The analysis of components that lead to increased work of breathing in chronic obstructive pulmonary disease patients

Background: This study is to explore the components and related mechanism responsible for the increase of work of breathing (WB) in chronic obstructive pulmonary disease (COPD) patients. Methods: Eight COPD patients and eight healthy volunteers were recruited in the study. The rebreathing method was used to increase end-tidal CO2 partial pressure (PetCO(2)) and stimulate the increase in ventilation (VE). The increase in VE, WB, and changes in the compositions of WB were observed and analyzed. The WB and its components were calculated using the Campbell diagram. Results: The inspiratory work (Wi) of breathing, a major component of total work of breathing (Wtot), in the COPD group was significantly higher than the control group during quiet breathing (P<0.05). As the minute VE increased, Wtot and Wi increased in a linear manner, and the slope of increase was significantly higher in the COPD group as compared to the normal group (P<0.05). The analyses of changes in overcoming airway resistance (Wrs) and lung/chest-wall elastance (Wel) indicated that the slope of increase (response to VE increase) of Wrs was not significantly different between the two groups (P>0.05) although the Wrs in the COPD group was always higher than the normal group (P<0.05). However, as the VE increased, the slope of the increase in Wel was significantly higher in the COPD group than the normal group. Work done to overcome the intrinsic PEEP (W-PEEPi), a component of the Wel, was not observed in the control group. However, W-PEEPi increased gradually as VE increased and accounted for 56% of Wel at the end of rebreathing trial in COPD group. Conclusions: Airway resistance was the main cause for increased WB during quiet breathing. As the VE increased, an increase of W-PEEPi became an important part of increased WB in COPD patients, so it is important to reduce dynamic hyperinflation in COPD patients.