Tissue resistance in the guinea pig at baseline and during methacholine constriction

Total lung resistance (RL), airway resistance (Raw), and tissue resistance (Rti) were measured in unconstricted and methacholine (MCh)-constricted guinea pigs while tidal volume, lung volume, and breathing frequency were varied. Measurements were made in tracheostomized ventilated guinea pigs with use of alveolar capsules. Relationships between Raw and Rti at different breathing frequencies, lung volumes, tidal volumes, 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 approximately 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%), indicating a greater tissue than airway constrictor response. Rti was also 0.5 log dose more responsive to intravenous MCh than Raw on the basis of the dose required to produce 100% increase in resistance from baseline (PD100). These data show that, in the guinea pig, Rti changes with lung volume, breathing frequency, and constrictor tone in a manner similar to other species previously reported and that Rti can be an important determinant of lung dysfunction during constriction, even in species for which it is small in relation to Raw at baseline.