Postnatal enlargement of human tracheobronchial airways and implications for particle deposition

Abstract

In support of predictions for inhaled particle deposition, morphometric measurements were taken on 20 replica airway casts of people aged 11 days to 21 years. Measurements of right upper lobe airway lengths, diameters, and branching angles were made such that a growth model suitable as input to predictive equations for particle deposition efficiency was obtained. The tracheobronchial airways growth was describable by linear regressions on body length. The length-todiameter ratio of growing airways did not change in any simple way as a function of airway generation. Airflow rates for a given state of physical activity for various ages were found from previously published data to be describable by linear regressions on body mass. Three states of physical exertion-low activity, light exertion, and heavy exertion-were used for modeling purposes. The computed particle deposition efficiencies indicate that under most circumstances smaller (younger) people will have greater tracheobronchial deposition efficiencies than larger (older) people. For example, tracheobronchial dose on a per kilogram body mass basis for 5-pmdiameter particles may be more than 6 times higher in the resting newborn than in the resting adult assuming equivalent deposition efficiencies above the larynx.Because little is known regarding age-related differ-(1913) published values for lengths and diameters (sagences in inhaled particle deposition, we have been inter-ittal and frontal) of the trachea and right and left main ested in developing mathematical predictions for particle bronchi for 12 children aged 1 month to 13 years, and a deposition in the tracheobronchial airways correspond-40-year-old adult. Engel's data show relatively rapid but ing to a range of ages. Such predictions are strongly decelerating growth of the measured structures up to dependent on airway anatomy, so measurements of the about age 7 years, but with only one child over age 10 growing airways were taken to develop a model upon years little can be said about growth after age 7. Also, which to base the mathematical predictions for depo-only two generations of airways were well described. sition.Although there is agreement on the sequence of events Although several investigators have provided quanti-during postnatal lung growth, many fundamental questative descriptions of the dimensions of adult human tions remain. There is general agreement (although perairways, few have described the manner in which these haps a lack of direct proof) that the full number of structures grow. Among the earliest morphometric bronchial airways are present at birth and that alveolarmodels for the complete adult tracheobronchial airways ization of bronchioles continues postnatally in a proxiwere those published by Weibel(1963). To generate these mal direction. Bronchial airway growth generally models, measurements from the trachea downward for parallels changes in stature, but there is confusion reabout 10 generations were made on a plastic replica cast garding details. This state of und...

Methods for modeling particle deposition as a function of age

“…Morphometric measurements on 20 in-situ casts for children aged 11 days to 21 years were used to construct airway length and diameter predictions as linear functions of body height (Phalen et al, 1985). The relationships proposed were:…”

“…Table 2 also provides average tracheobronchial dead space values because dead space variations will influence airway sizes and the inhaled aerosol deposition pattern. (Phalen et al, 1985;NCRP, 1997).…”

“…Many factors, that may or may not be significant, are usually ignored in the particle deposition (Phalen et al, 1985;NCRP, 1997). models.…”

“…Are fluid dynamic factors such as non-axial and turbulent flows included? Table 3 Estimated thoracic deposition efficiencies in the adult and 2-year old child tracheobronchial anatomical model of Phalen et al (1985) scaled to dead space volumes of 20 cm 3 and 40 cm 3 for a low level of physical activity …”

“…Pioneering work on adopting these computational methods to children was motivated by radiation protection considerations -from nuclear weapon or accident fallout, natural radon in homes, and other natural and anthropogenic sources (Hofmann et al, 1979;Crawford, 1982;Hofmann, 1982;Crawford and Eckerman, 1983). More recent modeling efforts were stimulated by an interest in other environmental contaminants including combustion products as well as inhaled medications (Phalen et al, 1985;Xu and Yu, 1986;Yu and Xu, 1987;Ferron et al, 1989;Martonen and Zhang, 1993).…”