Respiratory Tract Dimensions

It is important that various anatomic dimensions be considered when the biological effects of deposition are assessed. This is especially important when deposition is reported only as total or regional respiratory tract deposition, which is usually the case, and not deposition by unit surface area. Anatomic dimensions of the respiratory tract are listed in Table 1. The total surface area per airway generation increases rapidly, so surface area concentrations (determined using lung models) of deposited aerosol decline several orders of magnitude from large airways to the pulmonary region [6,7]. The magnitude of this decline depends mainly upon the size of the inhaled particle. For example, the surface concentration decreases more than 10-fold from airway generation 3 to generation 15 for a 1.0-mm particle and more than 100-fold for a 5-mm particle over the same generations. For all respirable sizes, surface concentration is highest in airway generation 3 [9]. Expression of deposition as mass in a region can be misleading. This concept has received relatively little attention in the study of the efficacy of therapeutic aerosol.

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