Pulmonary Hypertension: Pathophysiology

(10-Mark Answer - From Fishman's Pulmonary Diseases & Disorders + Murray & Nadel's Textbook of Respiratory Medicine)

I. DEFINITION

II. PATHOGENIC MECHANISMS - THE SIX-CATEGORY FRAMEWORK

III. VASCULAR REMODELING - THE CENTRAL PATHOLOGIC EVENT

A. Medial Changes

• Increased number and size of pulmonary arterial smooth muscle cells (SMCs) - medial hypertrophy
• Extension of smooth muscle into normally non-muscularized or partially muscularized small intra-acinar vessels (a hallmark feature)
• Eventually: medial atrophy, fibrosis, and luminal dilation in chronic severe disease

B. Intimal Changes

• Intimal hyperplasia from endothelial cell and SMC proliferation
• In severe PAH: plexiform lesions - disordered endothelial proliferation forming glomerulus-like tufts within obliterated vessel lumen
• In situ thrombosis contributes to intimal remodeling

C. Adventitial Changes

• Adventitial fibroblasts undergo hypoxia-induced proliferation
• Transdifferentiate into myofibroblasts (expressing α-smooth muscle actin)
• Contribute to wall stiffness and reduced vascular compliance

IV. GENETIC, CELLULAR, AND MOLECULAR MECHANISMS

1. Sustained vasoconstriction
2. Vascular remodeling
3. In situ thrombosis
4. Increased arterial wall stiffness

A. Calcium Dysregulation - The Master Trigger

• A rise in cytosolic free Ca²⁺ concentration ([Ca²⁺]cyt) in pulmonary arterial smooth muscle cells (PASMCs) is the major trigger for both vasoconstriction AND cellular proliferation
• Resting [Ca²⁺]cyt is increased in proliferating PASMCs compared to growth-arrested cells
• Hypoxia disrupts Ca²⁺ homeostasis - hypoxic pulmonary vasoconstriction (HPV) occurs even in isolated PASMCs without endothelium

B. Imbalance of Vasoactive Mediators

• Nitric Oxide (NO): Produced by endothelial cells; activates cGMP in PASMCs → relaxation. Also inhibits PASMC proliferation and platelet aggregation. In PAH, NO production is reduced. Transgenic animals overexpressing NOS are protected from hypoxia-induced PH; mice lacking eNOS gene develop severe PH with even mild hypoxia.
• Prostacyclin (PGI₂): Activates cAMP in PASMCs → vasodilation. Also inhibits proliferation and platelet aggregation. Deficiency documented in PAH patients. Prostacyclin analogues (epoprostenol) improve hemodynamics, exercise capacity, and survival.
• Vasoactive Intestinal Peptide (VIP): Promotes vasodilation and inhibits SMC proliferation; levels are reduced in PAH patients.

• Thromboxane A₂ (TXA₂): Arachidonic acid metabolite; causes vasoconstriction, platelet aggregation, and acts as a smooth muscle mitogen. Increased TXA₂ metabolites found in urine of PAH patients.
• Endothelin-1 (ET-1): Potent vasoconstrictor and smooth muscle mitogen. Levels elevated in plasma and lung tissue of PAH patients, correlating with disease severity and prognosis. Acts via ET-A receptors (vasoconstriction/proliferation) and ET-B receptors (vasodilation when on endothelial cells, vasoconstriction when on SMCs).
• Serotonin (5-HT): Implicated by the epidemic of PAH from appetite suppressants (aminorex, fenfluramine), which raise plasma 5-HT by inducing release from platelets. Serotonin transporter (SERT) overexpression increases PASMC proliferation.

C. Growth Factor Dysregulation

• Angiopoietin-1 / TIE2: Overexpressed in most forms of non-familial PAH; correlates with disease severity. Regulates PASMC hyperplasia.
• VEGF: Increased in PAH pulmonary vasculature, especially within plexiform lesions where its pro-angiogenic properties mediate disordered endothelial proliferation.
• PDGF (Platelet-Derived Growth Factor): Increased in PAH pulmonary vasculature; drives PASMC proliferation and migration. The tyrosine kinase inhibitor imatinib (which targets PDGF receptor) improved exercise capacity in the IMPRES study but had serious adverse effects (subdural hematoma).

D. Cellular Microparticles

• Vesicle fragments from endothelial cells, platelets, leukocytes released during activation or apoptosis
• Circulating endothelial microparticles are increased in PAH (marker of endothelial dysfunction) and correlate with survival
• Impair endothelium-dependent vasorelaxation, decrease NO production, and promote inflammatory signaling

E. Inflammation and Cytokines

• Strong association of systemic inflammatory disorders with IPAH implicates inflammation in vascular remodeling
• Elevated cytokines: TNF-α, IL-1β, IL-2, IL-4, IL-6, IL-8, IL-10, IL-12p70
• Higher IL-6 levels independently associated with mortality
• The chemokine fractalkine (CX3CL1) is elevated in CD4 and CD8 T cells
• Perivascular infiltrates of T cells, B cells, macrophages, mast cells surround plexiform lesions

F. Genetic Basis

• BMPR2 (Bone Morphogenetic Protein Receptor type 2) gene mutations: present in ~70% of heritable PAH and 20-25% of apparently idiopathic PAH
• BMPR2 normally promotes endothelial cell survival and inhibits SMC proliferation
• Loss of BMPR2 function → reduced apoptosis of SMCs → unchecked vascular remodeling
• Other mutations: ACVRL1 (ALK1), ENG (endoglin) - associated with hereditary hemorrhagic telangiectasia + PAH
• In situ thrombosis: platelet dysfunction, endothelial injury, and coagulation abnormalities all contribute

V. CONSEQUENCES FOR THE RIGHT VENTRICLE

1. Right ventricular hypertrophy (RVH) - compensatory, maintains cardiac output
2. RV dilatation - decompensation as afterload increases
3. Cor pulmonale - ultimately right heart failure

VI. HYPOXIC PULMONARY VASOCONSTRICTION (HPV) - A KEY MECHANISM

• HPV is an adaptive mechanism redirecting blood flow from poorly ventilated to well-ventilated lung regions - optimizing V/Q matching
• Hypoxia induces vasoconstriction via Ca²⁺ influx in PASMCs, independent of endothelium
• Chronic hypoxia (COPD, high altitude) → sustained HPV → pulmonary vascular remodeling → Group 3 PH
• The signaling pathways initiated by acute and chronic hypoxia relate to disrupted Ca²⁺ homeostasis and redox sensing

VII. INTEGRATION: COMMON FINAL PATHWAY

Initiating Trigger (genetic, hypoxia, shear stress, inflammation, drugs)
          ↓
Endothelial dysfunction
          ↓
Imbalance: ↓NO, ↓PGI₂ / ↑ET-1, ↑TXA₂, ↑5-HT
          ↓
Sustained vasoconstriction + PASMC proliferation (↑[Ca²⁺]cyt)
          ↓
Medial hypertrophy → Intimal proliferation → Adventitial fibrosis
          ↓
Plexiform lesions + In situ thrombosis
          ↓
↑PVR → ↑PAP → RV hypertrophy → RV dilatation → Cor pulmonale → Death

KEY EXAM POINTS (Summary)

• Definition: mPAP >20 mmHg at rest on RHC (6th WSPH, 2018)
• Six pathogenic mechanisms (Fishman's Table 72-2): Passive, Hyperkinetic, Obstructive, Obliterative, Vasoconstrictive, Idiopathic
• Three key vasoactive mediator deficiencies: NO, Prostacyclin, VIP
• Three key vasoactive mediator excesses: Endothelin-1, Thromboxane A₂, Serotonin
• Central molecular trigger: Ca²⁺ dysregulation in PASMCs
• Key genetic mutation: BMPR2 (70% heritable PAH)
• Central structural lesion: Plexiform lesion (pathognomonic of severe PAH)
• End-organ consequence: Cor pulmonale / RV failure