During human growth and development from infancy to adulthood, dramatic changes occur in the respiratory system. It is important to understand respiratory airflow in different age groups in age-specific treatment of respiratory disorders. This study numerically investigated the age-related effects on inspiratory and expiratory airflow dynamics in four-generation lung airway models under normal breathing conditions. Tracheobronchial airway models of infant (6 month old), child (5 years old), and adult (25 years old) from sixth to ninth generations were constructed for the study. Computational fluid dynamics (CFD) was used to solve the equations governing the airflow. Results of this study indicate that as age increases, airflow velocity, pressure, and wall shear stress decrease for both inspiration and expiration in this particular subregion of the respiratory tract. During inspiration, the splitting of velocity streamlines at bifurcations increases with age. The opposite situation merging happens during expiration, and it also increases with age. The level of splitting and merging of streamlines here reflects the influence of respiratory mechanics in the age groups. The computational models provide new information on characteristics and patterns of age-dependent respiratory airflow in the sixth to ninth generations of tracheobronchial airways and can be applied in other generations.
During growth and development from infancy to adulthood, there is a change in lung volume, airway size, alveoli size and number, shape and stiffness of the thorax, and respiratory muscle strength [
To model the airflow dynamics in the airways of different age groups using CFD, detailed information on changes of anatomical and physiological parameters during growth is required. Some researchers investigated such changes with age. Hofmann [
Numerous studies have been carried out to investigate the airflow characteristics and particle transport and deposition in bifurcating airways of different age groups. Deng et al. [
The objective of this study was to investigate quantitatively the inspiratory and expiratory airflow characteristics (velocity, pressure, and wall shear stress) in tracheobronchial airways (G6–G9) of infant, child, and adult using CFD modeling. Computational results of airflow characteristics and airflow patterns are compared for the different age groups. The influence of age-dependent changes in airway geometry and respiratory parameters on airflow dynamics is assessed in this subregion of the respiratory tract.
Symmetric in-plane tracheobronchial airway (G6–G9) models of infant (6 months old), child (5 years old), and adult (25 years old) were constructed based on the Weibel 23-generation pulmonary model [
The geometric dimensions of the airway models.
Generation |
|
|
|
| ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Infant | Child | Adult | Infant | Child | Adult | Infant | Child | Adult | Infant | Child | Adult | |
6 | 3.5 | 5.6 | 8.8 | 1.0 | 1.8 | 2.8 | 1.8 | 3.0 | 4.6 | 0.09 | 0.15 | 0.23 |
7 | 2.9 | 4.7 | 7.4 | 0.9 | 1.5 | 2.3 | 1.4 | 2.4 | 3.6 | 0.07 | 0.12 | 0.18 |
8 | 2.5 | 4.0 | 6.3 | 0.7 | 1.2 | 1.8 | 1.2 | 2.0 | 3.0 | 0.06 | 0.10 | 0.15 |
9 | 2.1 | 3.3 | 5.3 | 0.6 | 1.0 | 1.5 | — | — | — | — | — | — |
(a) Schematic view of the tracheobronchial airway model (G6–G9) of (A) infant, (B) child, and (C) adult and (b) cross sections and flow path line in an airway model.
After importing the 3D airway models in ANSYS Fluent 16.2 software, unstructured tetrahedral meshes with inflation layers were generated. A mesh independence study on the flow solution was performed by comparing the average velocity of five points on the axis of right seventh generation (on flow path line
Air within the human respiratory tract is considered to be a homogeneous, Newtonian, and incompressible fluid. The Womersley numbers at the inlet of G6 are about 0.25, 0.34, and 0.44 for infant, child, and adult, respectively, during inspiration under normal condition. These indicate that the unsteady effects of the flow fields are relatively minor [
The velocity inlet and pressure outlet conditions were used at the inlets and outlets of the airway models, respectively, while the no-slip condition was imposed on the walls for inspiratory and expiratory airflow modeling. In this study, the breathing parameters tidal volume and respiratory rate were obtained from [
Respiratory parameters used in the CFD modeling.
Age group | Tidal volume (mL) | Respiratory rate (breaths/min) | Minute ventilation (mL/min) | Inspiration-to-expiration time ratio | Inspiratory flow rate (L/min) | Expiratory flow rate (L/min) |
---|---|---|---|---|---|---|
Infant | 39 | 36 | 1404 | 1 : 1.5 | 3.51 | 2.34 |
Child | 181 | 20 | 3620 | 1 : 1.7 | 9.77 | 5.75 |
Adult | 500 | 14 | 7000 | 1 : 1.7 | 18.90 | 11.12 |
The finite-volume-based CFD software ANSYS Fluent 16.2 was used for modeling of airflow in the constructed airway models. The governing equations were solved using a pressure-based solver. The SIMPLE algorithm was applied in the CFD solver for the pressure-velocity coupling. The second-order discretization scheme was used for the pressure term, and the second-order upwind discretization scheme for momentum terms. The underrelaxation factors 0.3 and 0.5 are selected for pressure and momentum, respectively. A residual of
The geometric modeling method of human airways used in this study is similar to that of Deng et al. [
Comparison of average velocities in middle cross sections of generations of the adult airway model.
Computational results of inspiratory and expiratory airflow in the models of tracheobronchial airways (G6–G9) of infant, child, and adult are presented in this section. The airflow characteristics such as velocity, pressure, and wall shear stress are selected for discussion and comparison. To make clear visibility, the figures displayed in this section are not scaled according to the dimensions of the airway they represent. All colour plots of a flow characteristic are shown along the same scale for inspiration and expiration.
Figures
Velocity contours at midplane and middle cross sections (1–4) of G6–G9 generation of (a) infant, (b) child, and (c) adult airway models during inspiration.
Velocity contours at midplane and middle cross sections (1–4) of G6–G9 generations of (a) infant, (b) child, (c) and adult airway models during expiration.
The airflow patterns during inspiration and expiration for different age groups are presented using velocity streamlines. The splitting of velocity streamlines at all bifurcations increases with age during inspiration as illustrated in Figure
Streamlines at the first bifurcation of (A) infant, (B) child, and (C) adult airway models during (a) inspiration and (b) expiration.
Figure
Comparison of inspiratory and expiratory airflow velocities at the center of middle cross sections of G6–G9 generations for different age groups airway models.
Figure
Wall pressure distribution in the airway model of (A) infant, (B) child, and (C) adult during (a) inspiration and (b) expiration.
Pressure contours at midplane of (A) infant, (B) child, and (C) adult airway models during (a) inspiration and (b) expiration.
Figure
Comparison of inspiratory and expiratory airflow pressure drop at the center of middle cross sections of G6–G9 generations for different age group airway models.
The wall shear stress contours in the airway models of different age groups for inspiration and expiration are presented in Figure
Wall shear stress distribution in the airway models of (A) infant, (B) child, and (C) adult during (a) inspiration and (b) expiration.
The study showed that respiratory airflow dynamics in the airway generations G6–G9 of infant, child, and adult is different; the airflow characteristics (velocity, pressure, and wall shear stress) decrease with age during inspiration and expiration; and there is variation of airflow pattern among the three age groups and between the two phases of respiration. The influences of the respiratory mechanics on the airflow in the age groups were reflected in the distribution of the velocity streamlines in the CFD modeling. The study further showed that wall shear stress is very small in the airways of the age groups and relatively higher for infants. This supported evidence that infants are more sensitive to the damages in the airway walls.
There is no data used in the research.
The author declares that there are no conflicts of interest regarding the publication of this paper.
I would like to thank the Institute of Computer Centre, IIT Roorkee, for providing me with necessary software packages I used in this study including ANSYS Fluent 16.2 and MATLAB R2015a. I also thank the Department of Mechanical and Industrial Engineering, IIT Roorkee, for providing me SOLIDWORKS 2013 Software.