DISCUSSION
The results of our propensity matched study indicate that in patients with HF, having a NAHIL >30,000 euros is associated with an improved prognosis, decreased mortality and hospitalizations. The most common socioeconomic indicator used in previous studies has been individual (1), household (30), or neighborhood income (4, 10). However, income may inadequately reflect socioeconomic position, particularly after retirement. Only a minority of the studies utilizing income addressed this limitation by examining additional socioeconomic indicators such as those employed in our study (30-32).
In comparison to previous observational studies (1, 2-7), our study is characterized by its prospective design and comprehensive collection of data on a numerous cohort of patients with incident HF, included in the 15 years study period, with a valid echocardiographic study in all of them. Most of the patients were diagnosed with HF as outpatients, and they present socio-demographic and clinical characteristics representative of the habitual clinical practice in this type of disease. Both the follow-up over time of the variables and the evaluation of therapeutic compliance have been comprehensive. We set out to perform PS matching and a rigorous statistical analysis of the results, adjusting for covariates not considered by previous studies, that are closely related to SES and prognosis. As SES is a time-varying exposure, we have performed multivariate analyses considering time-dependent exposure to SES. Finally and more important, this is the first study on SES and prognosis of HF including objective measures of cardiac dysfunction, HFrEF vs. HFpEF.
The observed associations between SES and HF prognosis are not fully explained by health-related behaviors, lifestyle factors, or traditional cardiovascular risk factors (32-34). The literature suggests that, the independent associations of SES with cardiovascular outcomes and mortality, at least partially reflect the influence of chronic stressors in daily life, which disproportionately affect persons in low SES groups (35, 36).These chronic stressors might result in maladaptive physiologic coping mechanisms and chronic elevations in blood pressure and inflammation that can lead to physiologic injury of the vasculature and myocardium (8). Once HF is established, deprived groups face numerous challenges, including limited access to healthcare (10), transportation costs, affordability of drug regimens (33), inequalities in treatment (34), greater co-morbidity precipitating cardiac decompensation, fewer contacts with primary care (3), and consequently more reliance on secondary care. Problems are compounded by impaired health literacy, education, and social support (35), coupled with poor compliance with medications, diet, and lifestyle restrictions (36). Further, as in our study, education levels correlate with SES (37), patients with higher education may have better understanding and knowledge of their disease process and treatment (38), while patients with lower education levels are less likely to be adherent to therapeutic recommendations (39). In addition, lower education level has been shown to be associated with poor quality of life (40), anxiety (40), physical and emotional distress (41), and inability to actively participate in self-care recommendations (42). Higher levels of education have also been shown to be associated with higher levels of disease-specific knowledge, healthy lifestyle, and improved outcomes (43-45).
Limitations. Firstly, there are limitations derived from an observational study on the habitual clinical practice, which prevent us from discounting completely any residual factors of confusion not determined, and any bias in the selection of the patients, that might explain our results. To reduce the possible influence of these weaknesses in the design of our study, a PS analysis has been performed, adjusting for a wide range of covariates. We have no data on the levels of brain natriuretic peptide (BNP), which constitutes a valuable element for the diagnosis and prognosis of patients with HF (46, 47). In our analysis, we used household income rather than total wealth or education, but adjustment for the educational level, the marital status, the living status, dependency, number of households, occupational status, total wealth and properties value, allows us to more comprehensively capture the cumulative results of SES on cardiovascular health over the life course in participants (2, 16, 17). We also lack information about the functional status, presence of anxiety or depression, the quality of life of our patients (50), and the literacy of patients or the main care-giver (48). Finally, this is a study carried out in only one center, and in one specific area of the south of Spain, with a population uniformly of white race, with medium-low socioeconomic and educational level, which has universal public health insurance giving people open, cost-free access to the health system (visits, tests and medication), thus limiting generalizability. International geographic variations in event rates has been observed in studies on HF (49).
Acknowledgements: To the members of the Grupo para la atención médica integral y continua de Cádiz (GAMIC), without whose work this paper could not have been written. To Prof. José Almenara for the performance of the statistical analyses and, to Royston Snart for his professional help in the translation of the manuscript into English.