Acceleration Forces

This article appeared in the supplement Ambulance Safety Solutions sponsored by ZOLL Medical Corporation

Expert source: Michael Kurz, MD, Operational Medical Director, Henrico County (VA) Division of Fire & Director of Emergent Cardiac Care, VCU Health System

   We know terrible things can happen to providers who are unrestrained in the backs of ambulances involved in collisions. But for EMS personnel and their patients alike, negative consequences can start occurring long before a vehicle crashes.

   In fact, even normal operation of an ambulance can subject a standing, unrestrained provider to forces that push them off-balance. These may not be enough to topple or injure them, but may require them to shift their stance or throw out a hand to keep their balance. And that could mean interruptions in vital patient care procedures such as chest compressions during CPR.

   The finding was first presented last year in an investigation, "Description of the Acceleration Forces Affecting Balance of Prehospital Providers While Delivering Cardiopulmonary Resuscitation," that earned Best Original Resuscitation Science honors in the Moderated Poster Session of the American Heart Association's Scientific Sessions.

   "I was astounded," says coauthor Michael Kurz, MD, operational medical director for the Henrico Co. (VA) Division of Fire and director of emergent cardiac care for the Virginia Commonwealth University health system. "From my experience as a paramedic, I knew providing CPR in the back of a moving ambulance is challenging, and the forces that are applied are significant to your balance. But I wasn't ready for the magnitude of the forces applied in the average call we analyzed, or the amount of time providers are potentially exposed to off-balancing forces."

   Kurz and his colleagues began with previous studies that established thresholds at which standing subjects are pushed off balance. Acceleration that exceeds 0.6-0.93 m/sec.² is one baseline past which it's hard to stand still. Jerk, or the rate of change of acceleration (i.e., meters per second per second) greater than 0.5-0.6 m/sec.³ is another.

   They then used a Road Safety onboard monitoring system to record lateral and axial acceleration data during the transport of 50 cardiac arrest patients by the Richmond Ambulance Authority. They calculated acceleration and acceleration change vectors for every second of time in motion and determined the percentage with critical acceleration (greater than 0.93 m/sec.²) and jerk (greater than 0.6 m/sec.³) by identifying all events exceeding those thresholds.

   They found that acceleration topped 0.93 m/sec.² 49% of the time vehicles were in motion and jerk surpassed 0.6 m/sec.³ 26% of the time. Together, one or the other of the thresholds was exceeded 60% of the time. "The resulting loss of balance," the authors concluded, "may result in personal injury or the delivery of poor quality CPR."

   It's important to note that this is proxy data--these forces weren't being applied to actual providers treating patients. Richmond uses ZOLL AutoPulse units to provide automatic chest compressions, thus allowing caregivers to remain seated and restrained during in-transit CPR. But agencies that haven't mechanized their compressions can assume their providers who stand while moving are lurching around in uncontrolled ways. This was demonstrated in a recent video from Las Vegas that showed providers continually adjusting their balance on a controlled course moving at just 15-20 miles per hour.

   While there's no literature examining the potential injury risk to providers of this phenomenon, it's not hard to envision a capacity for damage. Says Kurz, "We can only imagine, in the back of a moving ambulance, exposed to the forces outlined in this research, while a provider has his hands occupied providing CPR or delivering some other care, that these off-balancing forces could easily cause injury."

   They likely don't do patients much good either. "Presumably, while these off-balance forces take place, a provider cannot deliver quality CPR," says Kurz. "Without quality CPR, the coronary perfusion pressure drops off dramatically, and it requires at least two seconds of quality CPR to get back to a level where there's a high probability of return of spontaneous circulation. So it could have a profoundly detrimental effect on the quality of CPR delivered in the back of the ambulance and the potential for restarting the hearts of cardiac arrest patients."

   A follow-up research project will attempt to develop human data. Rather than using force proxies, it will measure forces on actual providers in back and the quality of CPR they're delivering.

Solutions

   So what's a concerned agency to do about its people being pushed around this way?

   The first and most obvious answer is to keep them seated and restrained as much as possible. To this end, technologies and designs that prevent the need to unbuckle and stand can be useful and appropriate. Particular to CPR, automated chest compression devices not only relieve the provider from standing unrestrained, but can also ensure a consistent rate and depth of compressions for patients.

   Larger questions involve lights-and-sirens driving--Code 3 operation obviously produces greater forces than slower, nonemergency travel--and, clinically, termination of resuscitation in the field and the need to transport patients with CPR in progress.

   "We need to examine why we're even transporting patients with CPR in progress," says Kurz. "Recent research on termination-of-resuscitation guidelines raises the question of whether we should transport these patients at all. It may be that we should be terminating some in the field. And if transport is appropriate for some patients, what is the safest way to transport them, keeping in mind the safety of our providers, the safety of the public, and the quality of CPR and clinical care in the back of the moving ambulance?"

   But even that's no comprehensive solution. Even with field termination, some resuscitated patients may rearrest during transport. So if the need for compressions in the back of a moving ambulance can't feasibly be eliminated, what's the best we can do?

   "The goal is to minimize risk for everyone involved," says Kurz. "If you're talking about CPR for folks who have to be transported, a mechanical device that provides quality compressions independently of an individual provider--and that allows the provider to be restrained and provide other care while that device provides the CPR--is the best solution I'm currently aware of to reduce the danger of that transport environment."