Among patients who have witnessed an out-of-hospital cardiac arrest, the proportion with a "shockable" arrhythmia is markedly higher in public settings (60%) than in home settings (35%), according to a prospective cohort study reported in the Jan. 27 issue of the New England Journal of Medicine.
The use of AEDs by laypersons in public settings is known to improve survival, but such use in residential settings does not.
The reason for this discrepancy is not yet clear, but the implication is that the usefulness of resuscitation strategies might depend on the location where the cardiac arrest occurs, said Dr. Myron L. Weisfeldt of Johns Hopkins University, Baltimore, and his associates.
Thirty years ago, 70% of witnessed out-of-hospital cardiac arrests were characterized by so-called shockable arrhythmias – ventricular fibrillation or pulseless ventricular tachycardia that are amenable to treatment with an automated external defibrillator (AED). That rate has dropped dramatically and now stands at only 23%.
The use of AEDs by laypersons in public settings is known to improve survival, but such use in residential settings does not. To determine whether this discrepancy is related to the prevalence of shockable rhythms in these locations, Dr. Weisfeldt and his colleagues analyzed data from a population-based emergency medical services registry of out-of-hospital cardiac arrests.
The prospective, multicenter cohort study involved 12,930 adults treated by 208 EMS agencies at seven U.S. and three Canadian sites for nontraumatic cardiac arrest in which external defibrillation or chest compressions were attempted.
Ventricular fibrillation or pulseless ventricular tachycardia occurred in 3,336 cases, for an overall frequency of 26%.
A total of 3,451 patients had bystander-witnessed cardiac arrest in the home, of which 1,193 (35%) had initial shockable arrhythmias when EMS arrived and applied AEDs. In contrast, 600 of the 1,003 patients (60%) whose cardiac arrest occurred in public had shockable arrhythmias when EMS arrived.
Similarly, when an AED was applied by a lay bystander before EMS arrival, only 36% of the patients in home settings had a shockable rhythm, compared with 79% of those in public settings.
As would be expected with these findings, the rates of survival to hospital discharge reflected this discrepancy: When an AED was applied by a bystander in a public location, survival was 34%, but when the device was applied by a bystander in a home, survival was only 12%, the investigators said (N. Engl. J. Med. 2011 Jan. 27;364:313-21).
"Although the median time from the 911 call to EMS arrival was modestly longer for bystander-witnessed cardiac arrests at home than for those in public, the EMS response times were less than 7 minutes for more than 75% of the patients in both locations," they noted.
Thus, "it does not seem likely that the much lower frequency of ventricular fibrillation or pulseless ventricular tachycardia observed after cardiac arrest in the home would be accounted for by differences in EMS response time or other delays in the case of home-witnessed arrests," Dr. Weisfeldt and his associates said.
Studies in Japan and Denmark also have found this discrepancy between home and public settings. Although the reason is not yet clear, one possible explanation is that patients who have a cardiac arrest at home are typically older and more likely to have comorbidities that limit their outside activities. Thus, the location of the cardiac arrest might be a surrogate marker for underlying disease severity, the researchers said.
Their findings suggest that AEDs and the training of lay responders in their use should be targeted at public rather than home settings. In the home, prompt, bystander-delivered CPR or compression-only CPR might save more lives than "the widespread deployment of AEDs," they added.
This study was supported by the National Heart, Lung, and Blood Institute, the National Institute of Neurological Disorders and Stroke, the U.S. Army Medical Research and Materiel Command, the Canadian Institutes of Health Research–Institute of Circulatory and Respiratory Health, the American Heart Association, Defence Research and Development Canada, and the Heart and Stroke Foundation of Canada.
Dr. Weisfeldt reported receiving royalties from a patent for a pacemaker. His associates reported ties to Medtronic, Jolife, Philips, and Physio-Control.