Effect of aerosolized isoproterenol on resting myogenic tone in normals.

The effects of aerosolized isoproterenol on expiratory (exp) and inspiratory (insp) conductance (Gaw), maximal exp and insp flow (VEmax and VImax), and static elastic recoil pressure (Pst) were measured in 12 normals. Both exp and insp Gaw increased throughout the vital capacity (37% at 50% VC; P less than 0.01). VEmax increased only at 50% VC (9%; P less than 0.01). VImax and Pst did not change. Accordingly, a dichotomy existed between the Gaw and Vmax changes during both exp and insp. We do not attribute this dichotomy to loss of driving pressure or to volume-time-dependent behavior of airway tone. We interpret the increased exp and insp Gaw to indicate isoproterenol deposition within and bronchodilatation of larger central airways (trachea, main stem, lobar, segmental). Since insp Gaw increased and VImax did not, we conclude that the caliber of these central airways is not the exclusive deteminant of VImax, that the caliber of some more distal airways (subsegmental and beyond) did not change, and that these airways are important determinants of VImax. We conclude that non-uniform distribution of isoproterenol could account for the Gaw-Vmax dichotomy during inspiration, and that such non-uniform distribution coupled with resultant increased compliance and compressibility of the downstream segment could account for the Faw-Vmax dichotomy during expiration.