Delayed distension of contracted airways with lung inflation in vivo

A deep inspiratory sigh is one of the most severe dynamic stresses that lun gs normally experience. It typically is a very transient phenomenon, normal ly lasting only about 2 to 3 s. The airway response to a deep inspiration h as been shown to be different in asthmatic and normal individuals. When air way smooth muscle (ASM) is contracted in normal subjects, a deep inspiratio n results in a subsequent dilation of the airways. However, in asthmatic su bjects, a deep inspiration often results in little change in airway functio n, and sometimes results in an even further contraction of ASM. The mechani sm underlying this difference depends on the dynamic behavior of both ASM a nd the lung parenchyma. If the contracted muscle had slower dynamic respons es than the lung parenchyma, the timing of the deep inspiratory maneuver co uld affect the airway response. In the present study, we designed an experi ment to determine how well matched the dynamic response is of airways to th at of the lung parenchyma. The results clearly demonstrate that airways con tracted with methacholine dilate at about a rate four times slower than tha t of the lung parenchyma during rapid lung inflation and deflation. This ef fect may play a role in the unique response of asthmatic subjects to deep i nspiration. The mechanism of this dynamic slowness of contracted airways pr obably involves intrinsic: properties of the smooth-muscle contractile proc esses.