Influence of the Inhalation Route on Tracheal Flow Structures in Patient-Specific Airways using 3D PTV

Using 3D particle-tracking velocimetry on a patient-specific, refractive-index matched tracheal model, this study demonstrates that while the presence of nasal and oral cavities significantly alters tracheal flow structures compared to idealized conditions, the specific inhalation route (oral versus nasal) has a minimal impact on the resulting flow patterns.

The Gist

Imagine your respiratory system as a complex, winding highway system that leads from the outside world deep into your lungs. For years, scientists trying to understand how air (and the tiny particles it carries, like viruses or medicine) moves through this highway have been looking at the map with a major blind spot. They often ignored the "entrance ramps"—your nose and mouth—and just assumed the air arrived at the main highway (the trachea) in a perfectly smooth, organized line, like cars merging onto a freeway in perfect formation.

This paper asks a simple but crucial question: Does it actually matter if you breathe through your nose or your mouth? Does the path you take to get to the highway change how the traffic flows once you're on it?

Here is the story of how the researchers found the answer, explained in everyday terms.

The Experiment: Building a "Ghost" Lung

To study this without hurting real people, the team built a super-accurate, life-sized model of a human upper airway.

• The Material: They used a special clear silicone that looks like glass but feels like rubber.
• The Trick: To see inside this clear tube without the walls distorting the view (like looking through a funhouse mirror), they filled the model with a mixture of water and glycerin. This liquid has the exact same "optical density" as the silicone, making the walls virtually invisible.
• The Eyes: They used high-speed cameras and tiny floating particles (like microscopic glitter) to track exactly how the liquid moved. This is called 3D Particle Tracking, which is like having a swarm of invisible drones filming the traffic from every angle at once.

They tested two scenarios:

1. Steady Flow: Like holding a deep breath and blowing air in smoothly.
2. Oscillating Flow: Like normal, calm breathing (in and out).

They tested this at two speeds: a slow, quiet breath and a faster, more active breath.

The Big Discovery: The "Entrance Ramp" Doesn't Change the "Highway"

The researchers expected that breathing through the nose (which is twisty and full of little shelves) would create a very different traffic pattern than breathing through the mouth (which is a wide, open tunnel).

The result was surprising: Once the air passed the throat and entered the main windpipe (trachea), it didn't matter much where it came from.

• The Analogy: Imagine two cars entering a highway. One car comes from a narrow, winding country road (the nose), and the other comes from a wide, straight city avenue (the mouth). You might expect them to merge into the highway in totally different ways.
  • What actually happened: By the time they reached the main highway, both cars were driving in almost the exact same lane, at almost the exact same speed, and in almost the exact same formation. The "traffic jam" or "swirl" caused by the nose or mouth smoothed itself out very quickly.

What Did Change the Flow?

If the nose vs. mouth didn't matter, what did?

1. The Shape of the Road (Anatomy): The biggest factor was the shape of the throat and windpipe itself. The air didn't flow in a perfect, round circle like water in a pipe. Instead, it formed a weird, "M-shaped" pattern, hugging the sides of the tube. This is because the human throat is bent and twisted, not a straight pipe.
2. The Speed of Traffic (Reynolds Number): When the air moved faster (like during exercise), the flow became more chaotic and spread out, filling the whole tube more evenly. When it was slow, it stayed more concentrated in the center.
3. The Rhythm (Womersley Number): This is a fancy way of saying "how fast you are breathing in and out." Changing the rhythm of the breath changed the shape of the swirls slightly, but not enough to change the overall picture.

Why Does This Matter?

You might think, "If the nose and mouth don't matter, why study this?"

• For Medicine: If you are designing an inhaler for asthma or delivering medicine to the lungs, you don't need to worry as much about whether the patient is breathing through their nose or mouth. The medicine will likely reach the same spots in the lungs regardless.
• For Viruses: If you are trying to understand how a virus spreads through the air in a room, you can simplify your computer models. You don't need to simulate the entire complex maze of the nose to know how the air moves in the lungs; you just need to know the shape of the throat.
• For Reality: The study proved that previous models which assumed air enters the lungs in a "perfect, smooth line" were wrong. Real air is messy and asymmetrical right from the start, but it settles into a predictable pattern quickly.

The Bottom Line

Think of your airways like a river. Whether the water starts in a narrow, rocky creek (the nose) or a wide, open bay (the mouth), by the time it reaches the main river channel (the trachea), the water is flowing in the same direction, with the same speed, and the same shape.

The study tells us that while the entrance is complex and unique, the journey through the main windpipe is surprisingly consistent, no matter how you take your first breath.

Technical Summary

1. Problem Statement and Motivation

Respiratory diseases, including pandemics like COVID-19, rely heavily on the transport and deposition of aerosol particles within the airways. The flow field in the trachea dictates how these particles move into the lower respiratory tract.

• The Gap: Previous experimental and numerical studies often simplified the upper airway geometry (omitting the nasal/oral cavities, pharynx, and larynx) and assumed idealized, fully developed laminar inflow conditions at the tracheal inlet.
• The Question: Does the specific route of inhalation (oral vs. nasal) significantly alter the complex, three-dimensional flow structures in the trachea, or do the downstream anatomical features (pharynx, larynx) homogenize the flow such that the entry route becomes negligible?
• Objective: To quantify the influence of the inhalation route on tracheal flow structures under physiologically relevant steady and oscillatory conditions using a patient-specific model.

2. Methodology

A. Experimental Model

• Geometry: A patient-specific silicone model of the human respiratory tract was manufactured. It includes the oral cavity, nasal cavity, pharynx, larynx, trachea, and bronchial tree down to the 7th generation.
  • The base geometry was derived from CT scans of a healthy adult male.
  • The nasal cavity was integrated from a separate CT dataset and merged with the oral/laryngeal model.
• Manufacturing: The model was created using a lost-core approach. A high-resolution 3D-printed wax core was printed, assembled, and then encased in silicone (RTV-615). The wax was subsequently melted out, leaving a rigid, non-compliant airway replica with wall thicknesses between 8–100 mm.
• Fluid: To enable optical measurements, a water-glycerin mixture (56.75% glycerin) was used. This fluid matches the refractive index of the silicone model ($n=1.406$), eliminating optical distortions.
• Seeding: The fluid was seeded with Orgasol particles ($d_p \approx 47.7 \, \mu m$) with a Stokes number $St \ll 1$, ensuring they faithfully follow the flow.

B. Flow Conditions

The study simulated two primary breathing regimes:

1. Steady Inhalation: Simulating constant flow.
2. Oscillatory Flow: Simulating calm breathing (inhalation/exhalation cycles) using a sinusoidal waveform.

• Parameters:
  • Reynolds Number ($Re_{Tr}$): 400 (resting) and 1200 (moderate ventilation).
  • Womersley Number ($Wo$): 3 and 4.5 (representing oscillation time scales).
• Routes: Individual inhalation was achieved by sealing either the oral or nasal cavity with custom-designed plugs, ensuring defined inflow conditions.

C. Measurement Technique

• 3D Particle Tracking Velocimetry (PTV): The study utilized the Shake-The-Box (STB) algorithm to capture the inherently 3D and asymmetric nature of the flow.
• Setup:
  • Three high-speed cameras (one at $0^\circ$, two at $25^\circ$) captured the flow.
  • Illumination was provided by a high-power Nd:YLF laser ($527$ nm).
  • Measurements were taken in the trachea, specifically focusing on cross-sections leading up to the first bifurcation.
• Validation: PTV results were cross-validated against previous PIV (Particle Image Velocimetry) data, showing excellent agreement.

3. Key Contributions

1. Refractive-Index Matched 3D PTV in a Full Airway Model: This is one of the few studies to fully resolve the 3D flow field in a patient-specific model that includes the entire upper airway (nasal/oral cavities through to the trachea) using high-resolution STB-PTV.
2. Systematic Comparison of Routes: It provides a direct, controlled comparison of oral vs. nasal inhalation under identical downstream boundary conditions, isolating the effect of the entry route.
3. Oscillatory Flow Analysis: Unlike many studies focusing on steady flow, this work characterizes the unsteady, oscillatory flow structures (varying $Wo$) relevant to actual breathing cycles.

4. Key Results

A. Influence of Upstream Anatomy vs. Inhalation Route

• Anatomy is Dominant: The presence of the upper airways (nasal/oral cavities, pharynx, larynx) fundamentally alters the flow entering the trachea. Instead of a parabolic profile, the flow exhibits asymmetric, "M-shaped" velocity profiles and strong secondary vortices.
• Route Independence: Despite the complex upstream geometry, no significant difference was found between oral and nasal inhalation in the tracheal flow field.
  • Velocity profiles in sagittal and coronal planes were nearly identical for both routes.
  • Contour plots of absolute velocity magnitude showed the same arc-shaped patterns and high-velocity cores for both routes.
  • The difference in effective volume flux between routes was calculated to be less than 6%.

B. Effect of Reynolds Number ($Re$)

• $Re = 400$: The flow exhibits a more parabolic shape, though still skewed due to tracheal inclination. The high-velocity core is distinct.
• $Re = 1200$: The velocity profiles become flatter and more diffuse. The high-velocity core broadens, and shear layers widen due to enhanced momentum exchange and mixing. The flow becomes less sensitive to the Womersley number at higher $Re$.

C. Effect of Womersley Number ($Wo$)

• Oscillatory Dynamics: The $Wo$ number influences the shape of the velocity profiles, particularly at lower Reynolds numbers.
• Profile Shifts: At $Wo = 4.5$ (shorter periods), profiles became slightly wider, indicating higher sensitivity to unsteady effects.
• Exhalation: During exhalation, the flow is largely independent of the inhalation route (as flow originates from the bronchial tree). However, $Wo$ significantly affects vortex orientation and the redistribution of velocity peaks (e.g., transitioning from double-peak to single-skewed profiles).

D. Flow Structures

• Inhalation: Characterized by a central high-velocity jet surrounded by slower near-wall regions, forming an arc-shaped pattern in coronal planes.
• Exhalation: Characterized by complex vortex structures and secondary flows. The high-speed region tends to shift toward the left main bronchus side (negative $x^*$), suggesting flow dominance from that branch.

5. Significance and Implications

• Modeling Simplification: The finding that oral and nasal routes yield nearly identical tracheal flow structures suggests that for studies focusing only on the lower airways (trachea/bronchi), the specific entry route may be less critical than previously thought, provided the upstream geometry (larynx/pharynx) is accurately represented.
• Therapeutic Optimization: Understanding that the flow structure is robust to the entry route helps in designing aerosol-based therapies. Drug delivery strategies can focus on the downstream bifurcation dynamics rather than the specific inhalation method, assuming the upper airway geometry is preserved.
• Pathogen Transmission: The study confirms that the upper airway morphology (specifically the larynx and glottis) is the primary driver of flow asymmetry and particle transport mechanisms, rather than whether air enters through the nose or mouth.
• Methodological Benchmark: The successful application of 3D PTV (STB) in a refractive-index matched, patient-specific model sets a new standard for experimental respiratory fluid dynamics, moving beyond 2D PIV and idealized inlet assumptions.

Conclusion: While realistic upper airway geometry is essential for capturing physiologically accurate flow (creating asymmetric, non-parabolic profiles), the specific choice between oral and nasal inhalation has a negligible impact on the resulting flow structures in the trachea. The Reynolds number is the primary driver of flow behavior, followed by the Womersley number, while the inhalation route is a secondary factor.