Explain me a concept of explain me a concept of myocardial infraction myocardial infraction in fractioon

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Myocardial Infarction (MI) - "Heart Attack"

What Is It?

Myocardial infarction (MI) is necrosis (death) of heart muscle resulting from ischemia - inadequate blood supply to meet the metabolic demand of the myocardium. The 2018 joint task force of U.S. and European Cardiology groups defines it as "the presence of acute myocardial injury detected by abnormal cardiac biomarkers in the setting of evidence of acute myocardial ischemia."

It is commonly called a "heart attack" and is one of the leading causes of death worldwide. - Robbins & Kumar Basic Pathology, p. 353

Causes and Risk Factors

The vast majority (~90%) of MIs result from acute thrombosis of a coronary artery overlying a disrupted atherosclerotic plaque. The remaining ~10% occur from:

Coronary artery vasospasm
Embolism from mural thrombi (e.g., in atrial fibrillation) or valve vegetations
Severe fixed coronary atherosclerosis without thrombosis (especially subendocardial infarcts)
Disorders of small intramyocardial arterioles: vasculitis, amyloid deposition, sickle cell disease

Key risk factors: aging, male sex (women are relatively protected until menopause), hypertension, diabetes, dyslipidemia, smoking, and family history. About 10% of MIs occur before age 40. - Robbins & Kumar Basic Pathology, p. 353

Pathogenesis - How Does MI Happen?

The typical sequence of events:

An atheromatous plaque erodes or ruptures due to endothelial injury, intraplaque hemorrhage, or mechanical forces - exposing subendothelial collagen and necrotic plaque contents to circulating blood.
Platelets adhere, aggregate, and activate, releasing thromboxane A2, ADP, and serotonin - causing more platelet aggregation and vasospasm.
Coagulation is activated by exposed tissue factor, adding to the growing thrombus.
Within minutes, the enlarging thrombus completely occludes the coronary artery lumen.

Angiography performed within 4 hours of onset shows coronary thrombosis in almost 90% of cases. - Robbins & Kumar Basic Pathology, p. 354

Histology of coronary atherosclerosis and thrombosis:

Microscopic cross-sections of coronary arteries. Left: 60-70% narrowed by atherosclerosis. Right: Near-total occlusion from organized thrombus.

Coronary atherosclerosis (left: 60-70% narrowed; right: near-total occlusion). - Robbins & Kumar Basic Pathology

Microscopic image of a coronary artery almost completely occluded by a fresh thrombus sitting atop a fibrointimal plaque with cholesterol clefts.

Fresh coronary thrombus nearly occluding the arterial lumen. - Robbins & Kumar Basic Pathology

What Happens to the Myocardium After Occlusion

Immediate metabolic changes (seconds to minutes)

Aerobic metabolism ceases within seconds of obstruction
ATP drops rapidly; lactic acid and other toxic metabolites accumulate
Contractility is lost within minutes
These changes are reversible at this stage

Irreversible injury threshold (20-40 minutes)

If ischemia persists for 20-40 minutes, irreversible coagulative necrosis of myocytes occurs
The sarcolemmal membrane ruptures, leaking intracellular macromolecules (troponins, CK-MB) into the bloodstream - this is the basis of cardiac biomarker testing

Wavefront of necrosis

Irreversible injury starts in the subendocardial zone first (it is most distal from epicardial vessels and under highest intramural pressure). With prolonged ischemia, the wavefront moves outward toward the epicardium, driven by tissue edema and reactive oxygen species.

Oxygen requirements

The heart requires about 1.3 mL O2/100g of muscle/min just to survive. The normal resting left ventricle receives ~8 mL O2/100g/min. Even 15-30% of normal coronary flow may be enough to keep muscle alive. In the core of a large infarct with near-zero collateral flow, muscle death is inevitable. - Guyton & Hall Medical Physiology, p. 271

Types of MI

Type	Description
STEMI (ST-elevation MI)	Full-thickness (transmural) infarct; complete occlusion
NSTEMI (Non-ST-elevation MI)	Partial-thickness (subendocardial); partial or transient occlusion
Type 1	Spontaneous MI from plaque rupture/erosion
Type 2	MI secondary to supply-demand mismatch (e.g., severe anemia, tachycardia)
Types 3-5	MI related to cardiac death, PCI, or CABG procedures

(Fourth Universal Definition of MI, 2018)

ECG Changes in MI

The three major electrical abnormalities in acute MI are:

Defect in Infarcted Cells	Current Flow	ECG Change Over Infarct
Rapid repolarization (accelerated K+ channel opening)	Out of infarct	ST segment elevation
Decreased resting membrane potential (loss of intracellular K+)	Into infarct	TQ depression (appears as ST elevation)
Delayed depolarization	Out of infarct	ST segment elevation

The hallmark of acute MI is ST-segment elevation in leads overlying the infarcted area. Leads on the opposite side show ST depression (reciprocal changes).

After days to weeks, ST changes resolve. The dead, electrically silent tissue manifests as pathological Q waves - because the infarct area fails to contribute positive voltage during depolarization. - Ganong's Review of Medical Physiology, p. 534

Causes of Death After MI

1. Decreased Cardiac Output - Cardiogenic Shock

When healthy portions of the ventricle contract, the infarcted, non-functional muscle bulges outward instead of contracting inward - this is called "systolic stretch" (paradoxical wall motion). This wastes pumping force and dramatically reduces cardiac output.

Diagram of the heart showing normal contraction on one side and nonfunctional muscle undergoing systolic stretch (outward bulging) on the other side during systole.

Systolic stretch: nonfunctional muscle bulges outward when healthy muscle contracts. - Guyton & Hall Medical Physiology

Cardiogenic shock occurs when >40% of the left ventricle is infarcted. Mortality is 40-50% even with modern treatment.

2. Pulmonary Edema

Failure to pump blood forward causes damming of blood in the pulmonary circulation. Reduced cardiac output impairs renal perfusion and urine output, gradually increasing blood volume - leading to acute pulmonary edema, sometimes appearing days after the initial event.

3. Ventricular Fibrillation

In 80-90% of cardiac deaths after MI, death is from ventricular fibrillation (not mechanical failure). Factors promoting fibrillation:

K+ leaks out of ischemic cells → elevated extracellular K+ → increased irritability
Injury currents from partially repolarized ischemic muscle
Sympathetic activation (baroreceptor reflex response to low blood pressure) increases cardiac irritability
Local acidosis from lactic acid

Two peak periods for fibrillation: within 10 minutes of infarction, and again at ~1 hour. - Guyton & Hall Medical Physiology, p. 272

4. Cardiac Rupture (less common)

Necrotic myocardium can rupture, causing cardiac tamponade or acute mitral regurgitation from papillary muscle rupture.

Stunned vs. Hibernating Myocardium

Stunned myocardium: When reperfusion occurs before irreversible injury, the myocardium is saved but remains non-contractile for days due to persistent biochemical abnormalities. This is reversible.
Hibernating myocardium: Chronically hypoperfused but viable muscle that reduces its function to match supply. Contractility recovers with revascularization.

Cardiac Biomarkers

Sarcolemmal rupture in necrotic myocytes releases intracellular proteins into the bloodstream:

Biomarker	Rises	Peaks	Returns to Normal
Troponin I/T (most specific)	2-4 hrs	24-48 hrs	5-14 days
CK-MB	4-6 hrs	12-24 hrs	48-72 hrs
Myoglobin	1-2 hrs	6-9 hrs	24 hrs

Troponin is the gold-standard biomarker because it is highly sensitive and cardiac-specific.

Treatment Principles

The goal is to restore perfusion as quickly as possible (time = muscle):

Primary PCI (percutaneous coronary intervention / angioplasty + stent): treatment of choice for STEMI, ideally within 90 minutes of first medical contact
Thrombolysis: if PCI is not available within 120 minutes
Antiplatelet therapy: aspirin + P2Y12 inhibitor (clopidogrel, ticagrelor)
Anticoagulation: heparin
Beta-blockers: reduce cardiac work and arrhythmia risk
ACE inhibitors / ARBs: reduce remodeling post-MI
Statins: stabilize atherosclerotic plaques long-term

Summary in one sentence: MI is death of heart muscle due to prolonged ischemia, most often from coronary artery thrombosis on a ruptured atherosclerotic plaque, causing a cascade from ATP depletion to coagulative necrosis, manifesting clinically as chest pain, ECG changes, and rising cardiac biomarkers - and potentially leading to arrhythmia, pump failure, or death if not treated urgently.

Sources: Robbins & Kumar Basic Pathology; Guyton & Hall Textbook of Medical Physiology; Ganong's Review of Medical Physiology; Braunwald's Heart Disease

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