Morphometric changes after thermal and methacholine bronchoprovocations.

To determine whether there are distinctions in the location and pattern of response between different bronchoprovocations, we performed high-resolution computer-assisted tomography in 10 asthmatic subjects before and after isocapnic hyperventilation of frigid air (HV) and methacholine (Meth). The luminal areas of the trachea, main stem, lobar, and segmental bronchi were computed before and after each provocation and blindly compared. Both stimuli reduced the 1-s forced expiratory volume similarly (percent change in 1-s forced expiratory volume HV = 28.1 +/- 5.5%, Meth = 25.8 +/- 5.2%; P = 0.69) but did so in different fashions. Each provocation was associated with the development of both bronchial narrowing and dilation; however, more airways constricted with HV (67.7%) than with Meth (47.0%; P < 0.001). Furthermore, there was little concordance between either the magnitude or direction of change between stimuli in any region of the lung (r = 0.25). In general, the frequency of narrowing increased with branching. Constriction became more prominent in the lobar regions and increased further in the segmental branches, but a wide range of intensity existed. These data demonstrate that provocational stimuli evoke complex morphometric changes within the tracheobronchial tree and that different agonists produce different patterns. Thermal stimuli chiefly influence the segmental level, whereas the response to Meth develops more distally. Even within this distribution, the same airway does not respond in an identical fashion to different stimuli, so there does not appear to be a uniform trigger zone.