Selective deposition of inhaled aerosols to mechanically ventilated rabbits

We have studied selective deposition of tracer aerosols to specific sites in airways and peripheral regions of the rabbit lung by varying droplet size and breathing pattern. The different breathing patterns were controlled by a Servo Ventilator 900C and different droplet sizes (polydisperse) were generated by an air jet nebulizer (MA2) using two types of impactor vessels (MMD 2.3 and 4.1 microns). Three tracer aerosols were evaluated; Evans blue dye, 99mTc-DTPA and monodisperse fluorescent polylatex spheres (PLS). When we combined large droplets with "rapid-shallow" breathing (central deposition mode, CDM), 30% of the aerosol was deposited in the central airways. When small droplets were combined with "deep-slow" breathing (peripheral deposition mode, PDM) 60% was deposited in the peripheral part of the lung. The different detection techniques showed similar results but gave complementary information. Since detection of the radiolabelled aerosol was more sensitive than the other methods, less aerosol could be given allowing a more precise evaluation of the deposition, both from the macro autoradiographic images as well as from the well counter measurements. In order to investigate how far into the lung periphery the aerosol could be detected, we used PSL microspheres. PLS could be detected in the alveolar region by a fluorescent light microscope. However, a complete selectivity can not be obtained by aerosol delivery. The different technique used to reach selective deposition, showed that it is only possible to deposit the aerosol either more to the central or more to the peripheral parts of the respiratory tract in small subjects.