Using a Stifneck Select Collar$^{\mathrm{TM}}$ for hands-free semiautomatic blood flow measurements: a user study

This study demonstrates that a modified Stifneck Select Collar equipped with an inflatable cushion and ultrasonic Doppler sensor provides a robust, comfortable, and effective hands-free solution for measuring carotid blood flow during CPR, achieving usable signals in the majority of healthy volunteers without causing skin irritation.

The Gist

Imagine a person has suffered a cardiac arrest outside the hospital. Every second counts. Doctors and paramedics are performing CPR (chest compressions) to keep blood flowing to the brain, but they are flying blind. They don't know if their compressions are actually pushing enough blood to the brain or if the brain is starving for oxygen.

Currently, there is no simple way to "see" this blood flow in real-time without a massive hospital machine. This new paper presents a clever, low-tech solution to that high-tech problem.

Here is the story of their invention, explained simply:

The Problem: The "Jittery Hand"

Imagine trying to take a photo of a tiny, moving target (like a hummingbird's wing) while someone is shaking the camera violently. That is what trying to measure blood flow during CPR is like.

• The Target: The carotid artery in the neck (the main road for blood to the brain).
• The Shaking: The violent chest compressions of CPR shake the whole body.
• The Old Tools: Previous attempts used sticky patches (like band-aids) or necklaces. Sticky patches can rip off or hurt the skin if you move them; necklaces are too wobbly and slip around when the body shakes.

The Solution: The "Smart Neckbrace"

The researchers decided to stop fighting the shaking and instead harness it. They turned a standard emergency neck brace (the kind used to stop neck injuries, called a Stifneck) into a hands-free camera mount.

Think of it like this:

1. The Base: They took a rigid neck brace that locks the head in place. This stops the head from wobbling around, acting like a sturdy tripod for a camera.
2. The Mount: They 3D-printed a special holder that clips onto the brace. It's like a rail system on a train.
3. The "Air Pillow": This is the genius part. Between the sensor and the neck, they placed an inflatable air cushion (like a tiny, deflated balloon).
   • The Analogy: Imagine holding a balloon against a wall. If you blow a little air into it, it presses gently against the wall. If you blow more, it presses harder.
   • The Magic: The paramedic can pump air into this cushion to press the ultrasound sensor firmly against the neck. If they need to move the sensor, they just let the air out, slide the sensor along the rail, and pump the air back up. No sticky glue, no ripping skin.

The Experiment: The "Test Drive"

To see if this "Smart Neckbrace" worked, they didn't test it on real cardiac arrest victims (that would be too risky). Instead, they tested it on 102 healthy volunteers (like a test drive with normal drivers).

They put the brace on, inflated the air pillow, and tried to listen to the blood flow in the neck.

• Did it hurt? Almost no one felt pain. Most rated it a "1" out of 10 (where 1 is "no pain" and 10 is "ouch").
• Was it comfortable? People rated it about a "6.5" or "7" out of 10. It felt a bit like wearing a stiff collar, but not terrible.
• Did it work?
  • 92% of the time, they could hear the "whoosh" of the blood flow (the Doppler signal).
  • 73% of the time, they could see a clear graph of the blood flow on the screen.
  • 0% of the time did anyone get a skin rash or injury.

Why This Matters: The "Black Box" for CPR

If this device works in real emergencies, it acts like a dashboard for CPR.

• Right now: Paramedics guess if their CPR is good.
• With this device: They get a real-time "speedometer" for blood flow. If the blood flow drops, they know to change their technique immediately. If it stays steady, they know they are saving the brain.

The Catch (The "But...")

The device isn't perfect yet.

• The "Jugular" Problem: The neck brace covers the side of the neck where doctors usually put IV needles. In a real emergency, doctors need that space. The researchers admit they might need to redesign the brace later to leave a "window" open for needles.
• The "Manual" Problem: Right now, a human has to slide the sensor along the rail to find the perfect spot. In the future, they hope to make it automatic so the sensor finds the blood vessel by itself.

The Bottom Line

This paper describes a clever way to turn a rigid neck brace into a stable, hands-free ultrasound mount. It uses an air pillow to press the sensor gently against the skin, avoiding sticky glue and skin damage. While it still needs some fine-tuning for real-life emergencies, it's a promising step toward giving rescuers a "superpower" to see if their life-saving efforts are actually working.

Technical Summary

1. Problem Statement

Out-of-hospital cardiac arrest (OHCA) has low long-term survival rates (0–15%), heavily influenced by "no-flow time" (NFT)—the duration without blood circulation. Current CPR quality assurance lacks real-time, quantifiable feedback outside hospital settings.

• The Gap: While non-invasive ultrasound (US) Doppler can measure common carotid artery (CCA) blood flow to assess CPR effectiveness, existing portable devices require manual, hands-on operation.
• The Challenge: During CPR, the chest compressions generate forceful impulses that travel through the body, causing tissue movement. This makes it difficult to maintain precise sensor alignment with the carotid artery.
• Limitations of Existing Solutions:
  • Adhesives: Require firm skin contact but risk skin trauma upon removal, are difficult to reposition, and fail in wet conditions (sweat/blood).
  • Neckbands: Often lack robust skin contact and allow too many degrees of freedom, leading to signal noise and misalignment.
• Goal: Develop a hands-free, mobile, hazard-free sensor attachment that is robust, adjustable, usable without adhesives, and capable of withstanding CPR forces to provide real-time blood flow feedback.

2. Methodology

The authors developed a custom sensor attachment system integrated into a standard medical neck brace and validated it through a clinical study.

A. System Design & Components

The system modifies the Stifneck Select Collar™ (Laerdal) to hold a custom ultrasonic probe.

• Sensor Mount: A 3D-printed holder made of polylactic acid (PLA) with smoothed edges to minimize tissue damage. It accommodates a custom US probe (25mm x 41mm base) with three piezo-ceramics oriented at 25°, 0°, and -12° to capture tissue responses.
• Adjustable Positioning: The mount features a rail system with notches (5mm increments). This allows the sensor to be moved along the neck and documented relative to fixed landmarks, enabling reproducible positioning across different patients.
• Contact Pressure Mechanism: An inflatable cushion (TR-Band™, Terumo) is placed between the sensor rail and the back of the sensor holder.
  • It is inflated via syringe (up to 18ml) to apply adjustable, consistent pressure against the skin.
  • This eliminates the need for adhesives and ensures stable contact even during movement.
• Cable Management: A strain relief bracket prevents cable tension from pulling the sensor off the neck.
• Data Acquisition: A small computer with three synchronized US modules triggers pulses and records reflections from the target tissue.

B. Study Protocol

• Participants: 102 healthy volunteers (ages 19–83; 63 male, 39 female).
• Procedure:
  1. Volunteers lay supine; the Stifneck collar was applied.
  2. US gel was applied, and the sensor rail was positioned.
  3. The TR-Band cushion was inflated to secure the probe.
  4. Doppler signals (audible and visual flow curves) were verified.
  5. The sensor was repositioned along the rail to test multiple locations.
• Duration: Measurements lasted between 10 and 70 minutes (average ~31 mins).
• Evaluation Metrics:
  • Proband (Subject) Feedback: Pain (1–10 scale) and Comfort (1–10 scale).
  • User (Medical Expert) Feedback: Sensor support/stability (1–10 scale), signal audibility, flow curve usability, and skin lesion checks.
• Ethics: Approved by the University of Leipzig Ethics Committee; conducted in accordance with the Declaration of Helsinki.

3. Key Contributions

1. Novel Mechanical Interface: A hands-free sensor mount that utilizes a standard neck brace and an inflatable cushion for pressure, eliminating the need for skin adhesives.
2. Reproducible Positioning: The rail-and-notch system allows for precise documentation and repositioning of the sensor relative to neck landmarks, addressing the variability of patient anatomy.
3. Robustness to Motion: The rigid fixation to the collar and the adjustable pressure mechanism are designed to maintain alignment despite the mechanical forces generated during CPR.
4. Safety Validation: A comprehensive user study demonstrating that the device causes no skin trauma, circulatory restriction, or significant pain during prolonged wear.

4. Results

The system was evaluated on 102 healthy subjects with the following outcomes:

• Comfort & Pain:
  • Pain: Extremely low. Mean score 1.19/10 (SD 0.46). Over 80% of participants reported zero pain (score of 1).
  • Comfort: Mean score 6.52/10 (SD 1.78). Most participants rated it around 7/10, indicating it is tolerable for prolonged periods.
• Stability (Support):
  • Rated 9.95/10 (SD 0.32) by medical users.
  • 99 out of 102 cases received a perfect 10/10 rating for robust connection.
• Signal Quality:
  • Audible Doppler Signals: Achieved in 92.2% (94/102) of sessions.
  • Usable Flow Curves: Achieved in 73.5% (75/102) of sessions, allowing for qualitative analysis.
• Safety:
  • Skin Irritation: 0% incidence of skin lesions or irritation.
  • Physiological Impact: No reports of restricted circulation or respiration.
• Wear Duration: Average application time was 31.19 minutes, with a maximum of 70 minutes.

5. Significance and Conclusion

• Clinical Impact: This device provides a viable pathway for real-time, hands-free monitoring of carotid blood flow during CPR. This could allow first responders to quantify "no-flow time" and adjust CPR quality dynamically, potentially improving survival rates and neurological outcomes.
• Usability: The system is designed for rapid application and detachment (under 2 seconds), crucial in emergency scenarios. It overcomes the limitations of adhesives (skin trauma, repositioning difficulty) and neckbands (instability).
• Future Work:
  • Refining the optimal sensor position based on patient anatomy (neck size, vessel depth) to automate positioning.
  • Testing the system under actual CPR conditions (with chest compressions) to analyze noise interference and displacement risks.
  • Addressing the limitation that the Stifneck collar may obstruct jugular vein access; future iterations may require a modified collar design to ensure IV access remains available.

In summary, the authors successfully demonstrated a safe, robust, and user-friendly sensor attachment that enables hands-free ultrasonic Doppler measurements, laying the groundwork for advanced, data-driven resuscitation technologies.