The Four Minute Barrier

Jun 2nd, 2010 | By | Category: Response Times

Two carefully conducted studies in this issue—one of efficacy and the other of cost —are very significant cautionary notes to the literature on public access defibrillation. They reinforce two important points from the world literature on this topic:

1) If an intervention shortens the time from witnessed cardiac arrest to defibrillation to less than four minutes or so, remarkably high survival rates are seen. If, in contrast, the intervention results in shortening the time to defibrillation from, say, an average of 13 minutes to an average of 11 minutes, very little improvement in outcomes will be seen.

2) If admission to a hospital is increased by a defibrillation program but discharge rates and long-term survival are not, there will be bad news from conventional costs and effectiveness analyses.

In rural areas, where the time to arrival of emergency medical services (EMS) is more than 15 minutes, survival will not occur without defibrillators positioned nearby. Similarly, in large cities such as New York and Chicago, traffic and high-rise elevators are serious impediments to the EMS system. In Miami, a moderate-sized city with good EMS similar to those of van Alem et al’s Amsterdam, police vehicle defibrillators brought relatively small benefit. But defibrillation within four minutes resulted in a high survival rate.

Public access defibrillation is not a miracle in itself. When combined with cardiopulmonary resuscitation (CPR) in some settings, such as Las Vegas casinos, it can be. The system in the casinos resulted in a very large percentage of victims defibrillated within four minutes and a >50% long-term survival rate, which is effective and almost certainly cost-effective. Preliminaryresults of a large multi-center NHLBI study in the United States and Canada randomizing community sites to CPR instruction versus CPR instruction plus use of automated external defibrillators (AEDs) are also promising.  Training was provided to lay volunteers, and AED sites had twice the number of survivors.

Public access defibrillation has become possible only in the last 10 years, since the introduction of AEDs that are inexpensive, self-instructional, easy to operate, and automatic, requiring no diagnostic ability.  The AED papers in this issue also point out the limitations of public access defibrillation, whichpresent three challenges for the next generation of technology and therapeutic intervention.

The first is to be able to bring four-minute defibrillation into the home, where 70% of cardiac arrests occur. On the horizon are both relatively inexpensive implantable devices and reliable monitoring devices that can serve as the “smoke detector equivalent” for cardiac arrest. One can envision an implantable device with a global positioning sensor and the ability to communicate to a cell phone-like device carried by the patient. The cell phonewould immediately call an emergency response system centrally and page a local family member, neighbor, or friend. The family member would defibrillate, and the central system would dispatch the EMS.

Second, we need efforts to educate and train the public, emphasizing that after four to five minutes of cardiac arrest without defibrillation, bystander CPR is essential. It should be performed, even if a defibrillator is present, for two to three minutes before defibrillation.  This is a complicated message, but one established by Wik et al’s randomized trial.  With bystander CPR performed in those beyond five minutes of cardiac arrest, 14 out of 40 patients survived, versus two out of 41 patients in the control group that had defibrillation first.

Finally, in out-of-hospital as well as inpatient settings, we have new metabolic interventions to improve the poor survival rate of those admitted to the hospital alive. We already know that hypothermia can benefit those admitted with failure of immediate neurological recovery.  Many believe there are otherinterventions that will be discovered to interrupt the process of inflammation, sepsis, and ischemic-reperfusion injury that is the consequence of prolonged cardiac arrest.

Public access defibrillation is here to stay. It is a matter of where and when it is of enough value to be efficacious and cost-effective. The challenge is to bring its benefits to more individuals, combined with other interventions that will leadto clear improvement in long-term survival. There is reason for optimism and enthusiasm.

From bmj.com, 2004 by Myron L WeisfeldtWilliam Osler Professor of Medicine, chair

Editor’s Note: the usefulness of CPR in SCA has lately been found to be “insignificant”. See http://elevaed.com/?p=287 The four minute barrier remains.

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