When the deposition of aerosol boluses is used to estimate mean pulmonary a
irspace size, an implicit assumption is made that the inhaled particles are
distributed uniformly among normal and diseased lung regions. This assumpt
ion was examined in a series of dogs in which emphysema was experimentally
induced by exposure to papain. After the experimental disease had developed
for several weeks, boluses of fluorescent particles were inhaled, using a
breathing pattern similar to that used for aerosol measurements of airspace
size. The lungs were then excised and 18-20 tissue blocks were obtained fr
om each lung. A section from each tissue block was analyzed to determine th
e mean liner intercept (Lm), which was considered as an index of lung injur
y. In the same sections, the density of particles was determined by countin
g particles in a number of microscopic fields and dividing the particle cou
nt by the number of fields sampled, Correlation analysis generally revealed
a negative correlation of particle density with Lm, indicating fewer parti
cles being delivered to diseased regions. One lung, however, showed a posit
ive correlation between particle density and Lm. Correction for the fractio
nal deposition of aerosol in the lung: regions weakened but did not reverse
the relationship between particle density and Lm. A model calculation of t
he effect of the observed nonuniform distribution of aerosol on the determi
nation of airspace size found a negligible effect of uneven ventilation on
mean airspace size determination in this experimental preparation.