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On the causes of lung hyperinflation during bronchoconstriction.

Airway obstruction in asthma and chronic obstructive pulmonary disease (COPD) is often associated with lung hyperinflation. In this review, we examine the mechanisms that may cause functional residual capacity (FRC), residual volume (RV) and total lung capacity (TLC) to increase during acute and chronic airway obstruction. Normally, FRC at rest is determined by the static characteristics of the lung and chest wall. When airways narrow, FRC may be also be determined by dynamic factors. There are data suggesting that expiratory flow limitation during tidal breathing represents the starting trigger for FRC to increase, in order to allow breathing at higher flows. Indeed, the increase in FRC during induced bronchoconstriction in asthma is closely associated with the occurrence of flow limitation, i.e. the achievement of maximum flow during tidal breathing. Conversely, the decrease in FRC following bronchodilatation in COPD is closely associated with flow limitation disappearing or occurring at lower lung volumes. In normal young people, RV is determined by the static characteristics of the chest wall. During bronchoconstriction RV may also be determined by dynamic factors; therefore, changes in flow or airway calibre at low lung volumes may modulate RV during bronchoconstriction. During acutely induced bronchoconstriction, RV achieved with an expiration from TLC is less than with an expiration from tidal breathing, and this effect appears to be linked to the bronchodilator effect of the deep inhalation. The reasons for the increase in TLC during airway narrowing are not clear, but the duration of the bronchoconstriction by itself may play a role.