Exploring the differential association between greenness exposure and mortality: the case of individuals with coronary heart disease

Abstract Background Evidence suggests an inverse association between surrounding greenness and mortality among the general population and individuals with coronary heart disease (CHD). Little is known whether greenness-related mortality reduction differs between CHD and CHD-free individuals. Aim To compare associations of greenness and all-cause mortality between individuals with and without CHD. Methods Data from four Israeli cohorts were utilized: Two population-based cohorts derived from two national health surveys (n=3,246, inception years 1999–2001; n=1,799, 2005–2006) and two CHD patient-based cohorts (n=1,521, 1992–1993; n=12,784, 2004–2014). The two latter comprised patients hospitalized with an acute coronary syndrome (ACS, i.e., acute myocardial infarction or unstable angina pectoris) or stable coronary artery disease (i.e., undergoing percutaneous coronary intervention without ACS indication). Participants who self-reported preexisting CHD at baseline in the general population cohorts were excluded. Exposure to residential greenness was estimated using the Normalized Difference Vegetative Index (NDVI), a satellite image-based measure ranging from –1.0 to 1.0, with larger values indicating higher levels of vegetation density. NDVI was calculated within 100m, 300m, and 800m radii around each participant’s home address and averaged over the entire follow-up period. Data on all-cause mortality (last update: 2018) was retrieved from national registries. Cox models were constructed to assess association overall and by vulnerability level (CHD-free, stable disease, and ACS). All models were adjusted for harmonized covariates across cohorts, including age, sex, ethnicity, neighborhood socio-economic status, smoking, diabetes, hypertension, stroke, and year of study entry. Results A total of 17,217 participants were included in the study [mean (SD) age, 63.8 (15.3); 42.3% women]. Among them, 3,778 (22%) were free of CHD at baseline, 4,762 (28%) had stable disease, and 8,677 (50%) had ACS. During a median [interquartile range (IQR)] follow-up of 9 (5–12) years, 4,736 deaths occurred. The mean (range) period NDVI in the four cohorts was 0.11 (0.01–0.25). In the pooled analysis, an IQR increase in 300m-NDVI was associated with an adjusted hazard ratio (HR) of 0.93 [95% confidence interval (CI) 0.89, 0.97) for mortality. Stratification by vulnerability level showed heterogeneous results (p=0.03). While there was no association among CHD-free individuals (HR=1.11, 95% CI 0.98, 1.26), an inverse relationship was observed among CHD individuals, with a stronger association among ACS compared with stable disease individuals (ACS: HR=0.92, 95% CI 0.87, 0.98 vs. Stable: HR=0.95, 95% CI 0.87, 1.03). A similar pattern was seen across the different radii zones. Conclusion Residential green spaces exhibit a stronger association with reduced mortality risk in individuals with CHD compared to those without CHD, especially noticeable among ACS patients.