15.1 Pathophysiology of Atherosclerosis, Heart Failure, COPD, and Pneumonias

14.4 Pathophysiology of Atherosclerosis, Heart Failure, COPD, and Pneumonias

1. Atherosclerosis and Ischemic Heart Disease

Pathogenesis of Atherosclerosis

Atherosclerosis is a chronic inflammatory and proliferative response of the arterial wall to endothelial injury. The progression unfolds through six distinct pathological stages:

1. Endothelial Dysfunction & Injury: Triggered by hemodynamic strain, hyperlipidemia, smoking, or hyperglycemia. Permeability to circulating lipoproteins increases.
2. LDL Accumulation & Oxidation: Low-density lipoprotein (LDL) enters the subendothelial intima where it undergoes oxidation by reactive oxygen species (ROS). Oxidized LDL (oxLDL) is pro-inflammatory and cytotoxic.
3. Monocyte Recruitment & Foam Cell Formation: Endothelial cells express adhesion molecules (VCAM-1, ICAM-1). Monocytes migrate into the intima, differentiate into macrophages, and engulf oxLDL via scavenger receptors (CD36, SR-A). Lipid-laden macrophages become foam cells.
4. Fatty Streak Formation: Aggregations of subendothelial foam cells form visible yellow streaks, the earliest microscopic lesion of atherosclerosis.
5. Smooth Muscle Cell Migration & Matrix Synthesis: Platelet-derived growth factor (PDGF) and TGF-beta induce vascular smooth muscle cell (VSMC) migration from the media to the intima. VSMCs proliferate and deposit extracellular matrix (collagen, elastin), forming a fibrous cap over a central necrotic core rich in lipids and debris.
6. Complications: Plaque destabilization occurs via metalloproteinase secretion. Rupture exposes tissue factor, triggering acute thrombosis (causing unstable angina, MI, or stroke). Long-term media erosion causes vessel wall weakening and aneurysm formation (typically abdominal aortic aneurysm).

Ischemic Heart Disease (IHD) Spectrum

Ischemic heart disease results from an imbalance between myocardial oxygen demand and coronary blood supply.

Myocardial Infarction: Histological Timeline & Complications

Following acute coronary occlusion, myocardial tissue undergoes a sequential histological evolution:

• 0\u20134 Hours: Minimal light microscopic change; electron microscopy reveals wavy myocardial fibers and glycogen depletion.
• 4\u201324 Hours: Early coagulation necrosis, contraction band necrosis at edges, hypereosinophilic myocytes, and marginal edema. Risk of fatal ventricular arrhythmias (ventricular fibrillation) is highest.
• 1\u20133 Days: Extensive coagulation necrosis with loss of nuclei and cross-striations; dense neutrophilic infiltrate into the interstitium. Pericarditis may manifest as a friction rub.
• 3\u20137 Days: Neutrophils undergo apoptosis; macrophage influx phagocytoses necrotic debris. The infarcted wall is soft and structurally fragile.
• 7\u201314 Days: Proliferation of granulation tissue with rich capillary networks and collagen deposition at the margins.
• 2\u20138 Weeks: Progressive collagenization resulting in a dense, hypocellular fibrous scar.

Critical Post-MI Complications

• Free Wall Rupture (Days 3\u201314): Macrophage cleavage of necrotic myocardium leads to ventricular rupture, hemopericardium, and cardiac tamponade (Beck's triad: hypotension, muffled heart sounds, JVD).
• Papillary Muscle Rupture (Days 3\u20137): Ischemia (most commonly posteromedial papillary muscle supplied solely by PDA) causes acute severe mitral regurgitation and sudden pulmonary edema.
• Interventricular Septal Rupture (Days 3\u20135): Rupture creates a left-to-right shunt causing acute right heart failure and a new harsh holosystolic murmur.
• Dressler Syndrome (Weeks 2\u20138): Autoimmune pericarditis triggered by anti-myocardial antibodies, presenting with fever, pleuritic chest pain, and pericardial effusion.

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2. Heart Failure and Hypertensive Vascular Disease

Heart Failure Pathophysiology

Heart failure is a complex clinical syndrome resulting from structural or functional impairment of ventricular filling or ejection of blood.

• Left-Sided Heart Failure:
  • Systolic Failure: Reduced ejection fraction (HFrEF < 40%) caused by impaired myocardial contractility (e.g., post-MI, dilated cardiomyopathy).
  • Diastolic Failure: Preserved ejection fraction (HFpEF \u2265 50%) caused by impaired ventricular relaxation and decreased compliance (e.g., concentric hypertrophy from chronic HTN).
  • Manifestations: Left atrial pressure rises, backing up into pulmonary veins. Elevated pulmonary capillary hydrostatic pressure leads to pulmonary congestion and pulmonary edema. Symptoms include dyspnea, orthopnea, paroxysmal nocturnal dyspnea (PND), and S3 gallop. Histology demonstrates hemosiderin-laden alveolar macrophages ("heart failure cells") from extravasated red blood cells.
• Right-Sided Heart Failure:
  • Etiologies: Most commonly secondary to left-sided heart failure. Isolated right heart failure due to intrinsic lung disease or pulmonary vascular disease is termed cor pulmonale.
  • Manifestations: Systemic venous congestion leads to jugular venous distension (JVD), peripheral pitting edema, and passive congestion of the liver resulting in a nutmeg liver appearance (centrilobular necrosis with fatty peripheral hepatocytes). Severe cases progress to cardiac cirrhosis and ascites.

Hypertensive Vascular Pathology

Hypertension is defined as persistent blood pressure \u2265 130/80 mmHg (Stage 1 ACC/AHA) or \u2265 140/90 mmHg.

• Essential (Primary) Hypertension (90\u201395%): Multifactorial etiology combining genetic susceptibility, environmental factors (high sodium intake), and sympathetic/RAAS overactivity.
• Secondary Hypertension (5\u201310%): Identifiable underlying etiologies including renal artery stenosis (fibromuscular dysplasia or atherosclerosis causing activation of RAAS), Conn syndrome (primary hyperaldosteronism), Cushing syndrome, and Pheochromocytoma.
• Vascular Morphological Changes:
  • Hyaline Arteriolosclerosis: Seen in benign essential HTN and diabetes mellitus. Pink, homogeneous hyaline thickening of arteriolar walls due to extravasation of plasma proteins and smooth muscle matrix synthesis, resulting in luminal narrowing and end-organ ischemia (arteriolosclerotic nephrosclerosis).
  • Hyperplastic Arteriolosclerosis: Characteristic of severe/malignant HTN (BP > 180/120 mmHg). Concentric, laminated "onion-skinning" of smooth muscle cells and basement membrane duplication, often accompanied by fibrinoid necrosis (necrotizing arteriolitis) in kidney arterioles.

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3. Pulmonary Pathology: Obstructive, Restrictive, and Infectious

Chronic Obstructive Pulmonary Disease (COPD) & Asthma

COPD is characterized by persistent, non-reversible airflow limitation due to loss of elastic recoil or increased airway resistance.

Restrictive Lung Diseases

Restrictive diseases are characterized by reduced expansion of lung parenchyma and decreased total lung capacity (TLC, FEV1/FVC ratio is normal or increased > 80%).

• Idiopathic Pulmonary Fibrosis (IPF): Repeated epithelial damage and aberrant repair driven by TGF-beta leads to progressive subpleural interstitial fibrosis, fibroblastic foci, and honeycomb lung appearance on CT.
• Sarcoidosis: Systemic granulomatous disease characterized by non-caseating granulomas composed of epithelioid histiocytes and multinucleated giant cells containing asteroid bodies and Schaumann bodies. Associated with bilateral hilar lymphadenopathy, elevated serum ACE levels, and hypercalcemia (due to 1-alpha-hydroxylase activation by macrophages).
• Pneumoconioses:
  • Silicosis: Inhalation of silica particles by miners/sandblasters. Macrophage ingestion of silica causes phagolysosome rupture and fibrogenic cytokine release. Forms nodular fibrotic lesions in upper lobes with characteristic eggshell calcifications of hilar lymph nodes. Increases risk of tuberculosis.
  • Asbestosis: Inhalation of asbestos fibers in roofers/shipbuilders. Pathognomonic ferruginous bodies (golden-brown, dumbbell-shaped asbestos fibers coated with iron and protein). Manifests with parietal pleural thickening, ivory-colored pleural plaques, subpleural interstitial fibrosis (lower lobes), and markedly increased risk of bronchogenic carcinoma and malignant mesothelioma.

Pneumonia Patterns

Pneumonia is an infection of the lung parenchyma categorized by anatomical distribution and etiology:

• Lobar Pneumonia: Acute diffuse consolidation of an entire lung lobe, predominantly caused by Streptococcus pneumoniae (90%). Evolves through 4 classic stages:
  1. Congestion (Days 1\u20132): Vascular engorgement, intra-alveolar fluid with abundant bacteria, few neutrophils.
  2. Red Hepatization (Days 3\u20134): Massive neutrophilic exudation with extravasated erythrocytes filling alveoli; liver-like firm consistency.
  3. Gray Hepatization (Days 5\u20137): Disintegration of red blood cells; persistent fibrinopurulent exudate with macrophages.
  4. Resolution (Day 8+): Enzymatic digestion of exudate by macrophages, restoring normal lung architecture.
• Bronchopneumonia: Patchy, multifocal consolidation centered around bronchioles, often bilateral and basal. Common pathogens include Staphylococcus aureus (post-viral), Klebsiella pneumoniae (alcoholics, currant-jelly sputum), Pseudomonas aeruginosa (cystic fibrosis), and Haemophilus influenzae.
• Interstitial (Atypical) Pneumonia: Diffuse inflammation confined to alveolar septa without alveolar exudate. Presents with non-productive cough, low fever, and interstitial reticular infiltrates on chest X-ray. Common pathogens include Mycoplasma pneumoniae (cold agglutinins IgM), Chlamydia pneumoniae, Legionella pneumophila, and respiratory viruses (Influenza, RSV, SARS-CoV-2).