Retention of insoluble particles after local intrabronchial deposition in dogs

Recent studies have challenged the generally accepted hypothesis that bronchial particle clearance is complete within 24-48 h postdeposition. We studied bronchial retention of inert particles using a bronchoscope and microspray nozzle to localize deposition in a bronchus while avoiding alveolar deposition. Six-microliter aliquots (444 kBq) of submicrometer (number mean diameter = 0.22 mu m, geometric standard deviation = 1.75) technetium 99m-labeled (Tc-99m) sulfur colloid (SC) particles (n = 6) or the unbound radiolabel Tc-99m-pertechnetate ((TcO4-)-Tc-99m; n = 3) were sprayed onto a 5-mm-diam bronchus in halothane-anesthetized dogs. Radioactivity at the deposition site and clearance pathway was monitored externally with a gamma camera beginning immediately postspray. Bronchial retention of SC was 8.5 +/- 2.4 and 1.5 +/- 0.7% at 3 and 24 h postspray, respectively. Tracheal mucus velocity was measured at 10.4 +/- 2.2 mm/min. For comparison, clearance of inhaled submicrometer SC particles was also measured in the same dogs. Retention of inhaled aerosolized SC (peripheral lung deposition) was 98.1 +/- 1.1 and 76.3 +/- 1.8% at 3 and 24 h, respectively. (TcO4-)-Tc-99m cleared from the bronchi slightly more rapidly than did SC. Radioactivity was readily detected in the blood after deposition of (TCO4-)-T-99m but not Of SC. Thus SC cleared by mucociliary transport, whereas (TcO4-)-Tc-99m cleared predominantly by transepithelial absorption. We conclude that clearance of submicrometer particles from a 5-mm conducting airway is very nearly complete by 24 h, with similar to 92% of the clearance occurring within the first 3 h postdeposition.