Interaction of β₂-agonist and volume history in asthmatics and patients with chronic obstructive pulmonary disease

Objectives: This cross sectional study is aimed (a) to dissect out the b₂-agonist response from volume history (VH) response in asthmatics and chronic obstructive pulmonary disease (COPD) patients, (b) to find out whether or not VH response of airway resistance could be an indicator of the relationship between the airway and lung parenchymal hysteresis. Methods: The response of airway resistance' (Raw) as an indicator of airway tone was continuously recorded to various degree of volume history before and 10 minutes after inhalation of 200 microgram salbutamol (β₂) in 25 asthmatics, 25 COPD patients and 25 healthy volunteers as controls. Results: Important findings from this study are: (a) in asthmatics, VH alone decreased the average Raw by 28%. β₂ decreased it by 39% while VH + β₂ decreased it by 50%. However, 6 asthmatics out of 25 showed a bronchoconstrictor response to VH. (b) In patients with COPD, Raw response to VH, β₂ and VH + β₂ was evaluated to be 20, 12 and 12% respectively. However, 9 COPD patients out of 25 showed bronchoconstrictor response to VH. (c) In controls, Raw response to VH, β₂ and VH + β₂ was found to be 45, 5 and 48% respectively. Conclusion: This study shows that both VH and β₂ responses of Raw are additive in controls as well as asthmatics. However, such an effect is markedly reduced in patients with COPD. It may be proposed that VH response of Raw could be considered as an indicator of relative airway and lung parenchymal hysteresis determining the degree of airway tone.