In vivo hysteresis of airspace dimensions measured by aerosol recovery

In anesthetized mongrel dogs, we made measurements of single breath aerosol recovery (RC) at equal volume points on the inflation and deflation limb of the quasi-static pressure-volume (P-V) curve of the lungs. Using a 1.2 μm monodisperse aerosol, a large aerosol tidal volume (Vt), and a breathing period of 5 sec, we found that losses of particles were primarily due to sedimentation in pulmonary airspaces distal to anatomic dead space. Thus, the RC measurements could be related to a mean radius (R) of airspaces filled with aerosol over the course of the breath. Furthermore, at a given volume, differences between inflation and deflation limb RC could be attributed to differences in R for the two measurements (i.e., Ri vs Rd). We found that at isovolume, RC as measured from the inflation limb was larger than that measured from the deflation limb for low lung volumes (<0.75 TLC). However, the recoveries were similar as lung volume approached TLC (>0.75 TLC). These results implied that at the same volume, Ri>Rdexcept at volumes approaching TLC, i.e. a larger mean airspace dimension on the inflation limb than on the deflation limb at equal volume. The findings of this study support a model of nonuniform changes in airspace dimensions associated with in vivo inflation and deflation of the lungs.