The following information was extracted from a White Paper developed by Zoll Medical Corporation in support of their Full-Rescue AED by Lifesaving Resources, Inc. The full white paper is available as a download from http://www.zoll.com/workgroup. All five of the research papers to which it refers are also available as downloads from the workgroup.
According to Gerald, Dworkin, of Lifesaving Resources, Inc., this information should be of interest to all Lifeguard, Public Safety and Rescue personnel, as well as other first responders who are trained to respond to respiratory and cardiac emergencies. The need and use of AEDs is often advocated as being the end-all for all resuscitation response. However, according to the studies, only 50% of the cardiac arrest victims responded to have a shockable rhythm by the time rescuers applied AED electrodes. This, of course, is dependent upon the time of arrest to the time of response by first responders and the availability of this equipment.
When responding to a witnessed cardiac arrest, about half the time, an AED can be expected to advise NO SHOCK for the victim.
A victim of Sudden Cardiac Arrest (SCA) has collapsed and is unresponsive. An AED is found on the premises and electrodes have been properly attached. The AED has ordered everyone not to touch the victim. An ECG analysis of the victim's heart rhythm is in progress.
What is the probability that the AED will advise shocking this victim?
A shock will only be advised if the AED detects a ventricular fibrillating (VF) or ventricular tachycardia (VT) heart rhythm. A number of researchers in various parts of the world have, over the last few decades, investigated a large number of SCAs. None of them had set out to answer the question of how often an AED will recommend a shock, but all of them present data that allows us to make a reasonable estimate that about one half the time an AED will detect a VF or VT rhythm and advise a shock.
- Probably the largest and most thorough epidemiological study of cardiac resuscitation to date is a review of SCAs that occurred in King County, Washington, from 1990 to 1999. From a total of 5,213 cases of cardiac arrest during that time, 2,071 victims presented VT or VR, for a total of 40% for whom a shock would have been advised.
- A second study of SCAs in Finland, published in 2001, found that over a five year period 771 SCAs were witnessed and responded to. Of these, 442 victims presented VF and 2 presented VT heart rhythms, for a total of 58% of all victims for whom a shock would have been advised.
- Another study published in 2002 investigated 20 years of data from 1980 through 2000 in Seattle, Washington, and found that of 2,686 SCA incidents that occurred over the 20 year period, 1,365 presented VF or VT, for a total of 51% for whom a shock would have been advised.
- A fourth study published in 2003 reviewed SCAs from 1993 to 2001 in Basel, Switzerland, and found that of 380 cardiac arrests, 205 presented VR or VT, for a total of 54% for whom a shock would have been advised.
- A fifth study published in 2003 reviewed 19 years of data in Gothenberg, Sweden, where the reviewers found that in a total of 3,089 cases of SCA, 1,577 presented VR or VT, for a total of 51% for whom a shock would have been advised.
- Finally, a study published in 2004 reviewed SCAs in 17 different cities in Ontario Canada (the Ontario Prehospital Advanced Life Support Study Group, or "OPALS" study) and found that in 5,451 SCAs over a 36 month period, 1,819 presented VR or VT, for a total of 33% for whom a shock would have been advised.
Given the results of this research, it is believed that once a victim of SCA has been attached to an AED, the probability that the rescuer will be advised to shock the victim is approximately 50/50. This evidence fully supports the need for lay persons and rescue personnel to be fully competent in the ability to perform and support CPR, because once an AED has determined that no shock is advised, the most helpful thing the rescuer can do for the victim is to perform continuing, vigorous, and precise CPR.
More information is available at http://www.lifesaving.com