MI pathogenesis explain with pictures

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I now have all the key images and textbook content needed. Here is a comprehensive explanation:

Myocardial Infarction (MI) - Pathogenesis

Definition

MI ("heart attack") is necrosis of cardiac muscle resulting from prolonged ischemia. It is the death of myocytes when blood supply is cut off for long enough that irreversible injury occurs. The 2018 joint task force defines it as "acute myocardial injury detected by abnormal cardiac biomarkers in the setting of acute myocardial ischemia."

Robbins & Kumar Basic Pathology, Robbins Cotran Pathologic Basis of Disease

Step 1 - Atherosclerotic Plaque Formation

The fundamental prerequisite for most MIs is coronary atherosclerosis. Lipid-rich plaques develop in the coronary artery walls over years. The coronary involved before the thrombus was often <70% stenosed - meaning it was not severely flow-limiting beforehand; the acute event is what causes MI.

Microscopic: Coronary artery cross-sections (normal vs. severely diseased)

Coronary atherosclerosis - normal (left) vs. nearly occluded (right) with organized thrombus

eFig. 9.1 - Left: coronary narrowed ~65%. Right: almost fully occluded by previous organized thrombus. (Robbins & Kumar Basic Pathology)

Step 2 - Plaque Disruption (the Trigger)

The classic sequence:

Plaque erosion or rupture - caused by endothelial injury, intraplaque hemorrhage, or mechanical shear forces. This exposes subendothelial collagen and the necrotic lipid-rich core to the bloodstream.
Platelet adhesion and activation - platelets stick to exposed collagen, aggregate, and release thromboxane A2, ADP, and serotonin, causing further platelet recruitment and vasospasm.
Coagulation cascade activation - tissue factor exposure drives thrombin generation, fibrin deposition, and rapid thrombus growth.
Complete luminal occlusion - within minutes the thrombus can occlude the coronary artery entirely.

Angiography within 4 hours of MI onset demonstrates coronary thrombosis in nearly 90% of cases. After 12-24 hours this falls to ~60%, as some thrombi lyse spontaneously.

Microscopic: Coronary thrombosis on a ruptured plaque

Coronary thrombosis - nearly occluded artery with fresh thrombus and cholesterol clefts in the plaque

eFig. 9.2 - A markedly narrowed coronary artery nearly occluded by a fresh thrombus. Cholesterol clefts () visible in the fibrointimal plaque. (Robbins & Kumar Basic Pathology)*

Step 3 - Myocardial Response to Ischemia

Once the coronary is occluded:

Time	Event
Seconds	Aerobic metabolism stops; ATP drops; toxic metabolites (lactate) accumulate
Minutes	Loss of contractility (reversible)
20-40 min	Irreversible cell injury begins - coagulative necrosis of myocytes
Earliest sign	Sarcolemmal disruption → cardiac enzymes (troponin, CK-MB) leak into blood

Robbins & Kumar Basic Pathology

The sequence begins in the subendocardium (most vulnerable - furthest from epicardial vessels, highest intramural pressure) and spreads outward as a wavefront over 3-6 hours until the full wall thickness is involved (transmural infarct) if not treated.

Step 4 - Progression of Necrosis (Wavefront Phenomenon)

Progression of myocardial necrosis after coronary artery occlusion - subendocardial at 2hrs, transmural at 24hrs

FIG. 9.8 - Necrosis begins subendocardially (2 hrs) and expands to full wall thickness (24 hrs) without intervention. A thin subendocardial rim survives via oxygen diffusion from ventricular cavity. (Robbins & Kumar Basic Pathology)

Step 5 - Patterns of Infarction by Vessel

Transmural vs nontransmural infarct patterns based on vessel involved and degree of occlusion

FIG. 9.9 - Left: transmural infarcts from permanent occlusion of LAD, LCX, or RCA. Right: nontransmural patterns from partial obstruction, global hypotension, or small vessel disease. (Robbins & Kumar Basic Pathology)

Key vessel territories:

LAD occlusion (40-50% of MIs): anterior LV wall, anterior 2/3 of septum, apex
RCA occlusion (30-40%): right ventricle, posterior/inferior LV
LCX occlusion (15-20%): lateral LV wall

Step 6 - Morphological Evolution Over Time

Gross specimen (TTC stain):

Gross specimen - acute posterolateral MI shown by pale unstained area (necrosis, arrow) with anterior scar (arrowhead)

FIG. 9.10 - Acute MI of posterolateral LV: pale unstained area = necrosis (arrow); white glistening area = remote scar (arrowhead); hemorrhage at edge = ventricular rupture (). (Robbins & Kumar Basic Pathology)*

Microscopic evolution (H&E and Masson trichrome):

Microscopic features of MI at different stages: wavy fibers, neutrophils, macrophages, granulation tissue, collagen scar

FIG. 9.11 - (A) 1-day infarct: coagulative necrosis with wavy fibers. (B) 2-3 days: dense neutrophil infiltrate. (C) 7-10 days: macrophage phagocytosis of necrotic debris. (D) Granulation tissue with new capillaries. (E) Healed scar - dense blue collagen (Masson trichrome). (Robbins & Kumar Basic Pathology)

Complete Timeline Summary

Time Frame	Gross	Microscopy
0 - 30 min	None	None (reversible: mitochondrial swelling, glycogen loss)
30 min - 4 hr	None	Sarcolemmal disruption; wavy fiber change at border
4-12 hr	Occasional dark mottling	Onset coagulative necrosis; edema; hemorrhage
12-24 hr	Dark mottling	Coagulative necrosis; pyknotic nuclei; hypereosinophilic myocytes; early neutrophils
1-3 days	Yellow-tan infarct center	Loss of nuclei/striations; dense neutrophils
3-7 days	Hyperemic border; central softening	Dying neutrophils; early macrophage phagocytosis
7-10 days	Maximally yellow-tan, soft	Well-developed macrophage phagocytosis; early granulation tissue
2-8 weeks	Gray-white scar forming	Granulation tissue → fibrosis
>2 months	Dense white scar	Dense collagen scar; no regeneration

Robbins & Kumar Basic Pathology, Table 9.2

Reperfusion Injury

If blood flow is restored after irreversible injury has begun, reperfusion injury adds to myocyte death via:

Massive intracellular calcium influx → uncontrolled myofibril contraction → contraction band necrosis
Reactive oxygen species (ROS) from reperfused leukocytes damage membranes
"No-reflow" phenomenon - leukocyte aggregation occludes the microvasculature despite opening the epicardial vessel
Platelet and complement activation contributing to microvascular injury

Despite reperfusion injury, early reperfusion (within the 3-6 hour window) is the single most effective intervention to limit infarct size.

MI Without Atherothrombosis (~10% of cases)

Vasospasm (cocaine, ephedrine use; Prinzmetal angina)
Embolism from atrial fibrillation mural thrombus, valve vegetations, or intracardiac prostheses
Vasculitis, amyloid deposition, or sickle cell vaso-occlusion of small intramyocardial arterioles
Demand ischemia: severe fixed coronary disease + tachycardia, hypertension, or anemia
Robbins & Kumar Basic Pathology; Robbins Cotran & Kumar Pathologic Basis of Disease

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