The Effect of Compliance in the Upper Airway

Abstract

Nasal high flow therapy (NHFT) has been previously studied on anatomically correct physical airway models using a benchtop setup. In the current study it was desired to elaborate on previous work by incorporating physiological accuracy in the model by simulating the compliance of the internal soft tissues in the airway. Three tissues of interest were investigated: the soft palate, the tongue and the vocal folds. A multi-part mouth open airway model was designed to fit interchangeable compliant structures and fabricated using 3D printing. The model was tested with soft and rigid inserts to compare between compliant and rigid boundary conditions, respectively. Compliance was simulated by fabricating the soft inserts with a silicone resin that matched the elastic modulus of the relevant biological tissues. Pressure measurements were conducted along the airway at different points using pressure sensing probes that were inserted in specially designed taps located along the model. Breathing was simulated using a pulsatile pump and experiments were carried out for natural and NHFT assisted breathing. It was discovered that the soft palate was the only compliant structure that affected the pharyngeal airway pressures for all tested breathing cases. During natural breathing, the compliant soft palate caused pharyngeal pressures to be more negative at peak inspiration by 15.5 ± 5.7 % to 35.3 ± 12.1 % of the corresponding peak-to-peak pressures of the rigid airway model. Greater pressures were measured in the pharynx at peak expiration, with an increase by 3.8 ± 2.6 % to 10.7 ± 2.7 %. During NHFT assisted breathing at 30 L/min, the compliant airway experienced a greater peak inspiratory pressure at the velopharynx only with an increase of 8.0 ± 5.5 %, and globally increased peak expiratory pressures by 9.3 ± 1.1 % to 23.7 ± 5.5 %. During NHFT assisted breathing at 60 L/min, peak inspiratory and expiratory pressures were greater than corresponding airway pressures in the rigid model. Peak inspiratory pressures increased by 6.7 ± 4.6 % to 20.6 ± 7.6 % and expiratory pressures increased by 27.9 ± 4.4 % to 45.6 ± 10.6 %. It was hypothesised that flow induced motion of the compliant soft palate increased the resistance to the air flowing through the oral cavity and hence why it affected the pressures globally throughout the airway. The tongue and vocal folds showed no statistically significant difference, concluding that the compliance of these tissues does not affect breathing pressures. The capnography experiments concluded that, once again only soft palate compliance affected CO2 gas mixing; however, this was limited to the oral cavity region and only for the NHFT assisted conditions.