Impact of MetS on Long-Term Prognosis Among STEMI Patients Treated with pPCI—Ten-Year Follow-Up Study

Background/Objectives: Metabolic syndrome (MetS) affects more than 1.5 billion adults worldwide and is present in 37–70% of STEMI patients. Its ten-year prognostic value after primary PCI—particularly for heart failure, which is rarely examined as a primary endpoint—remains incompletely characterized. Methods: In total, 506 STEMI patients treated with primary PCI (December 2009–June 2010) were followed for ten years. MetS was defined at admission using AHA/NHLBI criteria. Co-primary endpoints were all-cause mortality, MACE, and hospitalization for heart failure. Multivariable Cox regression was adjusted for sex, age, LVEF, previous MI, Killip class, and multivessel disease. Four ML models were evaluated by 10-fold stratified cross-validation with SHAP-based feature, with a Fine–Gray subdistribution-hazard sensitivity analysis for heart failure. Feature attribution used TreeSHAP on XGBoost and permutation importance on a Random Survival Forest. Results: MetS(+) patients were older, more frequently female, and had higher SYNTAX scores (all p < 0.05). MetS was present in 216 patients (42.7%). It did not independently predict mortality (HR 1.09, p = 0.66) but did predict MACE (HR 1.47, p = 0.028) and heart failure hospitalization (cause-specific HR 2.86, 95% CI 1.57–5.22; Fine–Gray HR 2.61, 95% CI 1.44–4.75; both p ≤ 0.002). The null mortality finding coincided with differential statin discontinuation and a selective obesity paradox: in non-obese patients, MetS doubled mortality (42.9% vs. 21.1%, p = 0.008), while in obese patients, the effect disappeared (26.5% vs. 23.2%, p = 0.529). Two independent ML frameworks ranked the cumulative number of MetS criteria—rather than the binary diagnosis—among the leading individual-level features for heart failure prediction (Random Survival Forest c-index 0.843). Conclusions: In primary PCI-treated STEMI survivors, MetS independently predicts ten-year MACE and heart failure but not mortality. The number of MetS criteria at baseline, rather than the binary classification, was more strongly associated with heart failure risk; whether prospective modification of individual components reduces this risk requires dedicated interventional studies. The lean MetS-positive phenotype may represent a candidate subgroup warranting further investigation.