Effect of airflow and material models on tissue displacement for surgical planning of pharyngeal airways in pediatric down syndrome patients

Effect of tube length on the buckling pressure of collapsible tubes

BACKGROUND

The higher incidence of obstructive sleep apnea (OSA) in men than in women has been attributed to the upper airway being longer in men. The Starling resistor is the paradigm biomechanical model of upper airway collapse in OSA where a collapsible tube (representing the pharynx) is located between two rigid tubes (representing the nasal cavity and trachea). While the Starling resistor has been extensively studied due to its relevance to many physiological phenomena, the effect of tube length on tube collapsibility has not been quantified yet.

METHODS

Finite element analysis of a 3-dimensional collapsible tube subjected to a transmural pressure was performed in ANSYS Workbench. The numerical methods were validated with in vitro experiments in a silicone tube whose modulus of elasticity (361 ± 28 kPa) and dimensions (length = 100 mm, diameter = 22.2 mm, and wall thickness = 1.59 mm) were selected so that tube compliance was similar to pharyngeal compliance in humans during sleep. The buckling pressure (transmural pressure at which the tube collapses) was quantified in tubes of three different diameters (10 mm, 16 mm, and 22.2 mm) and ten length-to-diameter ratios (L/D = 4 to 13), while keeping the wall-thickness-to-radius ratio constant at 0.143.

RESULTS

The absolute value of the buckling pressure decreased from 4.7 to 3.3 cmH₂O (461-324 Pa) when L/D increased from 4 to 13. The buckling pressure was nearly independent from tube length for L/D >10.

CONCLUSIONS

Our finding that longer tubes are more collapsible than shorter tubes is consistent with the higher incidence of obstructive sleep apnea in males than females.

An Introduction to Biomedical Computational Fluid Dynamics

Computational fluid dynamics (CFD) is a tool that has been used by engineers for over 50 years to analyse heat transfer and fluid flow phenomena. In recent years, there have been rapid developments in biomedical and health research applications of CFD. It has been used to evaluate drug delivery systems, analyse physiological flows (e.g. laryngeal jet flow), facilitate surgical planning (e.g. management of intracranial aneurysms), and develop medical devices (e.g. vascular stents and valve prostheses). Due to the complexity of these fluid flows, it demands an interdisciplinary approach consisting of engineers, computer scientists, and mathematicians to develop the computer programs and software used to solve the mathematical equations. Advances in technology and decreases in computational cost are allowing CFD to be more widely accessible and therefore used in more varied contexts. Cardiovascular medicine is the most common area of biomedical research in which CFD is currently being used, followed closely by upper and lower respiratory tract medicine. CFD is also being used in research investigating cerebrospinal fluid, synovial joints, and intracellular fluid. Although CFD can provide meaningful and aesthetically pleasing outputs, interpretation of the data can be challenging for those without a strong understanding of mathematical and engineering principles. Future development and evolution of computational medicine will therefore require close collaboration between experts in engineering, computer science, and biomedical research. This chapter aims to introduce computational fluid dynamics and present the reader with the basics of biological fluid properties, the CFD method, and its applications within biomedical research through published examples, in hope of bridging knowledge gaps in this rapidly emerging method of biomedical analysis.

Airflow limitation in a collapsible model of the human pharynx: physical mechanisms studied with fluid-structure interaction simulations and experiments

The classical Starling Resistor model has been the paradigm of airway collapse in obstructive sleep apnea (OSA) for the last 30 years. Its theoretical framework is grounded on the wave-speed flow limitation (WSFL) theory. Recent observations of negative effort dependence in OSA patients violate the predictions of the WSFL theory. Fluid-structure interaction (FSI) simulations are emerging as a technique to quantify how the biomechanical properties of the upper airway determine the shape of the pressure-flow curve. This study aimed to test two predictions of the WSFL theory, namely (1) the pressure profile upstream from the choke point becomes independent of downstream pressure during flow limitation and (2) the maximum flowrate in a collapsible tube is V I max = A 3 / 2 ( ρ d A / d P ) - 1 / 2 , where ρ is air density and A and P are the cross-sectional area and pressure at the choke point respectively. FSI simulations were performed in a model of the human upper airway with a collapsible pharynx whose wall thickness varied from 2 to 8 mm and modulus of elasticity ranged from 2 to 30 kPa. Experimental measurements in an airway replica with a silicone pharynx validated the numerical methods. Good agreement was found between our FSI simulations and the WSFL theory. Other key findings include: (1) the pressure-flow curve is independent of breathing effort (downstream pressure vs. time profile); (2) the shape of the pressure-flow curve reflects the airway biomechanical properties, so that V I max is a surrogate measure of pharyngeal compliance.

How design characteristics of tracheostomy tubes affect the cannula and tracheal flows

OBJECTIVES

The aim of this study was to perform computational simulations of airflow within an anatomically accurate model of an adult trachea in different tracheostomy tube designs. We hypothesized that tracheal airflow in patients is significantly influenced by the geometry and size of these devices.

METHODS

The three-dimensional (3D) geometry of the trachea was reconstructed using computed tomography scans for an adult with no history of lung disease. 3D models of four cuffed tube designs, namely Tracoe, Portex, and Shiley Proximal and Distal tracheostomy tubes were generated using geometric modeling software. Transient simulations of airflow in the tube-airway assembly were performed for each tube using computational fluid dynamics (CFD).

RESULTS

Airflow velocity was higher for the Shiley tubes compared with Portex and Tracoe tubes. For all designs, the largest magnitude of inspiratory airflow turbulence was obtained midway in the trachea. The work of breathing, quantified by the resistance of the tracheostomy tube, was lowest for Tracoe. Maximum airway wall shear stress (WSS), defined as flow-induced frictional forces, occurred at the same spatial location in all cases. Low inspiratory WSS at the carina and high expiratory airway WSS at the cuff-airway interface were observed for the Tracoe and Portex tubes.

CONCLUSION

Our CFD model offers a promising approach not only for choosing a tracheostomy tube for a patient but for improving existing tracheostomy tube designs.

LEVEL OF EVIDENCE

NA Laryngoscope, 129:1791-1799, 2019.

Otolaryngologic management of Down syndrome patients: what is new?

PURPOSE OF REVIEW

The management of children with Down syndrome as it pertains to the otolaryngologist continues to evolve. Obstructive sleep apnea (OSA) has dominated the recent literature, but other topics including hearing loss, swallowing, and perioperative considerations are also reported.

RECENT FINDINGS

The prevalence of OSA in children with Down syndrome ranges from 57 to 73% in certain cohorts, and, whereas adentonsillectomy can decrease Apnea-Hypopnea Index, up to 80% may have persistent OSA. Surgical techniques involving reduction of the base of tongue are effective for those who fail adenotonsillectomy, and it is expected that drug-induced sleep endoscopy may improve outcomes. New technology is also on the horizon that can assist with diagnosis and treatment including computational modelling and upper airway stimulation. Children with Down syndrome may not respond to medical management of eustachian tube dysfunction as well as normally developing children. In addition, there is a high prevalence of inner ear anomalies, increasing the risk for sensorineural hearing loss.

SUMMARY

Questions remain pertinent to the otolaryngologist regarding the ideal management of children with Down syndrome. Additional studies are necessary, to optimize understanding and treatment of this complex population, in particular as opportunities develop with technological advances.

Biomechanics of the soft-palate in sleep apnea patients with polycystic ovarian syndrome

Highly compliant tissue supporting the pharynx and low muscle tone enhance the possibility of upper airway occlusion in children with obstructive sleep apnea (OSA). The present study describes subject-specific computational modeling of flow-induced velopharyngeal narrowing in a female child with polycystic ovarian syndrome (PCOS) with OSA and a non-OSA control. Anatomically accurate three-dimensional geometries of the upper airway and soft-palate were reconstructed for both subjects using magnetic resonance (MR) images. A fluid-structure interaction (FSI) shape registration analysis was performed using subject-specific values of flow rate to iteratively compute the biomechanical properties of the soft-palate. The optimized shear modulus for the control was 38 percent higher than the corresponding value for the OSA patient. The proposed computational FSI model was then employed for planning surgical treatment for the apneic subject. A virtual surgery comprising of a combined adenoidectomy, palatoplasty and genioglossus advancement was performed to estimate the resulting post-operative patterns of airflow and tissue displacement. Maximum flow velocity and velopharyngeal resistance decreased by 80 percent and 66 percent respectively following surgery. Post-operative flow-induced forces on the anterior and posterior faces of the soft-palate were equilibrated and the resulting magnitude of tissue displacement was 63 percent lower compared to the pre-operative case. Results from this pilot study indicate that FSI computational modeling can be employed to characterize the mechanical properties of pharyngeal tissue and evaluate the effectiveness of various upper airway surgeries prior to their application.