Forces Applied on the Glottis During Endotracheal Intubation: Effect of Technique, Stylet, and Experience. A Manikin-based study

This manikin-based study demonstrates that glottic forces during endotracheal intubation are primarily determined by the use of a stylet and the specific laryngoscopy technique employed, rather than by operator experience or anthropometric characteristics, highlighting stylet use as the most significant modifiable factor contributing to potential airway trauma.

The Gist

Imagine your throat is a narrow, delicate tunnel, and the goal of intubation is to slide a flexible hose (the breathing tube) through it without bumping into the walls. This study is like a high-tech crash test, but instead of cars, they tested how hard doctors push that hose through the "tunnel" of the throat using different tools and techniques.

Here is the breakdown of what they found, using some everyday analogies:

The Big Question: How hard are we pushing?

When a doctor puts a breathing tube in, they use two hands. The left hand holds a lighted stick (the laryngoscope) to lift the tongue and see the opening. The right hand pushes the tube through.

Most people assume that new doctors (residents or students) push too hard because they are clumsy, while experienced doctors (attendings) are gentle masters. They also wondered if using a stiff wire inside the tube (a stylet) makes the tube act like a rigid spear, causing more damage.

The "Crash Test" Setup

The researchers didn't use real people. They used a very realistic dummy (a manikin) with a sensor built right into the "throat" to measure exactly how many pounds of force were applied when the tube went through.

They tested 50 different people (30 experts, 20 less experienced) using 5 different methods:

1. Direct View: Looking straight in with a standard stick.
2. Video View: Using a camera on a stick (like a GoPro).
3. With or Without the "Stiffener": Using a wire inside the tube to make it rigid, or leaving it floppy.

They also simulated two scenarios:

• The "Easy Day": Just doing a normal intubation.
• The "Laryngospasm": A scary scenario where the throat muscles clamp shut, and the doctor has to push as hard as they can to get the tube in.

The Surprising Results

1. Experience Doesn't Equal "Gentleness"

The Myth: "New doctors push too hard; experts are gentle."
The Reality: It didn't matter who was doing it. Whether it was a seasoned surgeon or a medical student, they all pushed with roughly the same amount of force.

• The Analogy: Think of it like driving a car. A new driver and a pro driver might take different routes or make different mistakes, but if they both press the gas pedal to get up a hill, they end up applying the same amount of force to the road. The skill changes the success rate, but not necessarily the brute force used.

2. The "Stiffener" (Stylet) is the Real Force Multiplier

The Finding: This was the biggest discovery. Using the stiff wire inside the tube doubled the force applied to the throat.

• The Analogy: Imagine trying to push a wet noodle through a narrow pipe. It bends and slides easily. Now, imagine putting a metal rod inside that noodle. Suddenly, it's a rigid stick. If you push that stick against the pipe wall, it doesn't bend; it hits the wall with much more force.
• Why do doctors use it? Because that rigidity helps the tube turn corners and get through the first time (which is good!). But the trade-off is that if you miss the target, you hit the throat walls harder.

3. The Camera vs. The Straight Stick

The Finding: When using the video camera (Videolaryngoscopy) with a standard blade, doctors applied the least amount of force. However, when they used a super-curved camera blade, the force went back up.

• The Analogy: Using a standard camera blade is like using a smooth, curved ramp to guide a ball into a hole. Using the super-curved blade is like trying to guide the ball through a winding, S-shaped slide; it requires more twisting and pushing to get the ball to the exit.

4. Strong Hands Don't Matter

The Finding: They measured how strong the doctors' grips were. It didn't matter if you had the grip strength of a bodybuilder or a child; it didn't change how hard they pushed the tube.

• The Analogy: It's not about how strong your arm is; it's about the technique. Even a weak person can push a heavy box if they have the right leverage, and a strong person might push gently if they are careful. The force came from the tool, not the muscle.

The Takeaway for Everyday Life

If you are a patient or just curious about medical safety, here is the bottom line:

• Don't worry about the doctor's experience level regarding how "gentle" their hands are. The study shows they all apply similar force.
• The biggest factor is the tool. Using a stiff wire (stylet) inside the breathing tube makes the procedure more forceful. Doctors need to be very careful with this tool. It's a "high-risk, high-reward" tool: it helps you get the tube in faster (which is good), but it hits the throat harder if you miss (which is bad).
• The best "gentle" tool seems to be the video camera with a standard blade, as it requires the least amount of pushing force.

In short: The study tells us that the equipment and the technique are the main drivers of force, not the doctor's experience or their muscle strength. To keep patients safe, we need to focus on choosing the right tools and knowing exactly when to use the "stiffener" (stylet) and when to take it out.

Technical Summary

1. Problem Statement

Endotracheal intubation is a routine but high-risk procedure associated with iatrogenic injuries, including dental damage, postoperative sore throat, and rare but severe complications like arytenoid dislocation and tracheobronchial injury. While previous research has quantified the lifting forces exerted by the laryngoscope blade (left hand) on the tongue and vallecula, there is a critical knowledge gap regarding the glottic forces exerted by the right hand as the endotracheal tube (and stylet) is advanced through the vocal cords.

It is hypothesized that:

• Operator experience influences force application (less experienced operators may apply excessive force).
• The use of a stylet increases tube rigidity, potentially increasing contact forces on the glottis.
• Different laryngoscopy techniques result in varying force profiles.

2. Methodology

Study Design:
A prospective, manikin-based experimental study conducted at Hôpital Maisonneuve-Rosemont (Montréal, Canada).

Participants:

• Total: 50 operators.
• Groups:
  • Experienced (n=30): Attending physicians and senior residents (PGY 3+) in anesthesia, emergency medicine, or ICU.
  • Less Experienced (n=20): Junior residents (PGY 1-2), medical students with prior intubation experience, respiratory therapists, and general surgeons.

Experimental Setup:

• Manikin: High-fidelity airway training manikin equipped with a calibrated force sensor (Mark-10 Digital Force Gauge) positioned immediately below the vocal cords to measure right-hand transmitted forces.
• Techniques Tested (5 variations):
  1. Direct Laryngoscopy (DL) without stylet.
  2. DL with stylet.
  3. Videolaryngoscopy (VL) with Macintosh blade without stylet.
  4. VL with Macintosh blade with stylet.
  5. VL with Hyperangulated blade with stylet.
• Conditions:
  1. Standard: Routine clinical practice.
  2. Simulated Laryngospasm: Glottic entry blocked; operators applied maximum acceptable force to restore oxygenation.
  3. Physical Maximum (Subset): A subset of 10 experienced operators performed an absolute physical maximum force test.
• Anthropometrics: Grip strength and hand size were measured for all participants.

Statistical Analysis:
Due to non-normal distribution of force data (right-skewed), non-parametric methods were used:

• Mann-Whitney U test (Experience groups).
• Wilcoxon signed-rank test (Stylet effect).
• Friedman test with Conover post-hoc (Technique comparison).
• Spearman rank correlation (Anthropometrics).
• Effect sizes reported as rank-biserial correlations ($r$).

3. Key Contributions

This study provides the first comprehensive quantitative characterization of glottic forces transmitted via the endotracheal tube and stylet. It distinguishes between the lifting force (left hand) and the insertion force (right hand), offering new insights into the biomechanics of airway trauma.

4. Key Results

A. Operator Experience

• Finding: There were no statistically significant differences in peak glottic forces between experienced and less experienced operators across any technique or condition (all adjusted $p = 1.0$).
• Effect Size: Effect sizes were negligible to small ($|r| \le 0.27$).
• Implication: The commonly held belief that less experienced operators apply excessive force is not supported by data; differences in success rates likely stem from visualization and tube delivery efficiency, not force magnitude.

B. Effect of Stylet Use

• Finding: Stylet use consistently and significantly increased glottic force across all groups and conditions.
• Magnitude:
  • Standard Conditions: Median force increased by 3.4 N to 7.3 N.
    • DL: 6.1 N $\to$ 11.1 N.
    • VL (Mac): 4.8 N $\to$ 8.7 N.
  • Laryngospasm Conditions: Median force increased by 6.1 N to 7.0 N.
• Significance: This is the single largest modifiable factor influencing applied force, with large effect sizes ($r \ge 0.75$).

C. Technique Comparison

• Standard Conditions: Videolaryngoscopy with a Macintosh blade produced the lowest glottic forces (Median 8.7 N) compared to Hyperangulated VL (Median 10.3 N) and DL. The difference between Mac VL and Hyper VL was statistically significant ($p = 0.049$).
• Laryngospasm Conditions: The advantage of the Macintosh blade disappeared; all techniques yielded similar high forces (~19–20 N).
• Physical Maximum: The highest recorded median force was 31.1 N (Hyper VL with stylet).

D. Anthropometrics

• Finding: No significant correlation was found between grip strength, hand size, and applied force.
• Implication: Intubation force is determined by technique and equipment interaction, not the operator's physical capacity.

5. Significance and Clinical Implications

• Device Selection: For standard intubations, videolaryngoscopy with a Macintosh-style blade may minimize glottic trauma compared to hyperangulated blades or direct laryngoscopy, provided a stylet is used.
• Stylet Management: While stylets improve first-pass success (as shown in the STYLETO trial), they inherently double the force applied to the glottis. Clinicians should consider strategies to mitigate this, such as:
  • Partial withdrawal of the stylet once the tube tip passes the vocal cords.
  • Using malleable stylets rather than rigid ones where appropriate.
  • Careful attention to stylet tip positioning.
• Training Focus: Training curricula should shift focus from "reducing force" (as experience does not significantly alter force application) toward optimizing visualization and tube delivery mechanics.
• Future Directions: The authors suggest the development of force-sensing laryngoscopes for real-time feedback and further validation in live patient populations to correlate these force metrics with actual airway morbidity.

Conclusion: Glottic force is primarily driven by the intubation technique and stylet use, rather than operator experience or physical attributes. The use of a stylet is the most significant modifiable contributor to increased glottic force.