Key Point

Particle clearance from the airways appears to involve a brief fast phase followed by a slow phase of long duration.

GAUTING, GERMANY—A recent study by Winfried Möller, PhD, and colleagues provides new insight into how the body removes unwanted particles from the airways. Surprisingly, particle clearance is not completed in a day, as was previously believed, but over weeks or months, reported the study authors.¹

They made that determination by using the shallow bolus technique to observe mucociliary and long-term particle clearance in the airways of 13 healthy nonsmokers. “With the shallow bolus technique, a small amount of aerosol is inhaled at the end of inspiration,” explained Dr. Möller, Aerosol Biophysicist at the Institute for Inhalation Biology in Gauting, Germany. “That way, the aerosol can only penetrate into the airways to a limited volumetric depth.”

The inhaled aerosol used in the study contained ferromagnetic iron oxide particles, which have two major advantages over the technetium that is typically used to study particle clearance from the airways. First, ferromagnetic iron oxide is not radioactive; and second, it permits measurement of particle retention over hundreds of days versus only one day with technetium. The limited retention measurements available with technetium are the main reason why so little is known about particle clearance from the airways beyond one day, explained Dr. Möller.

He and his colleagues tried to confine the ferromagnetic iron oxide bolus to the bronchial airways of the study group by limiting the bolus front depth to the mean phase-1 dead-space volume of 164 mL. And indeed, the mean aerosol bolus front depth was calculated at 150 mL. Particle retention was measured over a nine-month period with magnetopneumography.

“Particle clearance from the airways showed a fast and a slow phase,” the authors related. In the fast phase, about half of the inspired ferromagnetic iron oxide particles were removed from the airways in a mean half-time of three hours, which is characteristic of mucociliary clearance, Dr. Möller pointed out.

The remaining particles were cleared much more slowly—in a mean half-time of 109 days. “It is not known why a large fraction of the particles are not subject to mucociliary clearance,” Dr. Möller told Pulmonary Reviews. “But it could be that after a day or so, all particles remaining in the airways are phagocytosed by macrophages and then cleared using the same kinetics seen in the alveolar space.”

While the results of the study suggest that current beliefs about airway particle clearance need to be reevaluated, they do not yet have any clinical implications because the study did not look at pathophysiological or pharmacological influences on long-term particle retention in the airways. The next step, said Dr. Möller, is to better understand the origins of such retention.

—Timothy Begany

Reference
1. Möller W, Häussinger K, Winkler-Heil R, et al. Mucociliary and long-term particle clearance in the airways of healthy nonsmoker subjects. J Appl Physiol. 2004;97:2200-2206.

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