Assessment of Airway Geometry with Inert Aerosols

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Abstract
When aerosol particles settle in the lungs through still air, the decline in their number concentration can be used to estimate the caliber of intrapulmonary airways. For this purpose, the lungs are filled with particles by inspiration of a monodisperse aerosol. Particles are then allowed to settle with velocity v onto airway surfaces during breath-holding and particle recovery is measured in the expired air. This technique is widely used either as a single-breath or bolus recovery technique. The first technique is limited to the lung periphery. Single breaths are used to measure particle recovery R as a function of breath-hold time t, and effective peripheral airspace dimensions are estimated by r=−2v/πd(In R(t))/dt. In the bolus recovery technique, aerosol boluses are inspired within a fixed volume of air into a selected volumetric lung depth and, after breath-holding, particle recovery is measured from this depth. The effective radius of airways at the selected lung depth is estimated using the above equation. This technique is not limited to the lung periphery but allows charting of intrapulmonary airways. A new technique is presente i thi paper: th single-breath concentration technique. The lungs are filled with particles by single tidal inspirations of monodisperse aerosols. Th effectiv radius of intrapulmonyr/ airways located at lung depth V is estimated from the decline in particle number concentratio c(V,t) during breath-holding observed in the expired air at the end of expiration of air volume V −= -π/iId(ln c(V,t))/dt Since only a few single-breath manoeuvres are required by this technique to assess airway dimensions at any lung depth, this technique is less time consuming than the bolus recovery technique and therefore more suitable for medical application. Also its spatial resolution is superior to that of the bolus recovery technique so that in proximal airways larger dimensions are obtained with the single-breath concentration than with the bolus recovery technique. In the lung periphery, airway dimensions established using the two techniques aie In close agreemet.