stable COPD (31 individuals) and asthma (32 individuals), who did not have exacerbation in the 6 months prior to study. Results: A persistent hyperinflation was found in all patients with COPD and in 30 (93.8%) patients with asthma. Significantly lower values of forced expiratory volume percentage of predicted (FVC%), forced expiratory volume in first second percentage of predicted (FEV1%), and FEV1/FVC ratio were found in COPD group (105.15±18.89%/91.90±16.74%, p<0.01; 82.82±24.23%/57.40±18.93%, p<0.01; 64.32±12.83/49.21±10.27, p<0.01; for FVC%, FEV1% and FEV1/FVC ratio, respectively). Significantly higher values of residual volume percentage of predicted (RV%), total lung capacity (TLC%), and residual volume/total lung capacity ratio percentage of predicted (RV/TLC%) were found in COPD group when compared to the group of patients with asthma (124.28±22.02%/153.10±25.36%, p<0.01; 146.53±36.59%/187.42±39.77%, p<0.01; 136.03±30.19%/163.13±27.85%, p<0.01; for RV%, TLC% and RV/TLC%, respectively). Significantly lower values of PImax and PImax% were found in COPD group (6.89±2.31kPa/8.66±2.60kPa, p<0.01; 65.89±21.94%/81.07±25.18%, p<0.01). Significant correlation in both groups was established between PImax% values and RV%, TLC% and RV/TLC% (R =-0.672, p<0.01; R = -0.544, p<0.01 and R =-0.594, p<0.01; respectively). Conclusion: In both stable asthma and COPD the strength of expiratory muscles is completely preserved, while the strength of inspiratory muscles is impaired in both diseases. The magnitude of persistent hyperinflation could be responsible for the respiratory muscle weakness. We suggested that additional training of inspiratory muscles could be of benefit in these patients.