Effect of Aerosol Size on the Clearance of DTPA from Lungs of Anesthetized Sheep

Abstract

ABSTRACT

Aerosolized 99mTc–diethylenetriamine pentaacetate (DTPA) clears uniformly from the air spaces in control sheep. However, in response to increased lung volume due to a positive end expired pressure (PEEP) of 10 cmH₂0, a portion of the DTPA clears very rapidly (fast compartment) while the remainder clears at a normal rate. We tested the hypothesis that the fraction of DTPA that cleared rapidly from the lungs was proportional to the fraction that deposited in the alveoli. We performed a compartment analysis of the clearance of DTPA delivered to anesthetized sheep in 4 different size aerosols. When the DTPA was delivered in a 1.0 micron aerosol, 77 ± 7% of the DTPA cleared rapidly. This fraction decreased with increasing aerosol size with only 36 ± 14% of the DTPA clearing rapidly for the 6.8 micron aerosol. A postmortem analysis of the site of deposition of the aerosols confirmed that the 1.0 micron aerosol deposited primarily in the alveoli and small airways (<1 mm diameter) and the 6.8 micron aerosol deposited primarily in 7–8 mm airways. Therefore, the fast compartment in the clearance of DTPA from lungs ventilated with a PEEP of 10 cmH₂O represents the fraction of the DTPA that deposited in the alveoli and small airways. This compartment analysis of the clearance rate may be useful as a noninvasive index of peripheral deposition of aerosols.