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Depth Perception


The importance of quality CPR for victims of cardiac arrest is beyond question. Real-time guidance and feedback on chest compression rates is already helping EMS teams perform better CPR. Now, more accurate feedback for chest compression depth is on the horizon, with a new application in development by Physio-Control.

Compression depth is one of the main determinants of coronary perfusion pressure, which in turn is a primary predictor of patient survival.(1) To adequately circulate a patient’s blood, rescuers must push hard enough—at least 2 inches for adults, according to the 2010 American Heart Association (AHA) Guidelines.

Devices on the market today use a single accelerometer to provide feedback on chest compression depth, which can be misleading if CPR is performed on patients on a soft surface such as thick rug with padding underneath, in a moving vehicle or on a hospital gurney with a flexible mattress—even if the patient is on a backboard or other rigid device.

“Recent studies and the AHA guidelines say today’s compression depth feedback devices have questionable accuracy on soft surfaces like a mattress or stretcher.(2–5) That’s because accelerometer-based CPR feedback devices cannot distinguish between compression of the chest and deflection of the underlying surface, like a mattress,” explains Isabelle Banville, PhD, principal scientist for Physio-Control. “We looked at all this and asked, how do we provide better accuracy? That’s why we developed the triaxial field induction technology.”

Triaxial field induction (TFI) technology measures changes in the strength of a very low-energy 3D magnetic field generated by a reference pad placed beneath the patient. A sensor on the patient’s chest continuously detects the 3D magnetic field and calculates the changing distance to the reference pad as the rescuer preforms compressions on the patient’s chest.

“It measures the relative distance from the chest sensor to reference pad, regardless of patient movement—such as a moving ambulance or the flexing of a mattress,” Banville explains. During a resuscitation attempt, TFI allows accurate compression depth target to be displayed so the caregiver knows to push harder if needed.

“At any time during CPR, with TFI technology, you know how effective you are at providing CPR. You know how deep you have compressed the chest on a patient,” Banville says. She expects the accurate real-time feedback to significantly improve EMS performance to the AHA guidelines.

“Just as important,” says Banville, “is the fact that more accurate compression depth measurements also mean that quality data will be used to assess current protocols and lead the way to future guidelines.”

The vagueness of current AHA guidelines (“at least 2 inches”) provides no concrete assistance to adequate performance and leaves medics in the field to estimate compression depth based on pediatric patient size (such as one-third the diameter of the chest). Capturing and analyzing accurate data could lead to more specific, evidence-based guidelines.

Today, EMS agencies use Physio-Control’s CODE-STAT 9.0 data review software in their post-event review efforts to improve performance and target training. Banville envisions that in addition to assessing compression rates and compression fractions, they will also use compression depth data.

Kathleen Klein is a medical writer with 25 years’ experience on topics ranging from cardiac resuscitation to neonatal medicine. Based in Seattle, she has worked for the University of Washington Health Sciences Center, Group Health Cooperative and onHealth.com.

Disclosure: The author is a contract writer for Physio-Control, Inc. but reports no other conflicts of interest with the sponsors of this supplement.

1. Edelson DP & Walsh D. The science of CPR: Identifying the factors key to improved patient outcomes. JEMS. 2010;35(12; supplement):8–10.

2. Perkins GD, Kocierz L, Smith SC, et al. Compression feedback devices over estimate chest compression depth when performed on a bed. Resuscitation. 2009;80(1):79–82.

3. Nishisaki A, Nysaether J, Sutton R, et al. Effect of mattress deflection on CPR quality assessment for older children and adolescents. Resuscitation. 2009; 80(5):540–545.

4. Fischer H, Gruber J, Neuhold S, et al. Effects and limitations of an AED with audiovisual feedback for cardiopulmonary resuscitation: a randomized manikin study. Resuscitation. 2011;82(7):902–907.

5. Berg RA, Hemphill R, Abella BS, et al. Part 5: adult basic life support: 2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation. 2010;122(18Suppl3):S685–S705.


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