Socioeconomic status and myocardial infarction : influence on secondary prevention and prognosis

Abstract

Background and Aims: In the wealth of research undertaken on myocardial infarctions (MIs), secondary prevention is less well studied. Incidence and death from MI have declined substantially in the past decades due to the identification of cardiovascular (CV) risk factors, methods to assess risk in the general population, development of efficient therapies that modify risk factor levels, and the introduction of revascularization therapies used in the acute phase of a MI. Meanwhile, secondary prevention in the large population of MI survivors performs poorly with suboptimal management and low achievement rates of the treatment targets recommended in major prevention guidelines. There is room for improvement. Links between socioeconomic status (SES) and CV risk factors, and first-ever MI have been reported for almost 100 years. Circumstances of contemporary secondary prevention after MI suggest that SES may be an important risk factor. With this thesis, aims were to improve knowledge on SES in secondary prevention care after MI and with regards to prognosis.

Material and Methods: This thesis was based a large nationwide cohort of men and women who attended routine revisits in the year after hospitalization for acute MI between 2005 and 2013 and were registered in the Swedish National Quality Registry for cardiac care. Clinical data collected on study participants was linked with data from national registries manged by government agencies on individual-level indicators of SES (disposable income quintiles, educational level, and marital status), claimed drug prescriptions, and recurrent atherosclerotic CV disease events (ASCVD; coronary heart disease death, nonfatal MI, fatal or nonfatal ischemic stroke) during long-term follow-up. Multivariable Cox regression models were used to estimate the association between SES and recurrent ASCVD and between on-treatment blood lipid levels (total cholesterol, low and high density lipoprotein cholesterol [LDL-C and HDL-C], and triglycerides) and recurrent ASCVD. The incremental predictive value of each blood lipid fraction was assessed by addition to a secondary prevention risk score for estimates of differences in C-index and measures of reclassification. The associations between SES and most secondary prevention activities and risk factor treatment targets recommended in major guidelines on secondary prevention were assessed in logistic regression models. Differences in sociodemographic, clinical, and therapeutic characteristics of participants and non-participants in clinical trials after MI were estimated in Poisson regression models and the association between clinical trial participation and recurrent ASCVD was estimated in Cox regression models. Mediation in the association between SES and recurrent ASCVD was assessed using sequential Cox regression models and a method for mathematically consistent estimates of causal mediating effects.

Results: Risk for recurrent ASCVD was lower among study participants with higher income, higher educational level, and in marriage. The strongest association with recurrent ASCVD was observed for income and the association was independent of differences in CV risk factor profile. With 97% in the cohort on statin therapy at the 2-month revisit, recurrent ASCVD was weakly associated with achieved levels of LDL-C and strongly associated with levels of triglycerides. The adopted secondary prevention risk score discriminated poorly in the study cohort (C-index <0.6) and measures of incremental predictive power were inconsistent. Rates of achieved risk factor targets 1 year after MI were overall low and worse in low SES groups. SES was associated with achieving smoking cessation and target levels of blood pressure levels and glycated hemoglobin, but not LDL-C. Correspondingly, rates of participation in programs within comprehensive cardiac rehabilitation were also low overall, and strongly associated with SES. Higher SES was also associated with more lipid profile measurements and intensification of statin therapy during the first year post-MI. Use of risk-modifying drug therapy was high overall. At discharge from initial care, higher SES was associated with receiving dual antiplatelet therapy. One year post-MI, high SES was associated with persistent use of statins, high statin intensity, and renin-angiotensin-aldosterone system inhibitors. The 10% of this cohort who participated in a clinical trial during the first year after MI (compared to those who did not) were more likely to be men, married, have an income in the highest quintile, a post-secondary education, a better risk profile, and their risk for recurrent ASCVD was lower. In the association between SES and recurrent ASCVD, risk attenuated in sequential analysis models, primarily from adjustment for risk factor profile and secondary prevention activities but a 37% higher risk remained in the lowest vs. highest income quintile after full extensive adjustments for plausible risk mediators. Estimated proportions of the excess risk for recurrent ASCVD in the lowest income quintile mediated through risk profile, physical training and patient education within cardiac rehabilitation were significant but small whereas optimal statin therapy was not a mediator of this risk.

Conclusions: SES, by proxy disposable income level, may be a better measure than on-treatment lipid levels in the assessment of risk for recurrent ASCVD within the post-MI population. More study is needed to improve secondary prevention risk prediction, for risk-based intensified treatment to those who would likely benefit most. Secondary prevention after MI performs poorly, especially among low-income groups. Observed SES disparities regarding participation in programs within cardiac rehabilitation were mediators for higher long-term risk of recurrent ASCVD events. Hence, interventions for improved cardiac rehabilitation participation in low-income groups may improve health equity. However, the mediating proportions were small and plausible effectiveness of interventions warrant evaluation of efficacy in clinical trials. Awareness of under-representation of low SES individuals among trial participants within the post-MI population must be taken into account in designing such confirmatory trials. Further study on pathways through which low SES is associated with secondary prevention achievements and higher risk for recurrent ASCVD is needed. Adherence to therapies and dietary habits may be important areas to study.