The Distribution of Inhaled Particles in Aerosol Measurements of Pulmonary Airspace Size

ABSTRACT

When the deposition of aerosol boluses is used to estimate mean pulmonary airspace size, an implicit assumption is made that the inhaled particles are distributed uniformly among normal and diseased lung regions. This assumption 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 from each lung. A section from each tissue block was analyzed to determine the mean liner intercept (Lm), which was considered as an index of lung injury. In the same sections, the density of particles was determined by counting particles in a number of microscopic fields and dividing the particle count by the number of fields sampled. Correlation analysis generally revealed a negative correlation of particle density with Lm, indicating fewer particles being delivered to diseased regions. One lung, however, showed a positive correlation between particle density and Lm. Correction for the fractional deposition of aerosol in the lung regions weakened but did not reverse the relationship between particle density and Lm. A model calculation of the effect of the observed nonuniform distribution of aerosol on the determination of airspace size found a negligible effect of uneven ventilation on mean airspace size determination in this experimental preparation.