EFFECTS OF VOLUME HISTORY ON AIRWAY CHANGES INDUCED BY HISTAMINE OR VAGAL-STIMULATION

The authors studied the effects of large inflations of the lungs and volume extremes on dead-space volume [V(D)] as an index of large-airway effects and pulmonary conductance [G(L)] plus dynamic compliance [C(dyn)] as an index of overall airway response. They used anesthetized, paralyzed, mechanically ventilated dogs and obtained equivalent decreases in G(L) with aerosol histamine and electrical stimulation of the vagi. Because V(D) was not changed after aerosol histamine but was markedly decreased during electrical stimulation of the vagi, this indicates that for an overall equivalent change in G(L) large airways are more affected by vagal stimulation than by histamine. Conversely, the predominant effects of histamine are on the more peripheral airways and air spaces. Large inflations of the lungs diminished the effects of histamine on G(L), but did not significantly alter the effects of vagal stimulation. The volume extremes of residual volume and total lung capacity did not significantly affect V(D) during control periods, after histamine, or during electrical stimulation of the vagi. In contrast, G(L) and C(dyn) were substantially altered by volume extremes during control periods and during induced airway constriction. These results suggest that with an equivalent overall airway response, there is a different distribution of responses within the tracheobronchial tree and that the large airway effects are not influenced by volume history.