NEJM: Shockable arrests occur more frequently in public locations
The majority of out-of-hospital cardiac arrest patients that respond to automated external defibrillators (AEDs) occur in public places rather than in the home. This may have some bearing on why the use of AEDs in the home setting have not proved beneficial, according to a study in the Jan. 27 issue of New England Journal of Medicine.

Thirty years ago, 70 percent of out-of-hospital cardiac arrests (OHCAs) were characterized by initial ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT), two conditions that respond favorably to AED shocks. Today the incidence of those two arrhythmias is 23 percent.

"This decline is of substantial importance for public health, since more than 300,000 Americans have an OHCA each year, with an estimated survival rate of 8 percent nationally, and the majority of survivors are in the subgroup of persons whose initial rhythm is VF or pulseless VT," according to the study.

To help determine if shockable arrests occur more frequently in the home or in public places, Myron L. Weisfeldt, MD, from Johns Hopkins University in Baltimore, and other investigators for the Resuscitation Outcomes Consortium (ROC) conducted a prospective cohort study of OHCAs in adults in 10 North American communities between 2005 and 2007.

Of the 12,930 evaluated OHCAs, 2,042 occurred in public and 9,564 at home. For cardiac arrests at home, the incidence of VF or pulseless VT was 25 percent when the arrest was witnessed by EMS personnel, 35 percent when it was witnessed by a bystander and 36 percent when a bystander applied an AED. For cardiac arrests in public, the corresponding rates were 38, 60, and 79 percent.

The adjusted odds ratio (OR) for initial VF or pulseless VT in public versus at home was 2.28 for bystander-witnessed arrests and 4.48 for arrests in which bystanders applied AEDs.

The rate of survival to hospital discharge was 34 percent for arrests in public settings with AEDs applied by bystanders versus 12 percent for arrests at home (adjusted OR, 2.49).

Researchers concluded that regardless of whether OHCAs are witnessed by EMS personnel or bystanders and whether AEDs are applied by bystanders, the proportion of arrests with initial VF or pulseless VT is much greater in public settings than at home.

As to why there is a difference in location of OHCA shockable arrhythmias, researchers suggested that those in the home are "typically older and more likely to have one or more chronic diseases that limit or preclude participation in activities outside the home."

Consequently, the location of an OHCA may be a "surrogate variable for underlying disease or disease severity and the corresponding risk of VF or pulseless VT."

Weisfeldt and colleagues noted that a strategy of increasing the frequency and quality of CPR in the home would likely be more beneficial than a strategy of the ready availability of an AED.

"Another strategy to improve survival is initial continuous chest compression without rescue breathing, which also may be more effective in cardiac arrest with VF or pulseless VT than in arrest with other initial rhythms," they said.