Myocardial Infarction

Myocardial infarction (MI) is death of heart muscle caused by prolonged myocardial ischemia, most often when a coronary artery is acutely occluded by thrombus forming on a disrupted atherosclerotic plaque. It is the central clinical event of coronary artery disease and the defining injury within the acute coronary syndromes.

Definition

Myocardial infarction is the death (necrosis) of cardiocytes due to prolonged ischemia, defined clinically by a rise and/or fall of cardiac troponin together with evidence of acute myocardial ischemia such as symptoms, electrocardiographic changes, or imaging findings.

Scope

This topic covers the concept and pathophysiology of myocardial infarction, its standardized definition by evidence of myocardial injury, and its main types and electrocardiographic categories. It describes the entity for reference and does not provide diagnostic cut-offs or treatment direction for individuals.

Core questions

• What distinguishes reversible ischemia from infarction (irreversible cell death)?
• How is myocardial infarction defined and detected with cardiac troponin?
• What separates type 1 (plaque-driven) from type 2 (supply-demand) infarction?
• What is the basis of the STEMI versus NSTEMI distinction?

Key concepts

• Myocardial necrosis
• Cardiac troponin
• Coronary occlusion
• Type 1 versus type 2 myocardial infarction
• STEMI and NSTEMI
• Infarct size and time-dependence
• Wavefront of necrosis

Mechanisms

Myocardial infarction typically results when a vulnerable coronary plaque ruptures or erodes and a thrombus occludes the artery, abruptly cutting off perfusion to the downstream myocardium (Hansson, 2005). Sustained ischemia first impairs contraction and then, after a critical duration, causes irreversible necrosis that spreads as a wavefront from the inner (subendocardial) layer outward, so that infarct size grows with the duration of occlusion (Anderson, 2017). The Fourth Universal Definition operationalizes infarction by requiring evidence of myocardial injury (a troponin rise and/or fall) together with signs of acute ischemia, and distinguishes type 1 infarction from atherothrombosis from type 2 infarction caused by an oxygen supply-demand mismatch without acute plaque disruption (Thygesen, 2018).

Clinical relevance

Myocardial infarction is a major cause of death and of heart failure from lost myocardium, and the time-dependence of necrosis underlies why ischemic time is a central concept in cardiology (Anderson, 2017; Byrne, 2023). This entry describes the entity for educational reference and is not a basis for diagnosing or treating any individual.

Epidemiology

Acute myocardial infarction is among the leading causes of death worldwide and a principal contributor to the burden of ischemic heart disease; incidence increases with age and cardiovascular risk factors, with fuller epidemiology covered in the parent area.

Evidence & guidelines

The Fourth Universal Definition of Myocardial Infarction provides the internationally adopted criteria and typology for the diagnosis, and the 2023 ESC acute coronary syndrome guidelines situate infarction within evidence-based management of the broader syndrome (Thygesen, 2018; Byrne, 2023).

History

The diagnosis of myocardial infarction evolved from clinical and electrocardiographic criteria toward biomarker-based definitions as cardiac troponin assays became available. Successive universal definitions, culminating in the 2018 fourth iteration, redefined infarction around evidence of myocardial injury and introduced the type 1 to type 5 classification (Thygesen, 2018).

Debates

Distinguishing type 1 from type 2 myocardial infarction in practice

With sensitive troponin assays, many troponin elevations reflect supply-demand mismatch (type 2) rather than acute plaque rupture (type 1); reliably separating the two affects classification and study but can be clinically ambiguous.

Key figures

• Kristian Thygesen
• Jeffrey L. Anderson
• David A. Morrow

Seminal works

• thygesen-2018
• anderson-2017

Frequently asked questions

Is every myocardial infarction caused by a blocked artery from a plaque?

Most are. Type 1 myocardial infarction results from acute plaque rupture or erosion with thrombosis. But type 2 infarction occurs when oxygen supply and demand are mismatched, for example from severe anemia or tachycardia, without acute plaque disruption.

Why does the timing of a myocardial infarction matter so much?

Ischemic muscle dies progressively over time once an artery is occluded, with necrosis spreading from the inner heart wall outward. The longer perfusion is absent, the larger the infarct, which is why ischemic time is emphasized in cardiology.