Elevation in lung volume and preventing catastrophic airway closure in asthmatics during bronchoconstriction

Asthma exacerbations cause lung hyperinflation, elevation in load to inspiratory muscles, and decreased breathing capacity that, in severe cases, may lead to inspiratory muscle fatigue and respiratory failure. Hyperinflation has been attributed to a passive mechanical origin; a respiratory system time-constant too long for full exhalation. However, because the increase in volume is also concurrent with activation of inspiratory muscles during exhalation it is unclear whether hyperinflation in broncho-constriction is a passive phenomenon or is actively controlled to avoid airway closure.

Using CT scanning, we measured the distensibility of individual segmental airways relative to that of their surrounding parenchyma in seven subjects with asthma and nine healthy controls. With this data we tested whether the elevation of lung volume measured after methacholine (MCh) provocation was associated with airway narrowing, or to the volume required to preventing airway closure. We also tested whether the reduction in FVC post-MCh could be attributed to gas trapped behind closed segmental airways.

The changes in lung volume by MCh in subjects with and without asthma were inversely associated with their reduction in average airway lumen. This finding would be inconsistent with hyperinflation by passive elevation of airway resistance. In contrast, the change in volume of each subject was associated with the lung volume estimated to cause the closure of the least stable segmental airway of his/her lungs. In addition, the measured drop in FVC post MCh was associated with the estimated volume of gas trapped behind closed segmental airways at RV.

Our data supports the concept that hyperinflation caused by MCh-induced bronchoconstriction is the result of an actively controlled process where parenchymal distending forces on airways are increased to counteract their closure. To our knowledge, this is the first imaging-based study that associates inter-subject differences in whole lung behavior with the interdependence between individual airways and their surrounding parenchyma.