During the 17-month study period, 15,549 persons experienced a nontraumatic OHCA and were treated by EMS in the study regions, and of these, 15,099 (97.1%) had a known outcome status. Of these, 1,330 arrests were excluded because they were witnessed by EMS (). Thus, the final cohort included 13,769 individuals (Group 1), among whom 2.1% (289) had an AED applied before EMS arrival (Group 3) and constituted the primary subgroup for analysis. In this group, an AED was applied by healthcare workers (32%), lay volunteers (35%), police (26%), or unknown (7%).
characterizes the 5 subgroups of non–EMS-witnessed cases (Groups 1 through 5). The same groups are identified in . The entire cohort included only 438 individuals who were younger than 18 years. The EMS response interval was similar across all groups, with a median response interval of 5.3 minutes (interquartile range, 4.0 to 6.9) for all non–EMS-witnessed cardiac arrests. Public location and bystander-witnessed arrests before EMS arrival also are shown in and were more common among those with placement of an AED before EMS arrival, especially among those who received an AED shock. In addition, the proportion of OHCA in which an AED was applied before EMS arrival varied across ROC sites, ranging from 1% to 7%.
Overall survival and by subgroup
shows the overall survival by subgroup. Overall survival was 7% among all EMS-treated OHCAs occurring before EMS arrival (Group 1; n = 13,769). Survival was 9% when CPR was performed before EMS arrival, but an AED was not applied (Group 2). Of the cases in which an AED was applied before EMS arrival (Group 3), 24% (69 of 289) survived, and of those who received a shock from an AED applied before EMS arrival (Group 4), 38% (64 of 170) survived. Survival was 40% with application of an AED by a lay person, 16% for health care workers, and 13% for police. Forty-one of the 289 AED applied arrests occurred in residential institutions where healthcare workers were present.
The survival of patients who had an AED applied before EMS arrival but no AED shock was 3.5%, and this rate was not different from the 2.3% (n = 2,887) survival among patients who received bystander CPR without an AED applied and had a nonshockable rhythm at presentation (p < 0.34). The rates reported exclude those with an unknown initial rhythm. Controlling for the covariates included in the model for the primary analysis did not change this conclusion (p < 0.60).
Logistic regression model
A logistic regression model was applied to all cases with complete information from Group 1 (n = 11,370). The unadjusted OR of survival associated with AED application before EMS arrival (with or without shock) was 4.74 (95% CI: 3.53 to 6.36). After adjusting for age, gender, bystander CPR performed, type of location, EMS response interval, witness status, initial rhythm, and ROC site, AED application before EMS arrival was associated with an OR of survival of 1.75 (95% CI: 1.23 to 2.50; p < 0.002). This OR was attenuated slightly to 1.62 (95% CI: 1.16 to 2.26; p < 0.007) when we used a stratified propensity score analysis to adjust for these covariates instead of multiple regression. In the reduced population that included only those who received treatment from a bystander, the complete-case OR was 1.68 (95% CI: 1.19 to 2.39; p < 0.004) and the propensity score OR was 1.73 (95% CI: 1.25 to 2.39; p < 0.002). Propensity score models predicted AED application well, with C statistics of 0.91 and 0.82, respectively, for the 2 models. Generalized estimating equation and generalized linear mixed model analyses that accounted for clustering by site produced similar estimates.
Of those cases with missing data (n = 2,399), 87% lacked witnessed status, bystander CPR status, or both (574 witnessed, 809 CPR, and 699 both). Thirty-six of the excluded cases were cases in which an AED was applied before EMS arrival. In the analysis of all 13,769 patients with survival data, using multiple imputation for missing covariate data, the OR of survival associated with an AED applied before EMS arrival was similar to that found in other analyses: 1.75 (95% CI: 1.27 to 2.41; p < 0.001).
Site-specific analysis of bystander AED use
When multiple regression was performed on complete cases with stratification by site, nearly all of the site-specific OR estimates were more than 1, but many were not significantly different from 1, likely because of the number of subjects receiving bystander AED application at individual sites was small (). This stratified model fit the data better than the model not stratified by site (p < 0.003). However, it required that a large number of ORs be estimated using a limited amount of data on bystander AED application.
Multivariate Analysis of Survival by Site: Cases With Complete Data Only
Post hoc subgroup analysis of location of AED application
presents survival by the type of location of the OHCA. Of the 288 OHCA in which AED application took place, 59% (171 of 288) occurred in public sites and 41% (117 of 288) occurred in private locations. Survival was 35% (59 of 171) among public location OHCAs with AED application, 20% (177 of 888) among public location OHCAs with bystander CPR but no AED application, 9% (10 of 117) among private location OHCAs with AED application, and 6% (204 of 3,510) among private location OHCAs with bystander CPR but no AED application. Although the absolute survival differences observed for AED application were greater in public settings than in private settings, the positive survival association of AED application did not differ significantly according to type of location (adjusted OR, 1.39 private and 1.86 public; p = 0.53 for interaction between AED and location).
Survival to Hospital Discharge by Type of Location
Extrapolated estimate of lives saved
We extrapolated survival in the approximately 21 million people in the ROC’s covered area to the entire population of the U.S. and Canada (330 million). We estimate that currently, 474 lives are saved per year by bystander application of an AED to those who experience an OHCA.