Aging effects on airflow dynamics and lung function in human bronchioles

Abstract

Background and objective: The mortality rate for patients requiring mechanical ventilation is about 35% and this rate increases to about 53% for the elderly. In general, with increasing age, the dynamic lung function and respiratory mechanics are compromised, and several experiments are being conducted to estimate these changes and understand the underlying mechanisms to better treat elderly patients.

Materials and methods: Human tracheobronchial (G1 ~ G9), bronchioles (G10 ~ G22) and alveolar sacs (G23) geometric models were developed based on reported anatomical dimensions for a 50 and an 80-year-old subject. The aged model was developed by altering the geometry and material properties of the model developed for the 50-year-old. Computational simulations using coupled fluid-solid analysis were performed for geometric models of bronchioles and alveolar sacs under mechanical ventilation to estimate the airflow and lung function characteristics.

Findings: The airway mechanical characteristics decreased with aging, specifically a 38% pressure drop was observed for the 80-year-old as compared to the 50-year-old. The shear stress on airway walls increased with aging and the highest shear stress was observed in the 80-year-old during inhalation. A 50% increase in peak strain was observed for the 80-year-old as compared to the 50-year-old during exhalation. The simulation results indicate that there is a 41% increase in lung compliance and a 35%-50% change in airway mechanical characteristics for the 80-year-old in comparison to the 50-year-old. Overall, the airway mechanical characteristics as well as lung function are compromised due to aging.

Conclusion: Our study demonstrates and quantifies the effects of aging on the airflow dynamics and lung capacity. These changes in the aging lung are important considerations for mechanical ventilation parameters in elderly patients. Realistic geometry and material properties need to be included in the computational models in future studies.

Fig 1. Human tracheobronchial airway model (a), and bronchioles and alveolar sacs.

G10 ~ G23 were region of interest where aging effects are considered in this study.

Fig 2. Boundary conditions for input, output, and alveolar sacs with computational mesh.

Ventilated waveform of flow rate at inlet G10, pressure waveform at all outlets (this figure shows pressure boundary condition at the outlet G20), and pressure waveform at the alveolar sac, were considered as boundary conditions for the 50-year-old and 80-year-old lungs. For the computational mesh, solid and fluid mesh elements of bronchioles were approximately 229,791 and 832,960; and for alveolar sacs they were approximately 82,530 and 201,432 respectively. The solid part was simulated on a single tissue layer.

Fig 3. Comparison of wall shear stresses in bronchiole airways for 50-year-old and 80-year-old during mechanical ventilation; contour of shear stress, normalized shear stress by generations, and normalized circumferential shear stress.

Fig 4. Comparison of wall shear stresses in alveolar sacs for 50-year-old and 80-year-old during mechanical ventilation; contour of shear stress, normalized shear stress by translational position, and normalized circumferential shear stress.

Fig 5. Comparison of pressure and wall shear stress by airflow for each age in the breathing cycle over the region of G10 ~ G23.

Mean values were computed by simulation for each age during inhalation, 0–0.4 s and exhalation 0.4–2.0 s. The pressure and shear stress at t = 0 is very close to 0 but not 0.

Fig 6. Comparison of strains in bronchiole airways between 50-year-old and 80-year-old for inhalation and exhalation at several positions; (a) longitudinal, and (b) circumferential.

Fig 7. Comparison of relative change of strains in alveolar sac between 50-year-old and 80-year-old for inhalation and exhalation at several positions; (a) longitudinal, and (b) circumferential.

Fig 8. Comparison of mechanical characteristics of airway wall in breathing cycle (inhalation: 0 ~ 0.4 second, exhalation: 0.4 ~ 2.0 second), (a) relative change of maximum strain between 50-year-old and 80-year-old, and (b) Von Mises stress of airway wall (tissue) for 50- and 80- year-old.

Fig 9. Variation of airway space: (a) airway space change with age at bronchiole region and (b) alveolar sacs.

Fig 10. Variation of Airway resistance and air-volumes with age.

Airway resistance was compared by Caio’s [6] and Koch’s [48] experiments, and volume also compared with Canadian Health Measures Survey (CHMS) [62] and Caio’s [6] study.

Fig 11. Comparison of relationship with volume-pressure loops for 50-year-old and 80-year-old.