[The effect of hyperinflation on respiratory muscles and breathing pattern in COPD]

Lung hyperinflation is a consequence of airway obstruction, increased airway resistance and compliance in patients with chronic obstructive pulmonary disease (COPD) which may result in respiratory muscle fatigue and deterioration of gas transfer. The aim of this study was to investigate the influence of hyperinflation on respiratory muscles, gas transfer and breathing pattern and compare the differences between mild and severe COPD. Twenty-eight COPD patients with radiological and tomographic evidence of emphysema were included in the study and they were divided into two groups according to the severity of COPD. Group I= FEV(1) < or = 49% (n= 16). Group II= FEV(1) > or = 50% (n= 12). Airflow rates were decreased and airway resistance was increased significantly in Group I. Maximal inspiratory pressure (MIP) was significantly reduced in Group I. FRC, RV and RV/TLC ratio were increased above 120% in both groups with more significant increase in Group I. Group I showed moderate hypoxemia (PaO(2) = 54.02 mmHg) with hypercapnia (PaCO(2)= 46.65 mmHg) whereas Group II patients were mildly hypoxemic (PaO(2)= 63.78 mmHg) with normocapnia. Parameters of breathing pattern were similar in both groups. Diaphragm height index (DHI) didn't showed significant difference between groups. But there were significant correlations between DHI and RV, FRC. MIP showed significant positive correlation with airflow rates and DLCO, negative correlation with lung volumes, positive correlation with PaO(2) and negative correlation with PaCO(2). FRC also negatively correlated with Ti and Ti/Ttot. In conclusion, hyperinflation present even in the mild forms of COPD causes inspiratory muscle weakness which in return results in impairment in gas transfer.