Influence of airways resistance on the nitrogen washout from the lung during forced rebreathing

The forced rebreathing method was used to determine the rate of change (apparent increase) of residual volume (RV) per breath in a group of 64 healthy non-smokers, 449 healthy smokers and 28 patients with chronic obstructive pulmonary disease (COPD). The mean FEV(1)% FVC in the patients was 48.28% (SD= = 8.9%). The rate of breathing into the bag-bottle system was maintained at 28 breaths per minute for all subjects. An asymptote in the nitrogen washout curve was reached within 5 breaths in healthy subjects as compared to between 7-13 in patients with COPD. The mean apparent increase in RV average 11ml/breath, 15ml/breath and 18ml/breath in healthy non-smokers, smokers and patients with COPD respectively. The mean apparent increase in RV in healthy non-smokers approximated a normal resting oxygen consumption per minute. It was higher in healthy smokers and patients with COPD than in healthy non-smokers. It is concluded that because of a high airway resistance, the forced rebreathing increases the work of breathing in order to overcome resistance to gas flow in the airways. More energy is required and oxygen is removed from the bag-bottle system without a corresponding replacement with carbon dioxide. The volume of the bag-bottle system progressively decreases resulting in a high apparent increase in RV.