Pulmonary Hypertension

• Endothelial dysfunction and imbalance of vasoactive mediators: The fundamental mechanism involves loss of the normal anti-proliferative, anti-inflammatory endothelial phenotype. Three key pathways become dysregulated: (1) Nitric oxide (NO) pathway—endothelial nitric oxide synthase (eNOS) expression decreases while phosphodiesterase-5 (PDE-5) activity increases, reducing cGMP-mediated vasodilation and anti-proliferation; (2) Prostacyclin pathway—decreased prostacyclin synthase (PGIS) expression reduces PGI₂ production, which normally maintains vasodilation and inhibits platelet aggregation; (3) Endothelin-1 (ET-1) pathway—increased ET-1 production via endothelial cells and macrophages, combined with enhanced endothelin receptor A (ETA) signaling, promotes vasoconstriction and smooth muscle proliferation. The net result is a pro-vasoconstrictor, pro-proliferative, pro-thrombotic state. This imbalance is central to all PH pathophysiology regardless of primary etiology and forms the rationale for pharmacologic intervention with PDE-5 inhibitors, soluble guanylate cyclase stimulators, and endothelin receptor antagonists.

• Smooth muscle cell proliferation and pulmonary vascular remodeling: In response to injury (hemodynamic stress, hypoxia, inflammation, genetic predisposition), pulmonary artery smooth muscle cells (PASMCs) undergo a phenotypic switch from a quiescent, contractile state to an activated, proliferative, and migratory state. Calcium signaling is abnormal, with altered expression of potassium channels (particularly KCNK3 mutations in heritable PAH) leading to increased intracellular calcium and enhanced proliferation. Growth factors (PDGF, FGF, VEGF) and inflammatory cytokines (IL-6, TNF-α) are upregulated. The medial layer of pulmonary arteries undergoes concentric hypertrophy; simultaneously, intimal fibrosis and plexiform lesions develop (particularly in IPAH), representing neointimal proliferation of endothelial cells. This structural remodeling progressively increases PVR according to the Hagen-Poiseuille equation (PVR = 8ηL/πr⁴), where vessel radius (r) becomes critically reduced. Unlike systemic hypertension where vessels dilate, pulmonary vessels become rigid and fixed, limiting the heart's ability to adapt.

• Right ventricular dysfunction and maladaptation: Initially, the right ventricle (RV) compensates for elevated afterload through concentric hypertrophy, increasing contractility via the Frank-Starling mechanism. However, this adaptation becomes maladaptive in chronic PH. The hypertrophied RV becomes increasingly stiff, impairing diastolic filling and increasing RV end-diastolic pressure (RVEDP). Interventricular dependence worsens—the bulging hypertrophied septum encroaches on the left ventricle, reducing left ventricular compliance and ultimately impairing left ventricular filling and systemic cardiac output. The RV becomes increasingly ischemic due to elevated wall tension and compression of the RV coronary circulation. Fetal genes reactivate (increased expression of B-type natriuretic peptide, atrial natriuretic peptide, and myosin heavy chain beta). Without intervention, the RV progresses to eccentric dilatation with reduced ejection fraction—the point of no return—ultimately causing RV failure with systemic hypotension, hepatic congestion, and death.

• Thrombosis and platelet dysfunction: Endothelial injury and stasis in dilated pulmonary vessels promote in situ thrombosis, which is pathognomonic in IPAH and contributes to increased PVR. Tissue factor expression is upregulated on endothelial cells. Conversely, platelet function is abnormal with increased reactivity and aggregation. The combination explains why anticoagulation is considered in certain PH subtypes despite absence of frank venous thromboembolism. Chronic thromboembolic PH (CTEPH) represents the extreme end of this spectrum, where organized thrombi replace the normal vascular architecture.

• Hypoxia and metabolic derangement: In diseases causing pulmonary parenchymal disease or ventilation-perfusion mismatch (COPD, ILD, sleep apnea), chronic hypoxia triggers pulmonary vasoconstriction via hypoxic pulmonary vasoconstriction (HPV) through mitochondrial oxygen sensing. Chronic activation of this normally protective reflex leads to sustained vasoconstriction and eventually to the fixed vascular remodeling described above. Metabolic acidosis from tissue hypoperfusion and lactate accumulation further impairs RV function. Additionally, hypoxia stabilizes hypoxia-inducible factor 1-alpha (HIF-1α), promoting proliferation and glycolytic metabolism in smooth muscle cells.

• Genetic predisposition and molecular mechanisms: Mutations in genes encoding bone morphogenetic protein receptor type 2 (BMPR2), ALK1, endoglin, KCNK3, and others account for approximately 80% of heritable PAH cases. BMPR2 loss impairs the normal anti-proliferative signaling in smooth muscle and endothelial cells. A "two-hit" model explains variable penetrance: genetic mutation alone is insufficient; additional environmental triggers (infection, toxin exposure, pregnancy) are typically required. Somatic mutations in EIF1AX and other genes have been identified in sporadic IPAH, suggesting clonal expansion of abnormal cells within pulmonary vessels.

• Group 1: Pulmonary Arterial Hypertension (PAH) includes idiopathic PAH (IPAH), heritable PAH (germline BMPR2, ALK1, EIF1AX mutations), drug/toxin-induced PAH (anorexigens historically; currently methamphetamine, cocaine), and PAH associated with connective tissue disease (CTD-PAH), congenital heart disease (CHD-PAH), portal hypertension (portopulmonary hypertension), HIV, and schistosomiasis. IPAH typically presents in middle-aged women (female:male ratio 1.7:1), while CTD-PAH (particularly systemic sclerosis) carries a 7-14% prevalence among SSc patients. Heritable PAH follows autosomal dominant inheritance with incomplete penetrance (~70%). Drug-induced PAH is now rare in developed countries but remains important historically.

• Group 2: PH due to left heart disease is the most common cause of PH overall, occurring in the setting of systolic heart failure, diastolic dysfunction, mitral valve disease, or aortic valve disease. The mechanism is passive pulmonary venous hypertension and secondary reactive arterial remodeling. When PVR remains elevated despite correction of left heart disease, the term "out of proportion" PH is used and carries worse prognosis.

• Group 3: PH due to chronic lung disease and/or hypoxemia includes COPD (affects 40% of patients with severe COPD but hemodynamically significant PH in only 5%), interstitial lung disease (IPD/ILD with UIP pattern), sleep-disordered breathing (obstructive sleep apnea, central sleep apnea), and high-altitude pulmonary hypertension. The unifying mechanism is chronic hypoxia triggering sustained vasoconstriction and remodeling. COPD-associated PH occurs through multiple mechanisms: parenchymal destruction reducing vascular bed, hypoxia, and direct inflammatory injury.

• Group 4: Chronic thromboembolic pulmonary hypertension (CTEPH) results from organized residual thrombi after acute pulmonary embolism, affecting 2-4% of PE survivors. Risk factors include recurrent PE, extensive clot burden, larger PE, and thrombophilia. CTEPH is unique because it may be curable via pulmonary endarterectomy (PEA) in operable patients.

• Group 5: PH with unclear or multifactorial mechanisms includes sarcoidosis, histiocytosis X, lymphangioleiomyomatosis (LAM), hemoglobinopathies (especially sickle cell disease), chronic kidney disease, fibrosing mediastinitis, and others. In sickle cell disease, PH develops in ~10% of patients through hemolysis-related endothelial dysfunction, chronic hypoxia, and in situ thrombosis.

• Additional risk factors and modifiers: Female sex (particularly for Group 1 PAH), family history, pregnancy (which worsens PAH and carries high maternal mortality), oral contraceptives and estrogen therapy, systemic infection (particularly HIV), appetite suppressant use, autoimmune disease, and portal hypertension all increase PH risk. Environmental exposures (chronic hypoxia at altitude, coal mining) and certain occupations increase Group 3 PH risk.

• Dyspnea on exertion (DOE): The cardinal symptom, often initially mild with high exertion tolerance and progressively worsening. Pathophysiologically reflects inability to increase cardiac output appropriately during exercise due to fixed PVR and limited RV reserve. Dyspnea is poorly correlated with degree of PH severity and can be profound even with moderate hemodynamic compromise. Patients often have normal resting oxygen saturation but desaturate with exertion due to impaired pulmonary perfusion-ventilation matching and limited cardiac output preventing sufficient oxygen delivery.

• Syncope and presyncope: Occur as disease advances, reflecting either exertional syncope (critical inability to increase cardiac output during exercise, causing cerebral hypoperfusion) or rest syncope (advanced RV failure with low systemic cardiac output and hypotension). Syncope in PH is ominous, indicating severely limited RV reserve and is a marker of high mortality. Mechanism may also involve arrhythmia (supraventricular tachycardia or atrial fibrillation occur as the dilated right atrium becomes electrically unstable).

• Chest pain: Occurs in 20-30% of PH patients, typically pleuritic or positional; mechanism includes RV ischemia from increased wall tension in the setting of compressed RV coronary circulation, acute pulmonary embolism (particularly in PAH where thrombosis occurs), or rarely pericarditis. Occasionally described as retrosternal pressure mimicking angina.

• Palpitations and irregular heartbeat: Result from tachycardia (compensatory response to low cardiac output), premature atrial contractions, or atrial fibrillation (which develops in ~15% of PAH patients and confers worse prognosis by worsening hemodynamics and increasing thromboembolism risk). Patients may report "heart racing" or awareness of forceful heartbeats.

• Fatigue and exercise intolerance: Often the first symptom noticed, reflecting low cardiac output and inadequate oxygen delivery to skeletal muscle. Distinguishing PH-related fatigue from depression or deconditioning is clinically important but challenging.

• Orthopnea and paroxysmal nocturnal dyspnea: Develop with overt right heart failure and elevated right atrial pressure causing systemic venous congestion, pulmonary edema from RV-mediated left ventricular dysfunction, or both. Less common than in left heart failure but important to recognize as indicating advanced disease.

• Peripheral edema and ascites: Reflect right atrial pressure elevation (typically when RAP >8-10 mmHg) causing systemic venous congestion, hepatic congestion, and transudation. Initially peripheral edema; with progression, ascites and anasarca develop. Hepatic congestion causes hepatomegaly with hepatic tenderness and elevated transaminases; severe congestion can cause liver dysfunction ("cirrhosis of the heart").

• Physical examination findings reflect the hemodynamic derangement and RV failure:
• Elevated jugular venous pressure (JVP): Often the earliest clinical sign of RV failure; notable for prominent CV waves (tricuspid regurgitation), hepatic pulsations, and lack of normal X descent. JVP >8 cm H₂O correlates with elevated RAP.
• Prominent RV heave (parasternal lift): Reflects RV hypertrophy and anterior displacement of the RV free wall against the chest wall; best appreciated at the left lower sternal border in inspiration.
• Loud P₂ (accentuated pulmonary component of S₂): Due to increased pulmonary artery pressure causing earlier closure of the pulmonary valve; P₂ becomes louder and single (fixed split S₂ may occur with severe RV failure and reduced RV ejection).
• Tricuspid regurgitation murmur: High-pitched holosystolic murmur at the left lower sternal border that increases with inspiration (Carvallo sign); reflects RV dilatation and tricuspid annular dilatation. Severity correlates with disease progression.
• S₃ or S₄ gallop: S₃ suggests RV dysfunction; S₄ reflects RV hypertrophy and reduced compliance.
• Hepatomegaly with hepatic tenderness: Often pulsatile hepatomegaly due to severe tricuspid regurgitation transmitting systolic pulses directly to the liver.
• Lower extremity edema and ascites: With advanced disease.
• Cyanosis and clubbing: Uncommon unless associated with congenital heart disease or prolonged hypoxia; suggest advanced PH with right-to-left shunting or chronic parenchymal lung disease.

• Important clinical variants in presentation:
• Acute presentations: Acute massive PE (Group 4) presents with acute dyspnea, hemoptysis (if infarction), hemodynamic collapse, and high mortality if untreated. Acute decompensation in chronic PH may occur with arrhythmia, additional PE, RV infarction, or sudden RV failure.
• Subtle presentations: Early PH may present only with dyspnea on exertion, normal resting vital signs, and normal oxygen saturation, making diagnosis easy to miss. High clinical suspicion is needed when DOE appears disproportionate to objective findings.
• Group-specific presentations: Portopulmonary hypertension occurs in 5% of cirrhosis patients and may worsen rapidly with portal vein thrombosis; schistosomiasis-associated PAH is common in endemic regions; sickle cell PH presents with painful crises and progressive dyspnea; CTEPH may have history of prior PE but many patients do not recall thrombotic events.

• Clinical evaluation and risk stratification: Detailed history should assess exercise tolerance (WHO functional class), syncope, chest pain, and symptoms of right heart failure. Risk stratification incorporates demographics (female sex, family history), comorbidities (autoimmune disease, HIV, portal hypertension, anorexigen use), and symptoms. Physical examination as described above provides important clues; notably, elevated JVP and hepatomegaly are concerning for hemodynamically significant PH. Functional class assessment uses WHO classification: Class I (no limitation), Class II (dyspnea with strenuous exercise), Class III (dy

Immediate stabilisation of decompensated RV failure (emergency)

• Treat the trigger and protect preload: sepsis, anemia, arrhythmia, or interrupted PAH therapy are the usual precipitants. Cautious diuresis (loop diuretic, e.g. furosemide) for congestion, but the stiff pressure-overloaded RV is preload-dependent — over-diuresis collapses cardiac output.
• Vasopressor before inotrope: norepinephrine restores systemic (and therefore RV coronary perfusion) pressure; dobutamine or milrinone augment RV contractility. Avoid agents that drop systemic pressure without unloading the pulmonary bed.
• Avoid intubation when possible: induction agents plus positive-pressure ventilation abolish venous return and can precipitate arrest. Correct hypoxemia and acidosis, both of which worsen hypoxic pulmonary vasoconstriction. VA-ECMO is a bridge to therapy or transplant.

Group 1 PAH — guideline-directed therapy (CHEST 2019 pharmacologic therapy guideline; ESC/ERS PH guideline)

• Vasoreactivity testing at right heart catheterisation (inhaled nitric oxide) identifies the ~10% acute responders, who are treated with high-dose calcium channel blockers (nifedipine, diltiazem, amlodipine). Verapamil is avoided for its negative inotropy. Non-responders must never receive CCBs — they cause systemic hypotension and shock.
• Initial oral combination therapy for treatment-naïve low/intermediate-risk patients: endothelin receptor antagonist (ambrisentan) plus PDE-5 inhibitor (tadalafil), the AMBITION strategy.
• Escalation: add a prostacyclin pathway agent — oral selexipag (IP receptor agonist) or inhaled treprostinil. Soluble guanylate cyclase stimulator (riociguat) is an alternative to a PDE-5 inhibitor, never combined with one.
• High-risk/WHO functional class IV: continuous IV prostacyclin (epoprostenol), the only PAH therapy with a survival benefit in IPAH.

Definitive/surgical

• Pulmonary endarterectomy is potentially curative for operable CTEPH plus lifelong anticoagulation; balloon pulmonary angioplasty and riociguat for inoperable disease.
• Lung or heart–lung transplantation for progression on maximal therapy; atrial septostomy as a bridge.

Disease-related

• Acute decompensated right ventricular failure and cardiogenic shock (emergency): afterload mismatch causes RV dilatation, leftward septal shift, and impaired LV filling; RV subendocardial ischemia follows because high wall tension plus systemic hypotension abolishes the normal systolic RV coronary perfusion gradient. Signals: hypotension, cool extremities, oliguria, rising lactate and BNP/NT-proBNP, worsening tricuspid regurgitation.
• Atrial arrhythmias: right atrial stretch produces atrial flutter or fibrillation in a ventricle wholly dependent on atrial kick; onset often manifests as abrupt syncope or shock rather than palpitations. Restoring sinus rhythm — urgent cardioversion if unstable — is the priority.
• Syncope and sudden cardiac death (emergency): fixed cardiac output cannot meet exertional demand; exertional syncope is a high-risk feature mandating urgent escalation.
• Massive hemoptysis (emergency): rupture of hypertrophied bronchial collaterals or dilated pulmonary arteries; managed with bronchial artery embolisation.
• Ortner (cardiovocal) syndrome: hoarseness from left recurrent laryngeal nerve compression by a dilated main pulmonary artery; the same aneurysmal PA can compress the left main coronary artery and cause angina.
• Congestive hepatopathy and cardiorenal syndrome: elevated right atrial pressure transmitted backward produces pulsatile hepatomegaly, transaminase and bilirubin elevation, cardiac cirrhosis, and venous congestion nephropathy.

Treatment-related

• Prostacyclin infusion catheter sepsis and pump/line interruption (emergency): epoprostenol has a very short half-life, so any interruption causes rebound pulmonary hypertensive crisis and death. Fever in a patient with an indwelling prostacyclin line is a bloodstream infection until proven otherwise. Class effects: jaw pain, flushing, diarrhea, headache.
• Endothelin receptor antagonists: teratogenicity (REMS with monthly pregnancy testing), peripheral edema, anemia, and hepatic transaminase elevation.
• PDE-5 inhibitors/riociguat: systemic hypotension, epistaxis; profound hypotension if combined with nitrates or with each other.
• Calcium channel blockers in non-responders: systemic vasodilation without pulmonary unloading precipitates shock.
• Over-diuresis: prerenal azotemia and underfilling of the preload-dependent RV.

• Echo screens, right heart catheterisation confirms: an estimated RV systolic pressure on transthoracic echo is never diagnostic. If the stem asks for the single best next step to establish the diagnosis after an abnormal echo, the answer is right heart catheterisation.
• The wedge pressure separates the groups: a normal pulmonary capillary wedge pressure with high pulmonary vascular resistance means pre-capillary disease (Groups 1, 3, 4); an elevated wedge means left heart disease (Group 2). This determines whether PAH-specific vasodilators are indicated or harmful.
• V/Q scan, not CT angiography, is the screening test to exclude CTEPH. This is the classic distractor — CTPA can miss chronic organised, recanalised thrombus. Every newly diagnosed patient needs a V/Q scan because CTEPH is the one surgically curable form (pulmonary endarterectomy).
• Vasoreactivity testing is only for Group 1, and only acute responders get high-dose calcium channel blockers. Giving a CCB to a non-responder causes systemic hypotension and RV collapse — a favourite wrong answer.
• Buzzwords that pin the etiology: plexiform lesions and BMPR2 mutation (autosomal dominant, incomplete penetrance) for heritable/idiopathic PAH; anorexigens, methamphetamine, and cocaine for drug-induced; systemic sclerosis (especially limited/CREST) for the most commonly tested connective tissue disease association.
• Exam findings: loud, single P₂; left parasternal heave; holosystolic murmur at the lower left sternal border that increases with inspiration (Carvallo sign); pulsatile hepatomegaly.
• Never abruptly stop epoprostenol — rebound pulmonary hypertensive crisis is fatal. A patient admitted for an unrelated problem keeps the infusion running.
• Pregnancy is contraindicated in PAH (high maternal mortality), and endothelin receptor antagonists are teratogenic; effective contraception plus counselling is expected in the vignette.
• In Group 3 (COPD, ILD), treat the lung disease and hypoxemia — long-term oxygen and disease-directed therapy — rather than reflexively reaching for a pulmonary vasodilator, which can worsen V/Q matching and hypoxemia.