Duration of deep inspiration and subsequent airway constriction in vivo

The effects of a deep inspiration (DI) in asthmatics differ from those observed in healthy subjects. When considering the effects of a DI, an implicit assumption is that all the airways are distending at the same rate as the lung parenchyma. However, with such rapid lung inflation, the ability of contracted airways to dynamically follow the lung parenchyma was recently shown to significantly lag the lung inflation. Another potentially important variable in the response of the individual airways to a DI that has not been well studied is the duration of the DI maneuver. The current study examines the effects of increasing duration at TLC during a DI on subsequent airway caliber. In five anesthetized and ventilated mongrel dogs, after DIs of increasing duration, changes in airway size were measured over the subsequent 5-minute period using high-resolution computed tomography. Results show that the duration of the maneuver is extremely important, leading to a qualitative change in the airway response. A long DI (> or = 30 seconds) caused subsequent airway dilation, while a shorter DI (< 30 seconds) caused bronchoconstriction. The precise mechanism underlying these observations is uncertain but seems to be related to intrinsic properties within the contracted airway smooth muscle.