In this study, we used an ML approach in a hypothesis-free manner to identify important factors of HF in ARIC study. This powerful method confirmed several well-established relationships and identified a variety of novel factors which have not been previously reported. We then used Cox regression analysis in an explorative manner to find independent factors and MR analysis to address causality. Our findings revealed that BMI, CAD, diabetes, and education, not only served as prognostic factors for HF, but potential therapeutic targets for the treatment and prevention of HF.
Established and novel prognostic factors for HF
HF prevalence has increased exponentially over the last three decades. This increase is attributable to several factors, including an aging population, and recent advances in the treatment of cardiovascular disease, leading to increased survival following an acute cardiac event.17 Prior studies have yielded inconsistencies in predictors of HF. Our ML results indicated that the 20 variables with the highest importance selected by RSF for HF are creatinine, glucose, age, previous CAD, systolic blood pressure, fibrinogen, albumin income, diabetes, magnesium, insulin, white blood cell, hemoglobin, sodium, education level, phosphorus, diastolic blood pressure, protein-c, heart rate and BMI. It confirmed several established risk factors as previously reported (e.g., age, history of CAD, diabetes, BMI, hemoglobin, total white blood cell, creatinine18 and hypertension).19-20 Yet to our surprises, a recent study in 20,254 US male veterans revealed that increased cardiorespiratory fitness was associated with progressively lower HF risk regardless of BMI, challenging BMI served as a well-established risk factors for HF.21 Our result was different from the above research, and we analyzed that it might due to the different population included.
As for glucose impartment, it was reported that there existed a positive, continuous, and independent association between fasting plasma glucose and risk for HF.22 The British Regional Heart Study carried out in older men demonstrated that serum magnesium was inversely related to risk of incident HF after adjustment for conventional CVD risk factors and incident MI.23 Our results also support a positive association between glucose and HF and a negative association between magnesium. A number of other well-established factors that have been reported in literatures, including smoking, atrial fibrillation, chronic obstructive pulmonary disease,1,19 were not observed among the top 20 predictors by ML approach. Though not selected, it did not mean that these traditional risk factors were not important for HF, as most of risk factors had adverse effects on cardiac structure, which ultimately would result in HF.
It was noteworthy that several novel predictors of HF were identified, including fibrinogen, albumin, income, education, phosphorus, protein-c. Fibrinogen was suggestive associated with incident HF that had preserved ejection fraction (HR 1.12; 95% CI 1.03-1.22; P=0.01).24 A prospective study of 3,366 men found that fibrinogen was associated with incident HF but this was abolished after adjustment for HF risk factors.25 So fibrinogen as a risk factor for HF was still controversial. Many epidemiological studies have suggested an inverse association between serum albumin level and HF. In the Health ABC study of 2,907 elderly individuals with a 9.4 years follow-up, low serum albumin level was associated with the development of new-onset HF, mainly with preserved ejection fraction, regardless of inflammatory markers, BMI and CHD.25 So low albumin level might serve as a novel predictor of increased risk of HF. In a very large population (N = 7,638,524) of chronic HF patients with access to universal healthcare, lower income was independently associated with higher mortality.27 Another study conducted in 54 countries reported that greater income inequality was associated with worse HF outcomes, with an impact similar to those of major comorbidities.28 More importantly, previous MR study demonstrated that genetic predisposition towards 3.6 years of additional education was associated with a one third lower risk of CAD, supporting that low education is a causal risk factor in the development of CAD,29 which is the major cause of HF. These studies suggested that socioeconomic status (income and education) might also affect HF besides the traditional risk factors.
Low serum magnesium and high serum phosphorus were identified independently associated with greater risk of incident HF in ARIC cohort,30 which was in accordance with our results. Our multivariable analysis revealed that protein C was slight negative associated with HF. Yet a prospective case-control study that involved 50 children demonstrated that there was a significant increase in plasma levels of cardiac myosin binding protein-C in patients with HF and this increase was associated with increased severity of HF, indicating positive association between cardiac myosin binding protein-C and HF. We analyzed the different associations might lie to different HF population and different kinds of protein-C.
In summary, by ML approach and multivariable analysis, we identified 13 traditional risk factors, including creatinine, glucose, age, previous CAD, systolic blood pressure, diabetes, magnesium, insulin, white blood cell, hemoglobin, sodium, diastolic blood pressure, heart rate and BMI, and 6 novel risk factors, including fibrinogen, albumin, income, education, phosphorus and protein-c.
Causality between the potential risk factors and HF
It would be of clinical value if the modifiable risk factors, such as BMI and education, were shown to causally lead to the development of HF.
As for causal factors of HF, hyperhomocysteinemia30 and elevated lipoprotein(a) levels32 were reported to be causally associated with HF. Because these two factors were not selected as top 20 variables, we did not analyze their causal estimate with HF. To our surprise, a recent MR study reported that though there was an observational association of CAD with HF, the genetically determined risk of CAD was significantly associated with HF with reduced ejection fraction but not with HF with preserved ejection fraction,33 indicating that HF with reduced and preserved ejection fraction should be treated differentially.
Our MR analysis showed that the genetically predicted BMI, CAD and diabetes was positive casually associated with HF, and education as a novel factor was also negative casually associated with HF. Among these four factors, education might serve as the source of some established risk factors, for education reflected socioeconomic circumstances and cognitive level. People with higher education level were usually with more self-management skills to maintain healthy status and access better health care. Previous findings also indicated a causal association between low educational attainment and increased risk of smoking,34 which was risk factors for both CAD and diabetes.