Disposition of 3-, 7-, and 13-μm microspheres instilled into lungs of dogs

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This study examined the lung retention and clearance of particles which differed only in their size. Radiolabeled polystyrene microspheres 3, 7, and 13 μm in real size, were instilled into the lungs of beagle dogs. A fiberoptic bronchoscope was used to distribute a 10-ml saline suspension of the microspheres among lung lobes of 24 dogs. The retention and clearance patterns of the microspheres were observed for 128 days after instillation. Tissue and excreta content of the radiolabel provided the basis for defining the fate of the microspheres or radiolabel dissolved from the microspheres. Approximately 2 to 3% of each size of the microspheres cleared the lung within a few days after the instillation procedure, primarily via the mucociliary escalator. Thereafter, the 3-μm microspheres cleared the lung with a biological retention half-time of 820 ± 54 days; 7- and 13-μm microspheres were retained with biological retention half-times in excess of several thousand days and too long to be determined in this study. Approximately 1.7% of the instilled burden of 3-μm microspheres translocated to tracheobronchial lymph nodes during this 128-day study. Approximately 0.2% of the 7-μm microspheres and none of the 13-μm microspheres accumulated in tracheobronchial lymph nodes. From these data, it was concluded that microspheres or particles 7 μm or more real diameter may be retained indefinitely in the lungs of dogs after deposition by instillation. Similar results are predicted for humans for large particles deposited in the lung by inhalation.