The role of the bronchial vasculature in soluble particle clearance

Although a role for the airway circulation in the clearance of inhaled particles is generally assumed, there is little information to confirm its importance. We studied the effects of decreased bronchial blood flow on the uptake of the soluble tracer technetium=99m-labeled diethylenetriamine pentaacetic acid (^99mTc-DTPA) from subcarinal airways in sheep (n = 7). The bronchial artery was cannulated and perfused with autologous blood at a control flow (0.6 mL/min/kg) or when the perfusion pump was stopped (no flow). ^99mTc-DTPA (6-10 μL) was delivered by a microspray nozzle inserted through a bronchoscope to a fourth-generation bronchus both during control blood flow conditions and no-flow conditions. Airway retention (by scintigraphy) and blood uptake were monitored for 30 min after the local deposition of ^99mTc-DTPA. During control flow conditions, 30 min after the delivery of the radiolabel, 21% of the tracer remained at the deposition site. Of the total delivered tracer, maximum blood uptake was 18% (n = 3). When bronchial perfusion was stopped, airway retention 30 min after deposition increased to 43%, and maximum blood uptake decreased to 7% of the total delivered tracer. Although mucociliary clearance was not directly measured, radiolabel tracer was observed to move progressively from the deposition site up to larger airways and contributed to the overall removal of tracer from the site of deposition during both flow conditions. However, these results demonstrate that decreased bronchial perfusion increases airway retention by limiting vascular uptake of the soluble tracer. These results emphasize the importance of normal perfusion of the airway vasculature for uptake of therapeutic agents delivered specifically to the conducting airways.