Case Studies in the Human Respiratory System - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 312

of the right main bronchus. This feature also affects small Stokes number particles,
where approximately 40 % of the particles introduced at the trachea inlet penetrate
past the sixth generation in the right lung airways. Therefore the efficacy of drug
delivery will be low if the occluded airway is deep in the left lung airways and will
lead to acute overdose in the localised regions of the right lung airways.

8.5.6 Closure

This case study moves beyond the nasal cavity to present the flow and particle depo-
sition patterns in the lung airways. Two models with six generations of the airway
tree from an acute asthma episode and following recovery of the same patient thirty
days apart were reconstructed from computed tomography (CT) scans in order to
investigate the effects of acute asthma on a realistic airway geometry, the airflow
patterns, the pressure drop, and the implications they have on targeted drug delivery.
The comparisons in the geometry found that in general the right side of the airway is
larger in diameter than the left side. The recovery of the airway was most significant
in the severely asthma-affected regions. Overall, the right airway exhibited greater
dilation in comparison with the left airway, especially from the fifth generation on-
wards. The required pressure difference at the inlet for the AA-model was nearly
twice the value for the recovered model. This suggests that during the period of an
acute asthma episode, the work of breathing for the patient in order to achieve the
same tidal volumes is double compared to that in the recovered state, which can lead
to respiratory muscle fatigue. Particle deposition patterns showed that the changes
in the airway had significant influence on flow patterns. A majority of the particles
deposited passed through or deposited in the right lung airways, as a consequence
of the biased carina bifurcation. It was also shown that the narrowing of the airway
magnifies the effects of the airway curvatures. This means that studies of therapeutic
drug delivery in the airway should consider the effects of airway narrowing and not
a recovered or a healthy airway.

8.6 Summary

Case studies were presented in this chapter to demonstrate the modelling strategies to
account for the inhalation of air and particles in the human respiratory system. In the
first case study, modelling requirements such as the flow regime (laminar/turbulent,
steady/unsteady) for steady inhalation and the heat transfer process were shown. The
first case study aimed to set a good foundation for the inclusion of particles into the
nasal cavity.
After establishing the airflow field, particles were introduced into the airway. The
modelling requirements for different particle morphology such as spherical, non-
spherical, submicron, and fibrous particles were shown. A link between the nasal
cavity geometry, the airflow field, and particle deposition was also found.