EFFECT OF LUNG AIRWAY BRANCHING PATTERN AND GAS-COMPOSITION ON PARTICLE DEPOSITION IN BRONCHIAL AIRWAYS .3. EXPERIMENTAL STUDIES WITH RADIOACTIVELY TAGGED AEROSOL IN HUMAN AND CANINE LUNGS

Particle penetration into lung airways during normal respiration is af fected by the exchange of inspired air and residual gas. In this study , particle penetration during the inspiratory phase was investigated u sing gamma-tagged monodisperse particles (0. 70, 0.90, 0.96, and 1.44 mum) suspended in various carrier gases having a wide range of kinemat ic viscosity. A 40-mL bolus of tagged aerosol was drawn into excised h uman and dog lungs at the end of a tidal breath, followed by a long br eathhold to allow for complete particle deposition hy sedimentation. T he lungs were then fixed, sectioned, and autoradiographed to determine tidal front locations. In human lungs, particles suspended in He-O2 p enetrated deeper than particles suspended in air, particles penetrated least in SF6-O2. Dog lungs, which have more asymmetrical airway branc hing patterns than human lungs, bad no significant particle penetratio n differences associated with carrier gas composition. It is concluded that particle penetration during the inspiratory phase is dependent o n factors that determine flow profile development, such as branching p attern and the Reynolds number of the carrier gas. The bolus front at the end of an air inspiration extended into about 10% of human lung ai rways of 1 mm diameter, and into about 0.1 % of 0.5-mm airways. It is concluded that rapid particle penetration to 1-mm airways during high- frequency oscillatory ventilation of lung casts is due to cumulative a xial core transport during multiple breathing cycles. Similarly, the d ispersion of an aerosol bolus from large airways to small airways duri ng in vivo breathholding studies appears to be due to oscillatory flow created hy the heartbeat.