Myocardial infarction (MI), commonly known as a heart attack, occurs when a portion of the heart muscle suffers damage or death due to a prolonged lack of blood flow. This interruption of blood supply deprives the tissue of the oxygen and nutrients it needs to survive. MI is not a single, uniform event but rather a spectrum of conditions with differing causes and severities. Standardized classification systems are used to categorize the specific type of event, ensuring effective treatment strategies and consistent medical research.
Establishing the Diagnostic Criteria
Diagnosing a myocardial infarction relies on recognizing a combination of clinical observations and objective biological measurements. The initial suspicion often arises from clinical symptoms, which typically include chest pain, shortness of breath, and sometimes nausea or unexplained weakness. These symptoms suggest a problem with blood flow to the heart but do not confirm tissue death.
Objective confirmation of heart muscle damage requires measuring specific substances released into the bloodstream. Cardiac biomarkers, particularly troponin, are proteins normally confined within heart cells. When heart tissue is injured or dies, these proteins leak into the circulation, and a rise and fall pattern in troponin levels above a certain threshold confirms myocardial injury.
The third component of diagnosis involves evaluating the heart’s electrical activity using an electrocardiogram (ECG). The ECG records electrical impulses and can show changes indicative of acute ischemia, such as ST-segment deviations or the development of new pathological Q waves. While symptoms raise suspicion, the classification and confirmation of an MI depend on these objective findings from the blood test and the ECG.
Classification Based on Electrocardiogram Findings
The initial ECG reading is a time-sensitive tool used for immediate triage, leading to the most widely recognized classification: ST-segment Elevation Myocardial Infarction (STEMI) and Non-ST-segment Elevation Myocardial Infarction (NSTEMI). This distinction is based on the appearance of the ST segment, the portion of the ECG tracing that connects the ventricular depolarization and repolarization phases.
A STEMI is diagnosed when the ECG shows a sustained elevation in the ST segment. This finding typically signifies a complete and persistent blockage of a major coronary artery, preventing blood flow to a large area of the heart muscle. Because the entire thickness of the heart wall is at risk, a STEMI is considered a medical emergency requiring immediate reperfusion therapy to reopen the blocked artery, often through a procedure like angioplasty.
In contrast, an NSTEMI occurs when there is evidence of myocardial damage, but the ECG lacks ST-segment elevation. The ECG may instead show subtle changes such as ST-segment depression or T-wave inversion, or it may even appear normal. An NSTEMI generally indicates a partial or transient blockage of a coronary artery. While still serious, the treatment pathway for NSTEMI is usually less time-critical than for STEMI, allowing for medical stabilization before deciding on invasive procedures.
Classification Based on Underlying Etiology
Beyond the ECG-based classification, a more comprehensive system, known as the Fourth Universal Definition of Myocardial Infarction, categorizes events based on the underlying cause or mechanism of injury. This system recognizes that heart muscle damage can result from various circumstances, leading to five distinct types of MI.
Type 1 Myocardial Infarction represents the classic event, caused by the rupture or erosion of an atherosclerotic plaque within a coronary artery. This plaque disruption leads to the formation of a blood clot (thrombus) that severely limits or completely obstructs blood flow, making it the most common cause of heart attack.
Type 2 Myocardial Infarction results from an imbalance between the heart muscle’s oxygen supply and its demand. Unlike Type 1, this type is not caused by acute plaque rupture, but by conditions that stress the heart, such as severe anemia, high or low blood pressure, or a rapid heart rhythm. In these cases, the oxygen demand exceeds the supply, even if the coronary arteries are only moderately narrowed.
Type 3 MI accounts for specific clinical situations where a patient dies suddenly with symptoms strongly suggestive of a heart attack, including presumed new ischemic ECG changes, before blood samples or troponin levels can become conclusive. This type acknowledges events where the patient’s condition prevents the full diagnostic workup.
Type 4 and Type 5 MIs are related to specific medical procedures. Type 4 MI is associated with percutaneous coronary intervention (PCI), a procedure often involving stenting to open blocked arteries. This category is divided into Type 4a (related to the procedure) and Type 4b (related to stent thrombosis). Type 5 MI is specifically linked to Coronary Artery Bypass Grafting (CABG) surgery, where the heart muscle damage is a complication of the surgical process.