Acute Myocardial Infarction

Introduction

Myocardial Infarction is when the heart muscle undergoes necrosis due to ischemia. The most common cause of myocardial infarction is atherosclerosis. The frequency of MI increases with age. MI affects the left ventricle most commonly, but it can also damage the atria or the right ventricle. Due to ischemia, MI can cause permanent damage to the muscle of the heart, it can impact systolic and diastolic function, and can increase risk for arrhythmia. Acute Myocardial infarction can be separated into 2 types: Non-ST-segment elevation MI (NSTEMI) and ST-segment elevation MI (STEMI).

Types of Myocardial Infarctions

Myocardial infarctions are classified into 5 types and is based on the circumstance and etiology.

Type 1: Spontaneous Myocardial Infarction. Type 1 is caused by ischemia stemming from a primary coronary event, this could be erosion, plaque rupturing.

Type 2: Myocardial infection that is caused by ischemia stemming from increased oxygen demand, or decreased oxygen supply. The increase in oxygen demand could be due to factors like hypertension, likewise, the decrease in oxygen demand could be due to an arrhythmia, hypotension, or coronary artery spasm.

Type 3: Myocardial infarction related to sudden unexpected cardiac death

Type 4a: Myocardial infarction associated with percutaneous coronary intervention

Type 4b: Myocardial infarction associated with a stent thrombosis

Type 5: Myocardial infarction associated with coronary bypass grafting

Clinical Manifestations

Acute myocardial infarction can present as:

- Dyspnea

- Fatigue

- Angina

- Substernal, visceral pain

- Silent MIs can present asymptomatic or as vague symptoms that the patient doesn't recognize as an illness (more common in diabetics or patients with known CAD)

- Syncope

- Diaphoresis

- In severe ischemia, the patient can express restlessness, apprehension and significant pain

- In right ventricular infarction, Kussmaul sign can be present with distended jugular veins, high right ventricle filling pressure, and hypotension.

Diagnosis

To diagnose MI, at least two of the following criteria should be met:

1. ST-segment changes or a Left bundle branch block
2. Symptoms of Ischemia
3. Pathological Q waves on ECG
4. Regional wall motion abnormality on imaging
5. Intra-coronary thrombus in angiography or at autopsy

ECG must be done immediately. ECG is a very specific diagnostic tool. On ECG, there can be peaked T waves which can indicate early ischemia that will progress to an ST elevation. However, not all patients may present with ST-segment elevations, this can indicate a NSTEMI. Which is why it's important to also assess cardiac biomarkers.

It's also important to test for cardiac troponin, and creating kinase-MB in the cardiac biomarker tests. The troponin test is the most specific for early acute MI detection. Troponin isoforms T and I are measured and observed. Troponin peaks at 12 hours and stays elevated for seven days. Regarding CK-MB, in the test an isoenzyme of creatine kinase is found in the myocardium and peaks at 10 hours and then normalizes in 2 to 3 days.

A echocardiogram can be used to assess the cardiac wall motion, the degree of valve abnormality, presence of cardiac tamponade, and ischemic mitral regurgitation. A cardiac angiography can also be utilized to detect any obstructions in the coronary vessels.

Sources

Mechanic OJ, Gavin M, Grossman SA. Acute Myocardial Infarction. [Updated 2023 Sep 3]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459269/

Kumar, V., Abbas, A. K., & Aster, J. C. (Eds.). (2018). Robbins basic pathology (10th ed.). Elsevier.

Sweis R, Jivan A. Acute Myocardial Infarction (MI). MSD Manual Professional Edition. Published February 2024. Retrieved December 19, 2025. https://www.msdmanuals.com/professional/cardiovascular-disorders/coronary-artery-disease/acute-myocardial-infarction-mi

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