In this population-based study of out of hospital cardiac arrest, women suffered a cardiac arrest at an older age than men, and were more likely to present with PEA or asystole and to arrest at home, all established determinants of worse survival. However, women were significantly more likely to have return of spontaneous circulation. When survival was examined within each presenting arrhythmia (VF/VT or PEA), women were also more likely to survive compared to men. In a multivariable analysis, after adjusting for circumstances of arrest, disease burden, race and median household income, female gender was independently associated with higher likelihood of survival from hospital discharge. This is the first comprehensive evaluation of gender differences in survival from SCA by incorporation of variables such as race, socioeconomic status and the lifetime clinical history, elements that had not been considered in earlier analyses. The findings indicate that in addition to ROSC and survival to hospital admission, survival to hospital discharge is also better in women presenting with VF and PEA, compared to men. The disparate findings from the only other study with survival to hospital discharge as an endpoint [11
] could be explained by differences in analysis methodology (PEA and asystole were treated as one group) as well as the comprehensive nature of the present analysis.
Why do women have a survival advantage over men when it comes to sudden cardiac arrest? The specific reasons responsible for this phenomenon are likely to require further evaluation but there are several possibilities. It has been reported that women who suffer a cardiac arrest are less likely to have underlying coronary artery disease [28
] or a diagnosis of structural heart disease [22
] and these could be contributing factors. It is also possible that higher estrogen levels in women are protective and influence SCA outcome. For example, women of reproductive age (13–49 years) have a better survival from SCA compared to men and women of other ages and this phenomenon could be explained by the cardioprotective effects of endogenous estrogen [26
]. In the present study, we have found that women < 50 years were more likely to survive to hospital discharge compared to men or women ≥ 50 years of age. Recently, we have identified a novel genomic locus that has protective effects against SCA [29
]. It is possible that genetic variations that are gender-specific (i.e. found on the X-chromosome) may play a role in the pathophysiology and outcome of cardiac arrest. Additionally, we observed no significant gender differences in the availability of bystander CPR, witnessed cardiac arrest or the response time. We have recently reported that the clinical correlates of PEA are multifactorial but also gender- and race-specific [24
Earlier studies that evaluated race and survival have also reported mixed results. Some report lower survival among African Americans [14
], but more recent studies have found no significant association after adjusting for SES and prior cardiac and functional status [16
]. The incorporation of lifetime clinical history (i.e., disease burden) in our study showed a trend, of borderline significance, toward lower survival in African Americans. These findings correlate with significantly higher frequency of PEA among African Americans in the present study as well as in a separate analysis that we have published [24
] and lower proportion of VF [14
], both scenarios that decrease likelihood of survival. In our study population, the majority of subjects were white (86%) with African Americans representing only 7%. Therefore, the borderline significance in the survival from SCA could be explained by small numbers of African Americans in this population. Either way, the reasons for higher prevalence of PEA among African Americans need to be investigated in further detail. Consistent with earlier studies [4
], we report that out-of-hospital SCAs that were witnessed, occurred in public places, and presented with VF were associated with better survival.
There are some limitations associated with this analysis. Survival information was unknown for 3.5% of subjects with a slightly higher proportion in men compared to women (4.0% vs. 2.3%).However, this was not statistically significant (p=0.11). In addition, when a conservative scenario was considered by assuming that all subjects with unknown information survived the cardiac arrest, female gender remained a significant determinant of survival. Inhospital course and management of the disease may provide more insights regarding potential mechanisms of gender differences in survival but this information was not available for this analysis. Additionally, the race distribution in our population is mainly White non-Hispanic and therefore the findings of this study need to be replicated in a larger multi-ethnic study of SCA.