How does the absence or presence of subglottal medialization affect glottal airflow?

OBJECTIVES: Our previous work has shown that the symmetric, smooth, convergent shape of the subglottis reduces turbulent airflow at the glottal entrance. Medialization thyroplasty may alter the glottal shape and is very likely to introduce some degree of glottal asymmetry, which could result in increased turbulence and a reduction in voice quality. This study reports the effects of medializing and not medializing the subglottis in silicone models of human cadaveric larynges. METHODS: In experiment 1, silicone models of 4 human cadaveric larynges were created. The subglottis was then completely medialized in all 4 models. Hot-wire anemometry was used to measure velocity and turbulence profiles at the entrance and exit of the subglottis. In experiment 2, 1 model was created to accommodate incremental medialization of the glottis without any medialization of the subglottis. Airflow characteristics were likewise measured. RESULTS: In experiment 1, the average maximum turbulence intensity (TI) at the exit of the larynx was less than the TI of incoming tracheal airflow for all 4 larynges. In experiment 2, incremental medialization of the glottis did not affect the TI for medialization up to 35%. However, the TI significantly increased for medialization of 53%. CONCLUSIONS: Medialization of the subglottis does not significantly affect the turbulence reduction properties of the subglottis, even though subglottal asymmetry is introduced. On the other hand, large amounts of medialization of the glottis only (with no subglottal medialization) can introduce significant amounts of turbulence.

Acoustic characteristics of phonation in "wet voice" conditions.

A perceptible change in phonation characteristics after a swallow has long been considered evidence that food and/or drink material has entered the laryngeal vestibule and is on the surface of the vocal folds as they vibrate. The current paper investigates the acoustic characteristics of phonation when liquid material is present on the vocal folds, using ex vivo porcine larynges as a model. Consistent with instrumental examinations of swallowing disorders or dysphagia in humans, three liquids of different Varibar viscosity ("thin liquid," "nectar," and "honey") were studied at constant volume. The presence of materials on the folds during phonation was generally found to suppress the higher frequency harmonics and generate intermittent additional frequencies in the low and high end of the acoustic spectrum. Perturbation measures showed a higher percentage of jitter and shimmer when liquid material was present on the folds during phonation, but they were unable to differentiate statistically between the three fluid conditions. The finite correlation dimension and positive Lyapunov exponent measures indicated that the presence of materials on the vocal folds excited a chaotic system. Further, these measures were able to reliably differentiate between the baseline and different types of liquid on the vocal folds.

Radiographic anatomy of the infracochlear approach to the petrous apex for computer-assisted surgery.

OBJECTIVE: 1) To define the surgical anatomy and dimensions of the infracochlear approach to the petrous apex through the use of high-resolution computed tomography and 2) use of digitized images of cadaveric temporal bones for computer simulation of infracochlear access using the Ohio Supercomputer Center/Ohio State University temporal bone simulator. BACKGROUND: The petrous apex is a surgically challenging area to access. Many routes have been described and used successfully in clinical practice. However, these routes have not been defined with the aim of application in computer-assisted surgery. The infracochlear approach, due to its access via a transcanal route, affords the opportunity for its potential application in minimally invasive computer-assisted surgery. METHODS: High-resolution computed tomographic scans were performed on 102 cadaveric skulls (204 temporal bones). Standard measurements were taken using an open-source picture archiving and communication system software of the maximum height, width, and depth of the infracochlear approach. In addition, the maximum diameter of a circular fenestration that could be created in the infracochlear space without breaching the basal turn of the cochlea, internal carotid artery, or the jugular bulb was used to simulate a drill path. In addition, 5 temporal bone specimens (3 left, 2 right) underwent high-resolution computed tomography, with the digitized images being used to create simulated temporal bones for infracochlear surgical access; the transcanal infracochlear approach was then performed by the same surgeon on the cadaveric bone. RESULTS: The mean height, width, and depth of the infracochlear space in temporal bones with nonpneumatized petrous apices were 7.2 +/- 0.4, 9.4 +/- 0.8, and 17.5 +/- 1.0 mm, respectively. Corresponding dimensions in pneumatized petrous apices were 7.6 +/- 0.4, 10.1 +/- 1.1, and 18.6 +/- 0.8 mm, respectively. The mean diameter of the circular fenestra in the nonpneumatized petrous apices was 5.1 +/- 0.4 compared with 5.7 +/- 0.6 mm in pneumatized petrous pieces. This was statistically significant (unpaired t test; p value = 0.04). The time to perform a simulated infracochlear approach to the petrous apex ranged from 3.1 to 12.6 minutes (mean, 6.1 minutes). The time to perform the same approach on the cadaveric bone ranged from 4.32 to 14.1 minutes (mean, 9.3 minutes). CONCLUSION: Temporal bones with pneumatized petrous apices have an overall larger infracochlear space. The mean diameter of a circular infracochlear path that would avoid damage to vital structures was sufficiently large in both pneumatized and nonpneumatized petrous apices to have a potential application as a safe approach in computer-assisted surgery. Such an application is feasible with mating of a robotic system with computed tomographic- or magnetic resonance imaging-guided imagery, which is the next phase of this study.

Unsteady laryngeal airflow simulations of the intra-glottal vortical structures.

The intra-glottal vortical structures developed in a static divergent glottis with continuous flow entering the glottis are characterized. Laryngeal airflow calculations are performed using the Large Eddy Simulation approach. It has been shown that intra-glottal vortices are formed on the divergent wall of the glottis, immediately downstream of the separation point. Even with non-pulsatile flow entering the glottis, the vortices are intermittently shed, producing unsteady flow at the glottal exit. The vortical structures are characterized by significant negative static pressure relative to the ambient pressure. These vortices increase in size and strength as they are convected downstream by the flow due to the entrained air from the supra-glottal region. The negative static pressures associated with the intra-glottal vortical structures suggest that the closing phase during phonation may be accelerated by such vortices. The intra-glottal negative pressures can affect both vocal fold vibration and voice production.

Influence of gender on pharyngeal airway length in obese adolescents.

OBJECTIVES: Although pharyngeal airway length has been implicated in an increased male predisposition for obstructive sleep apnea (OSA) in adults, data in obese children and adolescents are lacking. Our objective was to determine the influence of gender on pharyngeal airway length in obese adolescents, and to apply computational simulations to better understand the effect of pharyngeal airway length on the airway's predisposition to collapse in this select group. METHODS: Obese subjects without OSA were recruited from our Sleep Center. Their pharyngeal airway length was measured on midline sagittal magnetic resonance images as the distance between the hard palate and the base of the epiglottis. Computational fluid dynamics analysis was used to study the effect of pharyngeal airway length on airflow characteristics. The gender groups were compared for anthropometric measurements and pharyngeal airway length by an unpaired Student's t-test. RESULTS: Our study group included 18 female and 16 male obese adolescents with a mean (+/-SD) age of 14.7 +/- 2.3 years and a mean body mass index of 38.9 +/- 6.9 kg/m2. The groups did not differ in age, body weight, or normalized pharyngeal airway length (0.44 +/- 0.08 mm/cm in girls versus 0.44 +/- 0.11 mm/cm in boys; p = 0.9). The computational fluid dynamics simulation indicated that the 3-dimensional flow field and airway wall pressures were not significantly affected by pharyngeal airway lengthening of up to 10 mm. CONCLUSIONS: Our results indicate that in obese adolescents, there is no influence of gender on pharyngeal airway length, and pharyngeal airway length alone does not significantly affect the airway's predisposition to collapse. These findings suggest that pharyngeal airway length may not explain the increased male gender predisposition for OSA in this select group.

Adding the third dimension--a new tool for constructing 3D models of the airway from 2D bronchoscopic video.

OBJECTIVE: To develop a new method of using existing bronchoscopic video technology to generate a 3D model of the airway for clinical purposes. DESIGN: Prospective pilot study with clinical correlation. METHODS: A Storz 7200 bronchoscope was used to obtain video of a standardized tube. The images were then processed using "open source" tools which detected feature points. A three dimensional model was then constructed using these feature points. An in-house 3D image program was then used to compare the 3D model with the standardized tube. Video from a representative airway patient who had previously had a CT of the chest and a bronchoscopic examination was also analyzed using this technique. The 3D model was correlated with CT images to clinically validate this technique. SETTING: Tertiary care hospital. PATIENTS: One airway patient video was used for clinical validation. OUTCOME MEASURES: (1) Average diameters of the 3D video derived tube model were compared to the actual tube and (2) a cross section of the 3D video derived patient model was compared to the patient CT derived model. RESULTS: Repeated measures on standardized tubes demonstrated that is it possible to construct an airway model using this novel technique with a less than 5% error. Further, it is possible to construct a 3D model from patient video using existing bronchoscopic technology. CONCLUSIONS: It is possible to extract 3D data from a sequence of 2D images. Further, this 3D model can be used for the purposes of management and planning and is quantitatively accurate and reliable. Initial data suggests that these measurements correlate with actual airway size and may provide a better instrument with which to make surgical decisions.

Validation of computational fluid dynamics methodology used for human upper airway flow simulations.

An anatomically accurate human upper airway model was constructed from multiple magnetic resonance imaging axial scans. This model was used to conduct detailed Computational Fluid Dynamics (CFD) simulations during expiration, to investigate the fluid flow in the airway regions where obstruction could occur. An identical physical model of the same airway was built using stereo lithography. Pressure and velocity measurements were conducted in the physical model. Both simulations and experiments were performed at a peak expiratory flow rate of 200 L/min. Several different numerical approaches within the FLUENT commercial software framework were used in the simulations; unsteady Large Eddy Simulation (LES), steady Reynolds-Averaged Navier-Stokes (RANS) with two-equation turbulence models (i.e. k-epsilon, standard k-omega, and k-omega Shear Stress Transport (SST)) and with one-equation Spalart-Allmaras model. The CFD predictions of the average wall static pressures at different locations along the airway wall were favorably compared with the experimental data. Among all the approaches, standard k-omega turbulence model resulted in the best agreement with the static pressure measurements, with an average error of approximately 20% over all ports. The highest positive pressures were observed in the retroglossal regions below the epiglottis, while the lowest negative pressures were recorded in the retropalatal region. The latter is a result of the airflow acceleration in the narrow retropalatal region. The largest pressure drop was observed at the tip of the soft palate. This location has the smallest cross section of the airway. The good agreement between the computations and the experimental results suggest that CFD simulations can be used to accurately compute aerodynamic flow characteristics of the upper airway.

Role of subglottal shape in turbulence reduction.

OBJECTIVES: In previous work, we found that airflow at the superior edge of the vocal folds, in the excised canine larynx, can be laminar even when the tracheal airflow is predominantly turbulent. Turbulent flow directly above the folds may lead to an irregular or "rough" voice. Thus, it is important to determine the mechanism of turbulence reduction. From fluid mechanics, it is known that a smoothly converging duct will reduce turbulence. In this study, we tested the hypothesis that the majority of the turbulence reduction is due to the smooth converging shape of the subglottis. METHODS: In 3 excised canine larynges, hot-wire anemometry was used to measure the turbulence intensity (TI) below the cricoid cartilage and 2 to 3 mm above the superior edge of the vocal folds. Laminar flow was seen when the TI was approximately less than 2%. For our measurements, flow into the subglottis had an average TI of more than 20% (high turbulence) in the shear layer and a TI of more than 15% in the center of the jet. The larynges were tested under steady conditions (folds not phonating) with the vocal processes approximated. RESULTS: For the center of the jet, there is moderate turbulence below the cricoid cartilage and laminar flow 2 to 3 mm above the folds. For the shear layer, there is very high turbulence below the cricoid cartilage and low turbulence 2 to 3 mm above the folds. CONCLUSIONS: The smooth converging shape of the subglottis can produce a significant reduction in turbulence. These findings may have important voice implications for operations that may change the subglottal shape (such as vocal fold medialization or airway reconstruction).

Flow fields and acoustics in a unilateral scarred vocal fold model.

OBJECTIVES: From prior work in an excised canine larynx model, it has been shown that intraglottal vortices form between the vocal folds during the latter part of closing. It has also been shown that the vortices generate a negative pressure between the folds, producing a suction force that causes sudden, rapid closing of the folds. This rapid closing will produce increased loudness and increased higher harmonics. We used a unilateral scarred excised canine larynx model to determine whether the intraglottal vortices and resulting acoustics were changed, compared to those of normal larynges. METHODS: Acoustic, flow field, and high-speed imaging measurements from 5 normal and 5 unilaterally scarred canine larynges are presented in this report. Scarring was produced by complete resection of the vocal fold mucosa and superficial layer of the lamina propria on the right vocal fold only. Two months later, these dogs were painlessly sacrificed, and testing was done on the excised larynges during phonation. High-speed video imaging was then used to measure vocal fold displacement during different phases. Particle image velocimetry and acoustic measurements were used to describe possible acoustic effects of the vortices. RESULTS: A higher phonation threshold was required to excite the motion of the vocal fold in scarred larynges. As the subglottal pressure increased, the strength of the vortices and the higher harmonics both consistently increased. However, it was seen that increasing the maximum displacement of the scarred fold did not consistently increase the higher harmonics. The improvements that result from increasing subglottal pressure may be due to a combination of increasing the strength of the intraglottal vortices and increasing the maximum displacement of the vocal fold; however, the data in this study suggest that the vortices play a much more important role. CONCLUSIONS: The current study indicates that higher subglottal pressures may excite higher harmonics and improve loudness for patients with unilateral vocal fold scarring. This finding implies that therapies that raise the subglottal pressure may be helpful in improving voice quality.

Modeling flow in a compromised pediatric airway breathing air and heliox.

OBJECTIVES/HYPOTHESIS: The aim of this study was to perform computer simulations of flow within an accurate model of a pediatric airway with subglottic stenosis. It is believed that the airflow characteristics in a stenotic airway are strongly related to the sensation of dyspnea. METHODS: Computed tomography images through the respiratory tract of an infant with subglottic stenosis were used to construct the three-dimensional geometry of the airway. By using computational fluid dynamics (CFD) modeling to capture airway flow patterns during inspiration and expiration, we obtained information pertaining to flow velocity, static airway wall pressure, pressure drop across the stenosis, and wall shear stress. These simulations were performed with both air and heliox (helium-oxygen mixture). RESULTS: Unlike air, heliox maintained laminar flow through the stenosis. The calculated pressure drop over stenosis was lower for the heliox flow in contrast to the airflow case. This led to an approximately 40% decrease in airway resistance when using heliox and presumably causes a decrease in the level of effort required for breathing. CONCLUSIONS: CFD simulations offer a quantitative method of evaluating airway flow dynamics in patients with airway abnormalities. CFD modeling illustrated the flow features and quantified flow parameters within a pediatric airway with subglottic stenosis. Simulations with air and heliox conditions mirrored the known clinical benefits of heliox compared with air. We anticipate that computer simulation models will ultimately allow a better understanding of changes in flow caused by specific medical and surgical interventions in patients with conditions associated with dyspnea.

Role of vortices in voice production: normal versus asymmetric tension.

OBJECTIVES: Decreasing the closing speed of the vocal folds can reduce loudness and energy in the higher frequency harmonics, resulting in reduced voice quality. Our aim was to study the correlation between higher frequencies and the intraglottal vorticity (which contributes to rapid closing by producing transient negative intraglottal pressures). METHODS: Using six excised canine larynges (three with symmetric and three with asymmetric, periodic vocal fold motion), intraglottal vorticity was calculated from 2D velocity fields measured using particle imaging velocimetry. RESULTS: There is a strong correlation between intraglottal vorticity and acoustic energy in the higher frequencies; in periodic asymmetric motion, the vorticity and higher frequencies are both reduced. CONCLUSIONS: For unilateral vocal fold paralysis, these findings suggest one reason why periodic, asymmetric motion, may produce an abnormal voice. Further study will help determine when and why reinnervation, as opposed to medialization, may result in better voice quality.

Modeling flow in a compromised pediatric airway breathing air and heliox.

OBJECTIVES/HYPOTHESIS: The aim of this study was to perform computer simulations of flow within an accurate model of a pediatric airway with subglottic stenosis. It is believed that the airflow characteristics in a stenotic airway are strongly related to the sensation of dyspnea. METHODOLOGY: Computed tomography images through the respiratory tract of an infant with subglottic stenosis, were used to construct the three-dimensional geometry of the airway. By using computational fluid dynamics (CFD) modeling to capture airway flow patterns during inspiration and expiration, we obtained information pertaining to flow velocity, static airway wall pressure, pressure drop across the stenosis, and wall shear stress. These simulations were performed with both air and heliox. RESULTS: Unlike air, heliox maintained laminar flow through the stenosis. The calculated pressure drop over stenosis was lower for the heliox flow, in contrast to the airflow case. This lead to an approximately 40% decrease in airway resistance when using heliox, and presumably causes a decrease in the level of effort required for breathing. CONCLUSIONS: CFD simulations offer a quantitative method of evaluating airway flow dynamics in patients with airway abnormalities. CFD modeling illustrated the flow features and quantified flow parameters within a pediatric airway with subglottic stenosis. Simulations with air and heliox conditions mirrored the known clinical benefits of heliox as compared with air. We anticipate that computer simulation models will ultimately allow a better understanding of changes in flow caused by specific medical and surgical interventions in patients with conditions associated with dyspnea.

Large Eddy Simulation and Reynolds-Averaged Navier-Stokes modeling of flow in a realistic pharyngeal airway model: an investigation of obstructive sleep apnea.

Computational fluid dynamics techniques employing primarily steady Reynolds-Averaged Navier-Stokes (RANS) methodology have been recently used to characterize the transitional/turbulent flow field in human airways. The use of RANS implies that flow phenomena are averaged over time, the flow dynamics not being captured. Further, RANS uses two-equation turbulence models that are not adequate for predicting anisotropic flows, flows with high streamline curvature, or flows where separation occurs. A more accurate approach for such flow situations that occur in the human airway is Large Eddy Simulation (LES). The paper considers flow modeling in a pharyngeal airway model reconstructed from cross-sectional magnetic resonance scans of a patient with obstructive sleep apnea. The airway model is characterized by a maximum narrowing at the site of retropalatal pharynx. Two flow-modeling strategies are employed: steady RANS and the LES approach. In the RANS modeling framework both k-epsilon and k-omega turbulence models are used. The paper discusses the differences between the airflow characteristics obtained from the RANS and LES calculations. The largest discrepancies were found in the axial velocity distributions downstream of the minimum cross-sectional area. This region is characterized by flow separation and large radial velocity gradients across the developed shear layers. The largest difference in static pressure distributions on the airway walls was found between the LES and the k-epsilon data at the site of maximum narrowing in the retropalatal pharynx.

What can vortices tell us about vocal fold vibration and voice production.

PURPOSE OF REVIEW: Much clinical research on laryngeal airflow has assumed that airflow is unidirectional. This review will summarize what additional knowledge can be obtained about vocal fold vibration and voice production by studying rotational motion, or vortices, in laryngeal airflow. RECENT FINDINGS: Recent work suggests two types of vortices that may strongly contribute to voice quality. The first kind forms just above the vocal folds during glottal closing, and is formed by flow separation in the glottis; these flow separation vortices significantly contribute to rapid closing of the glottis, and hence, to producing loudness and high frequency harmonics in the acoustic spectrum. The second is a group of highly three-dimensional and coherent supraglottal vortices, which can produce sound by interaction with structures in the vocal tract. Present work is also described that suggests that certain laryngeal pathologies, such as asymmetric vocal fold tension, will significantly modify both types of vortices, with adverse impact on sound production: decreased rate of glottal closure, increased broadband noise, and a decreased signal to noise ratio. SUMMARY: Recent research supports the hypothesis that glottal airflow contains certain vortical structures that significantly contribute to voice quality.

Computational fluid dynamics analysis of upper airway reconstructed from magnetic resonance imaging data.

OBJECTIVES: We performed flow computations on an accurate upper airway model in a patient with obstructive sleep apnea and computed the velocity, static pressure, and wall shear stress distribution in the model. METHODS: Cartesian coordinates for airway boundaries were determined from cross-sectional magnetic resonance images, and a 3-dimensional computational model of the upper airway was constructed. Flow simulations were then performed within a FLUENT commercial software framework. Four different flow conditions were simulated during inspiration, assuming the steady-state condition. The results were analyzed from the perspectives of velocity, static pressure, and wall shear stress distribution. RESULTS: We observed that the highest axial velocity was at the site of minimum cross-sectional area (retropalatal pharynx) resulting in the lowest level of wall static pressure. The highest wall shear stresses were at the same location. The pressure drop was significantly larger for higher flow rates than for lower flow rates. CONCLUSIONS: Our results indicate that the presence of airway narrowing, through change in the flow characteristics that result in increased flow velocity and reduced static pressure, can itself increase airway collapsibility. Additionally, the effects of wall shear stress on airway walls may be an important factor in the progression over time of the severity of obstructive sleep apnea.

Using particle imaging velocimetry to measure anterior-posterior velocity gradients in the excised canine larynx model.

OBJECTIVES: To quantify the anterior-posterior velocity gradient, we studied the velocity flow fields above the vocal folds in both the midcoronal and midsagittal planes. It was also our purpose to use these fields to deduce the mechanisms that cause the anterior-posterior gradient and to determine whether the vortical structures are highly 3-dimensional. METHODS: Using the particle imaging velocimetry method for 5 excised canine larynges, we obtained phase-averaged velocity fields in the midcoronal and midsagittal planes for 30 phases of phonation. The velocity fields were determined synchronously with the vocal fold motion recorded by high-speed videography. RESULTS: The results show that immediately above the folds, there is no significant anterior-posterior velocity gradient. However, as the flow travels downstream, the laryngeal jet tends to narrow in width and skew toward the anterior commissure. Vortices are seen at the anterior and posterior edges of the flow. CONCLUSIONS: The downstream narrowing in the midsagittal plane is consistent with and is probably due to a phenomenon known as axis switching. Axis switching also involves vortices in the sagittal and coronal planes bending in the axial plane. This results in highly 3-dimensional, complex vortical structures. However, there is remarkable cyclic repeatability of these vortices during a phonation cycle.

Computational modeling of upper airway before and after adenotonsillectomy for obstructive sleep apnea.

Adenotonsillectomy, the first-line surgical treatment for obstructive sleep apnea (OSA) in children, is successful in only 50% of obese children. Computational fluid dynamics tools, which have been applied to differentiate OSA patients from those without OSA based on the airway flow characteristics, can be potentially used to identify patients likely to benefit from surgical intervention. We present computational modeling of the upper airway before and after adenotonsillectomy in an obese female adolescent with OSA. The subject underwent upper airway imaging on a 1.5 Tesla magnetic resonance imaging (MRI) scanner, and three-dimensional airway models were constructed using airway boundary coordinates from cross-sectional MRI scans. Our results using computational simulations indicate that, in an obese child, the resolution of OSA after adenotonsillectomy is associated with changes in flow characteristics that result in decreased pressure differentials across the airway walls and thus lower compressive forces that predispose to airway collapse. Application of such findings to an obese child seeking surgical treatment for OSA can potentially lead to selection of the surgical procedure most likely to result in OSA resolution. Effective intervention for OSA in this high-risk group will result in reduction in morbidity and the public health concerns associated with OSA.