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Background and aim: Despite robust evidence for lipid-lowering therapy in secondary prevention, significant gaps persist between clinical trial findings and real-world implementation following acute coronary syndrome. This thesis aims to investigate the relationship between lipid levels and long-term prognosis after MI, and to characterize long-term secondary prevention monitoring in clinical practice.

Methods: This thesis utilizes Sweden's comprehensive registry infrastructure, using the myocardial infarction registry SWEDEHEART and national health registers to examine lipid management across the pathway from acute hospitalization through long-term follow-up. Paper I examined early LDL-cholesterol changes and outcomes in 40,607 myocardial infarction patients. Paper II investigated the ”cholesterol paradox” in 63,168 statin-naïve patients with first myocardial infarction. Paper III assessed timing of non-HDL-cholesterol target achievement in 56,262 myocardial infarction patients. Paper IV evaluated long-term risk factor management in 6,083 patients followed in hospital and primary care for three years post-acute coronary syndrome.

Results: In Paper I, early aggressive LDL-cholesterol lowering was associated with reduced rates of cardiovascular events and mortality, with benefits apparent within six months and a 25% event reduction per 1 mmol/L decrease. Paper II revealed that the apparent cholesterol paradox—where low cholesterol at presentation predicts higher mortality—occurs likely because patients with lower baseline LDL-cholesterol were older with more comorbidities and higher non-cardiovascular mortality risk, while cholesterol reduction remained beneficial regardless of baseline levels. In Paper III, early achievement and sustained maintenance of non-HDL-cholesterol targets was associated with better outcomes compared with delayed attainment, challenging current guideline directed stepwise treatment approaches. Paper IV demonstrated deteriorating long-term risk factor control, with half of patients lacking LDL-cholesterol measurements and not achieving targets at three years. Male patients and those receiving structured follow-up had better monitoring.

Conclusions: This thesis validates the real-world effectiveness of intensive lipid-lowering therapy while revealing critical implementation gaps. The cholesterol paradox represents methodological artifacts rather than clinical concern, supporting universal treatment regardless of presenting levels. Early intensive combination therapy may be superior to current guideline-directed stepwise approaches and there remains substantial room for improvement in population-level implementation. Future strategies should focus on early intensive treatment algorithms and structured long-term monitoring systems.