Determinants of regional distribution of a bolus inhaled from residual volume

Abstract

The volume of lung at residual volume (RV) which fails to receive an inhaled tracer bolus (EXV) was quantitated in 13 normals by comparison of a scintigram of the distribution of a tracer bolus inhaled from RV (BORV) with a scintigram at RV of lung equilibrated with the tracer (EQRV). EXV was found in the dependent lung in the erect position in all subjects but also occurred to a lesser degree at the apex in 11 of 13 subjects. Basal EXV ranged from 1 to 7% of TLC, and unlike apical EXV increased with age (r= 0.91, P less than 0.01). EXV in the decubitus position shifted largely to the dependent lung with none remaining in the original erect apical and basal locations, demonstrating that gravity determined EXV location. Nitrous oxide, which is highly diffusible, failed in four subjects to carry the tracer to basal EXV even though perfusion was documented to persist to this area, implying basal EXV airways were closed, not narrowed. In one of the four subjects apical EXV was readily definable. Nitrous oxide carried tracer into this region, implying patent apical EXV airways.