This study shows that shockable arrhythmias (ventricular fibrillation or pulseless ventricular tachycardia) are a relatively infrequent presentation of out-of-hospital cardiac arrest (with an overall incidence of 26%) and account for a remarkably low proportion of both EMS-witnessed arrests (25%) and bystander-witnessed arrests (35%) in the home. The frequency of shockable arrhythmias was higher for bystander-witnessed cardiac arrests in a public location (60%), particularly those in which an AED was applied by a bystander in a public location (79%) (). Therefore, as might be expected, the rate of survival to hospital discharge was significantly higher when an AED was applied by a bystander after a cardiac arrest in a public location (34%, vs. 12% for arrests at home; adjusted model P = 0.04).
The limitations of this study should be acknowledged before we consider its implications and possible explanations for the findings. First, we did not have access to the ECG recordings from bystander-applied AED and cannot confirm independently that all shocked rhythms were ventricular fibrillation or pulseless ventricular tachycardia. However, AED rhythm-detection algorithms are considered to be highly sensitive and specific for a shockable arrhythmia, since a shock advisory is strongly correlated with its presence and a no-shock advisory with its absence.10,11
Second, it is possible that delays in calling for EMS help were responsible for the low frequency of ventricular fibrillation or pulseless ventricular tachycardia as the initial rhythm among cardiac arrests at home, including those witnessed by a bystander and those for which a bystander applied an AED. Ascertaining the delay between the time of the witnessed collapse and the call to EMS can be challenging in both the public setting and the home setting, since one must rely on accurate recollections by witnesses. Nevertheless, it is unlikely that such a delay would be greater today than it was in an earlier era, when ventricular fibrillation or pulseless ventricular tachycardia was the initial rhythm in 70% of all cardiac arrests.1,2
With respect to EMS delays, 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.
A spline-fit analysis (data not shown) relating the incidence of initial ventricular fibrillation or pulseless ventricular tachycardia to EMS response time in the case of bystander-witnessed cardiac arrests in public indicated that the frequency of this arrhythmia diminished from 60% to no less than 50% as the EMS response time increased from zero to 7 minutes. Therefore, 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. EMS response time was also not significantly related to the incidence of initial ventricular fibrillation or pulseless ventricular tachycardia in the multivariate analysis (). Furthermore, the frequency of these arrhythmias was similar (25%) for cardiac arrests in the home that were witnessed by EMS personnel, and in such cases, one would expect that the first rhythm was documented promptly after the event.
Survival data reported for the population groups in this study are consistent with previous reports on successful bystander-applied AED shocks and witnessed cardiac arrests in both public and non-public locations.12-14
Among the patients in our study who received AED shocks from bystanders in public locations, the survival rate was 42%. This compares favorably with results from a study of cardiac arrests in casinos in which the approximate survival rate was 53% among patients who received AED shocks after the arrests were promptly recognized by means of video cameras on the gaming floor.12
Similarly, in a study of cardiac arrests that occurred in Chicago airports, the survival rate was 60% among patients who received AED shocks delivered by bystanders.14
Studies in Osaka, Japan,15
and in Copenhagen16
came to similar conclusions regarding the incidence of ventricular fibrillation or pulseless ventricular tachycardia in public or workplace settings versus nonpublic ones. However, these studies did not specifically address arrests involving bystander-applied AEDs, nor did they exclude unwitnessed cardiac arrests, for which the interval between the arrest and the initial ECG is likely to be prolonged.
The results of this study have a number of important implications for public health and community strategies to improve survival after cardiac arrest. First, because only 20 to 30% of cardiac arrests in the United States and Canada occur in public settings, our findings suggest that AED programs and education in AED use by lay responders should be focused on these sites.17,18
Second, our findings suggest that the incremental benefit in survival from the use of AEDs in the home, as compared with a strategy that increases the frequency and quality of CPR by bystanders in the home, is likely to be small. The rate of survival after cardiac arrest in the home for the 1219 cases in which a bystander witnessed the event and performed CPR was 10%, which is similar to the 12% survival rate associated with use of a bystander-applied AED in the home. Increasing the rate of CPR by bystanders in the home, perhaps with dispatch assistance, might yield a benefit similar to that achieved with the use of home AEDs.19-21
Another strategy to improve survival is initial continuous chest compression without rescue breathing, which may also be more effective in cardiac arrest with ventricular fibrillation or pulseless ventricular tachycardia than in arrest with other initial rhythms. In experimental studies that propose continuous compression, ventricular fibrillation or pulseless ventricular tachycardia models of cardiac arrest are used.22
Two recently published studies in humans showed no significant difference in survival between patients who were randomly assigned, on the basis of dispatchers’ instructions to bystanders, to receive continuous compression without rescue breathing and those assigned to receive standard CPR with rescue breathing.20,21
In one of the two studies, continuous compression without rescue breathing was associated with increased survival among patients with arrests due to cardiac causes20
; in the other study, there was a trend toward increased survival with continuous compression and no rescue breathing among patients with arrests characterized by ventricular fibrillation or pulseless ventricular tachycardia.21
If arrests characterized by ventricular fibrillation or pulseless ventricular tachycardia have better outcomes with continuous compression alone, this could be the more effective resuscitation strategy in the public setting, whereas rescue breathing along with compression might be of greater importance in the home, where the frequency of ventricular fibrillation or pulseless ventricular tachycardia is lower.23
Why is the initial recorded cardiac-arrest rhythm different when cardiac arrest occurs in a public location rather than in the home? One explanation is that the person who has a cardiac arrest in the home is typically older and more likely to have one or more chronic diseases that limit or preclude participation in activities outside the home. Thus, the location of an out-of-hospital cardiac arrest may be a surrogate variable for underlying disease or disease severity and the corresponding risk of ventricular fibrillation or pulseless ventricular tachycardia. For example, treatment with an implanted defibrillator is known to have a smaller effect on survival among patients with more severe heart failure than among those with less severe heart failure, suggesting that the incidence of shockable arrhythmias (ventricular fibrillation or pulseless ventricular tachycardia) differs between these two groups.24
In conclusion, our study shows that the frequency of ventricular fibrillation or pulseless ventricular tachycardia as the initial recorded rhythm is lower among patients with witnessed cardiac arrests in the home than among those with witnessed arrests in a public setting. This finding adds strength to the argument for putting AEDs in public locations. Although the role of AEDs in cardiac arrests that occur in the home will probably continue to evolve, the relatively low incidence of shockable arrhythmias in this setting suggests that a treatment strategy that emphasizes prompt, bystander-delivered CPR of high quality (e.g., with the assistance of a dispatcher) should be as effective in saving lives as the widespread deployment of AEDs in homes.