TISSUE RESISTANCE IN THE GUINEA-PIG AT BASE-LINE AND DURING METHACHOLINE CONSTRICTION

Total lung resistance (RL), airway resistance (Raw), and tissue resist ance (Rti) were measured in unconstricted and methacholine (MCh)-const ricted guinea pigs while tidal volume, lung volume, and breathing freq uency were varied. Measurements were made in tracheostomized ventilate d guinea pigs with use of alveolar capsules. Relationships between Raw and Rti at different breathing frequencies, lung volumes, tidal volum es, and levels of constriction were compared with previously reported values in other species. Our results demonstrate that, at fixed tidal volume, Rti was inversely related to breathing frequency (Rti similar to f(-0.64), where f is breathing frequency in Hz) and increased with increasing lung volume. Rti was a significantly greater percentage of RL after MCh administration (40-50%) than at baseline (15-35%), indica ting a greater tissue than airway constrictor response. Rti was also 0 .5 log dose more responsive to intravenous MCh than Raw on the basis o f the dose required to produce 100% increase in resistance from baseli ne (PD100). These data show that, in the guinea pig, Rti changes with lung volume, breathing frequency, and constrictor tone in a manner sim ilar to other species previously reported and that Rti can be an impor tant determinant of lung dysfunction during constriction, even in spec ies for which it is small in relation to Raw at baseline.