MECHANICS OF BREATHING IN CHRONIC OBSTRUCTIVE PULMONARY DISEASE

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Abstract
Respiratory mechanics have been studied in 47 patients with chronic obstructive pulmonary disease and in 11 patients with bronchial asthma. The former group were arbitrarily categorized as having bronchitis or emphysema on the basis of the steady-state diffusing capacity, those with a low diffusing capacity being considered to have pulmonary emphysema. The positive correlation in these 66 patients between the diffusing capacity and the retractive force of the lung was used as further evidence that both of these measurements reflected the amount of emphysema present. The major feature that distinguished the patients with asthma from those with chronic obstructive pulmonary disease was the fact that, although the resting airway resistances were similar, the patients with asthma had less reduction of maximal expiratory flow rate and less increase of expiratory resistance at small lung volumes and during forceful expiration than the patients with chronic obstructive pulmonary disease. This was interpreted as confirmation of the hypothesis that airway obstruction in asthma is due to relatively fixed narrowing of larger airways, whereas, in patients with chronic obstructive pulmonary disease, the smaller, more distensible airways are obstructed. Evidence was also presented that airway obstruction became slightly more severe during forceful expiration at small lung volumes in patients with emphysema than in those with bronchitis. This is in keeping with the hypothesis that airway obstruction in the former group is related to loss of support of the small airways rather than exclusively to fixed obstruction within them.