Acute Myocardial Infarction: Ischemic Cascade Explained

Myocardial infarction refers to the irreversible necrosis of the heart myocytes. It leads to impaired cardiac function due to obstruction of the coronary artery, leading to inability to supply the increased demand.

MI is typically categorized based on ECG changes, especially the ST phase, as:

• ST-elevation myocardial infarction (STEMI) – usually due to complete coronary occlusion

• Non-ST-elevation myocardial infarction (NSTEMI) – usually due to partial occlusion

Coronary Atherosclerosis: The Underlying Substrate

Atherosclerotic plaques occluding the coronary arteries are responsible for the ischemic necrosis of the heart.

A. Endothelial Dysfunction

Endothelial damage occurs due to various factors, such as:

• Smoking

• Diabetes

• Hypertension

• Hyperlipidemia

Endothelial dysfunction leads to:

• Increased permeability of LDL through the basement membrane

• Recruitment of monocytes and platelets

• Increased cellular adhesion

B. Fatty Streak Formation

• LDL becomes oxidized in the intima.

• Monocytes differentiate into macrophages.

• Lipid-laden macrophages (foam cells) accumulate, forming fatty streaks.

C. Fibrous Plaque Formation

Smooth muscle cells migrate from the media to the intima and proliferate. They synthesize extracellular matrix components (collagen and proteoglycans), forming a fibrous cap over a lipid-rich necrotic core.

Stable plaques have thick fibrous caps. Vulnerable plaques have thin caps and large lipid cores, predisposing them to rupture.

2. Platelet Aggregation and Thrombus Formation

This is the most critical part of the mechanism.

A. Plaque Disruption

Mechanical stress and inflammatory degradation of the fibrous cap (via macrophage-derived metalloproteinases) cause rupture. This exposes highly thrombogenic materials, such as:

• Collagen

• Tissue factor

B. Platelet Activation

Exposure of subendothelial collagen leads to:

1. Platelet adhesion

2. Platelet activation

3. Release of ADP and thromboxane A₂

4. Platelet aggregation

C. Coagulation Cascade Activation

Tissue factor activates the extrinsic pathway of coagulation, resulting in thrombin generation and fibrin formation.

A thrombus forms, which may:

• Partially occlude the vessel (NSTEMI)

• Completely occlude the vessel (STEMI)

4. Myocardial Ischemia and Cellular Injury

Once coronary blood flow decreases significantly:

A. Oxidative Stress and Mitochondrial Dysfunction

Acute ischemic phase → Interruption of oxygen supply to the myocardium → Impaired mitochondrial electron transport chain (ETC) function → Overproduction of superoxide anions.

B. Metabolic Changes (Within Seconds)

• Aerobic metabolism ceases.

• ATP production declines.

• Cells switch to anaerobic glycolysis.

• Lactate accumulates, causing intracellular acidosis.

C. Loss of Contractility (Within 60 Seconds)

Reduced ATP impairs:

• Actin–myosin cross-bridge cycling

• Calcium handling

This results in early systolic dysfunction.

D. Ionic Pump Failure

ATP depletion disrupts:

• Na⁺/K⁺ ATPase

• Ca²⁺ ATPase

Consequences:

• Intracellular Na⁺ accumulation

• Calcium overload

• Cellular swelling

5. Progression to Irreversible Injury

If ischemia persists beyond approximately 20–40 minutes:

• Mitochondrial dysfunction becomes irreversible.

• Sarcolemmal membrane integrity is lost.

• Cell necrosis occurs.

Necrotic myocytes release intracellular proteins into circulation, including:

• Cardiac troponin I and T (most specific markers)

• CK-MB

6. Wavefront Phenomenon of Necrosis

In a complete occlusion (STEMI), necrosis progresses from:

1. Subendocardium (most vulnerable due to highest oxygen demand and lowest perfusion pressure)

2. Toward the epicardium

This transmural progression may take several hours.

7. Inflammatory Response and Healing

A. Acute Inflammatory Phase (0–3 days)

• Neutrophils infiltrate necrotic tissue.

• Cytokines amplify inflammation.

• Myocardium is structurally weakened.

B. Macrophage Phase (3–7 days)

• Macrophages remove necrotic debris.

• Risk of myocardial rupture is highest during this period.

C. Granulation and Scar Formation (Weeks)

• Fibroblasts proliferate.

• Collagen deposition replaces necrotic myocardium.

• Permanent fibrotic scar forms (non-contractile).

8. Reperfusion Injury

Restoration of blood flow is essential but may paradoxically cause additional injury due to:

• Reactive oxygen species

• Calcium overload

• Inflammatory activation

Clinically, this may manifest as reperfusion arrhythmias.

9. Hemodynamic and Electrical Consequences

MI can lead to:

• Reduced stroke volume and cardiac output

• Cardiogenic shock (if a large territory is involved)

• Ventricular arrhythmias

• Heart failure

• Mechanical complications (papillary muscle rupture, septal rupture, free wall rupture)

10. Summary of Pathophysiologic Sequence

1. Endothelial dysfunction → atherosclerotic plaque

2. Plaque rupture → platelet activation and thrombus formation

3. Coronary occlusion → myocardial ischemia

4. ATP depletion → ionic imbalance → necrosis

5. Inflammation → scar formation

6. Permanent loss of contractile myocardium

Clinical Correlation

In STEMI, complete occlusion produces transmural infarction with ST-segment elevation on ECG. In NSTEMI, partial occlusion produces subendocardial infarction without ST elevation.

Early reperfusion therapy (e.g., PCI or thrombolysis) aims to interrupt this pathophysiologic cascade and preserve viable myocardium.

References

1. Robbins & Cotran Pathologic Basis of Disease. Kumar V, Abbas AK, Aster JC. Elsevier; 2020.

2. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine. Zipes DP, Libby P, Bonow RO, Mann DL, Tomaselli GF, editors. Elsevier; 2021.

3. Thygesen K, Alpert JS, Jaffe AS, et al. Fourth Universal Definition of Myocardial Infarction (2018). European Society of Cardiology / American College of Cardiology / American Heart Association / World Heart Federation. Circulation. 2018;138:e618–e651.

4. American Heart Association. 2023 AHA/ACC Guideline for the Management of Acute Coronary Syndromes. Circulation. 2023.