Pulmonary Vascular Disease

SECTION 1

Historical Context & Motivation

The recognition of pulmonary vascular disease as a distinct clinical entity evolved gradually over more than a century. Early pathologists noted thickened pulmonary arteries and right ventricular hypertrophy at autopsy but lacked the physiologic tools to understand the underlying hemodynamics. It was not until the development of cardiac catheterization and advances in pulmonary physiology that clinicians could directly measure pulmonary artery pressures and begin classifying these disorders systematically. Today, pulmonary vascular disease encompasses a wide spectrum of conditions—including pulmonary hypertension, pulmonary embolism, and pulmonary vasculitis—that collectively represent a major source of morbidity and mortality worldwide.

1891

Romberg's Autopsy Observations

Ernst von Romberg described "pulmonary vascular sclerosis" at autopsy, recognizing thickened pulmonary arteries as a primary disease process rather than a consequence of left heart failure.

1929

Forssmann's Self-Catheterization

Werner Forssmann performed the first human cardiac catheterization on himself, laying the groundwork for invasive hemodynamic assessment of pulmonary pressures that would later define pulmonary hypertension.

1951

Dresdale Describes Primary Pulmonary Hypertension

David Dresdale and colleagues published the first clinical series of patients with idiopathic pulmonary hypertension, establishing it as a distinct clinical diagnosis separable from secondary causes.

1995

Prostacyclin Therapy Approved

Intravenous epoprostenol became the first FDA-approved targeted therapy for pulmonary arterial hypertension, demonstrating improved survival and ushering in the modern era of PAH-specific pharmacotherapy.

2018

6th World Symposium on Pulmonary Hypertension

The hemodynamic definition of pulmonary hypertension was updated to a mean pulmonary artery pressure (mPAP) > 20 mmHg, lowering the threshold from 25 mmHg and refining the classification into five clinical groups.

The central question that drives the study of pulmonary vascular disease is: How do we distinguish among the diverse etiologies of elevated pulmonary artery pressure and vessel pathology, and how do we tailor treatment to the underlying mechanism? Understanding this requires integrating hemodynamic data, imaging, and clinical context—skills that are directly tested on USMLE Step 2 CK.

SECTION 2

Core Principles & Definitions

Pulmonary vascular disease encompasses any pathology that impairs blood flow through the pulmonary arterial, capillary, or venous bed. The pulmonary circulation is normally a low-pressure, low-resistance system compared to the systemic circulation. Normal mean pulmonary artery pressure (mPAP) is approximately 14 ± 3 mmHg, and the entire right ventricular output traverses the lungs at roughly one-sixth the pressure required systemically. When this vascular bed is compromised—by obstruction, obliteration, or vasospasm—the right ventricle faces increased afterload, ultimately leading to right heart failure if untreated. Grasping these foundational hemodynamic concepts is essential before approaching the differential diagnosis.

Pulmonary Hypertension (PH)

Defined hemodynamically as mPAP > 20 mmHg at rest measured by right heart catheterization. PH is classified into five WHO groups based on etiology: pulmonary arterial hypertension (Group 1), left heart disease (Group 2), lung disease/hypoxia (Group 3), chronic thromboembolic (Group 4), and multifactorial/unclear mechanisms (Group 5).

Pulmonary Embolism (PE)

Acute obstruction of pulmonary arteries, most commonly by thrombus originating from deep veins. PE acutely increases right ventricular afterload and V/Q mismatch. Classified as massive (with hemodynamic instability), submassive (RV dysfunction without shock), or low-risk.

Pulmonary Vascular Resistance (PVR)

Calculated as (mPAP − PAWP) / CO, where PAWP is pulmonary arterial wedge pressure and CO is cardiac output. PVR distinguishes pre-capillary from post-capillary causes. Normal PVR is < 2 Wood units; elevated PVR (≥ 3 WU) with normal PAWP defines precapillary PH.

Right Ventricular Adaptation

The thin-walled right ventricle is exquisitely sensitive to afterload. Chronic pressure overload leads to compensatory hypertrophy, but eventual dilation and failure ensue. RV function is the strongest predictor of survival in pulmonary hypertension.

V/Q Mismatch

Ventilation-perfusion mismatch arises when blood flow is redirected away from occluded or obliterated vessels. Areas with ventilation but no perfusion (dead space) increase the alveolar-arterial oxygen gradient and contribute to hypoxemia in PE and CTEPH.

✦ KEY TAKEAWAY

Think of the pulmonary circulation as a wide, compliant highway designed for high-volume, low-pressure traffic. Pulmonary vascular disease is analogous to a highway losing lanes: whether by chronic narrowing (pulmonary arterial hypertension), sudden blockade (pulmonary embolism), or downstream congestion (left heart disease), the result is the same—traffic backs up and the right ventricle, acting as the pump feeding that highway, must work harder. The key clinical challenge is identifying where and why the lanes are lost, because treatment differs dramatically by cause.

SECTION 3

Visual Explanation — WHO Classification of Pulmonary Hypertension

The WHO classification organizes pulmonary hypertension into five groups based on etiology and hemodynamic profile. The critical distinction for management is between pre-capillary PH (low PAWP, elevated PVR) and post-capillary PH (elevated PAWP), as PAH-specific vasodilator therapies are only indicated for Group 1 and selected Group 4 patients.

This diagram highlights the fundamental organizational framework for pulmonary hypertension. Group 1 (pulmonary arterial hypertension) and Group 4 (chronic thromboembolic pulmonary hypertension) are the two groups with the most specific targeted therapies. Group 2, caused by left heart disease, is by far the most common cause of pulmonary hypertension overall, and its management centers on treating the underlying cardiac pathology—diuretics, afterload reduction, and valve repair/replacement—rather than PAH-specific agents, which can be harmful in this context. Group 3 PH from lung disease is managed primarily by treating the underlying pulmonary condition and correcting hypoxia. For USMLE purposes, the ability to distinguish these groups based on catheterization data (particularly the PAWP and PVR) is repeatedly tested.

SECTION 4

Hemodynamic Framework & Pathophysiology

The hemodynamic assessment of pulmonary vascular disease relies on a set of interrelated measurements obtained during right heart catheterization (RHC), the gold standard for diagnosis. Understanding the mathematical relationships between mean pulmonary artery pressure, pulmonary arterial wedge pressure, cardiac output, and pulmonary vascular resistance is essential for interpreting catheterization data and classifying pulmonary hypertension.

PULMONARY VASCULAR RESISTANCE

PVR = (mPAP − PAWP) / CO

PVR = pulmonary vascular resistance (Wood units), mPAP = mean pulmonary artery pressure (mmHg), PAWP = pulmonary arterial wedge pressure (mmHg), CO = cardiac output (L/min). Normal PVR is approximately 0.5–1.5 Wood units. A PVR ≥ 3 WU with mPAP > 20 and PAWP ≤ 15 defines pre-capillary pulmonary hypertension.

TRANSPULMONARY GRADIENT

TPG = mPAP − PAWP

The transpulmonary gradient reflects the pressure drop across the pulmonary vascular bed. A TPG > 12 mmHg in the setting of elevated PAWP suggests a combined pre- and post-capillary component (as in Group 2 PH with pulmonary vascular remodeling).

DIASTOLIC PRESSURE GRADIENT

DPG = dPAP − PAWP

DPG = diastolic pressure gradient, dPAP = diastolic pulmonary artery pressure. A DPG ≥ 7 mmHg is more specific than TPG for identifying a superimposed pre-capillary component in patients with post-capillary PH, since it is less affected by stroke volume and left atrial compliance.

The pathophysiology of pulmonary arterial hypertension (Group 1) involves a triad of vasoconstriction, vascular remodeling, and in situ thrombosis. At the molecular level, there is endothelial dysfunction with decreased production of vasodilators (nitric oxide, prostacyclin) and overexpression of vasoconstrictors (endothelin-1, thromboxane A₂). This imbalance promotes smooth muscle hypertrophy, intimal fibrosis, and formation of plexiform lesions—the pathognomonic histologic finding. These three pathways correspond directly to the three major classes of PAH-specific therapy: phosphodiesterase-5 inhibitors/soluble guanylate cyclase stimulators (nitric oxide pathway), prostacyclin analogues and IP receptor agonists (prostacyclin pathway), and endothelin receptor antagonists (endothelin pathway).

⚡ HIGH-YIELD FOR STEP 2

When a question stem provides catheterization data, first check the PAWP. If PAWP > 15 mmHg, the PH is post-capillary (Group 2)—treat the left heart. If PAWP ≤ 15 mmHg, calculate PVR: if ≥ 3 Wood units, it is pre-capillary PH and you should consider Groups 1, 3, 4, or 5. Always look for underlying causes (COPD, ILD, PE history, connective tissue disease) before diagnosing idiopathic PAH.

SECTION 5

Pulmonary Embolism & Chronic Thromboembolic Disease

Pulmonary embolism represents the most common acute pulmonary vascular emergency, and chronic thromboembolic pulmonary hypertension (CTEPH) is its long-term vascular complication. An estimated 2–4% of patients who survive an acute PE go on to develop CTEPH, where organized thrombus and secondary arteriopathy lead to progressive pulmonary hypertension. Understanding both the acute and chronic presentations is essential, as the diagnostic and therapeutic approaches differ significantly.

Diagnostic and management algorithm for acute pulmonary embolism. Hemodynamic instability directs toward immediate systemic thrombolysis, while stable patients are risk-stratified using clinical probability scores and D-dimer before proceeding to CT pulmonary angiography.

Acute PE: Key Clinical Features

The classic triad of dyspnea, pleuritic chest pain, and hemoptysis is present in a minority of patients with acute PE. More commonly, patients present with sudden-onset dyspnea and tachycardia out of proportion to exam findings. The Wells criteria and the revised Geneva score are validated clinical decision tools that stratify patients into low, intermediate, and high pre-test probability categories. In low-to-intermediate risk patients, a negative age-adjusted D-dimer effectively excludes PE without imaging. High clinical suspicion or an elevated D-dimer warrants CT pulmonary angiography, which is the diagnostic modality of choice with sensitivity and specificity both exceeding 95%.

CTEPH: The Chronic Sequel

CTEPH (WHO Group 4) develops when organized thrombus fails to resolve after acute PE, leading to fibrous obstruction and secondary small-vessel arteriopathy. The hallmark screening test is a V/Q scan showing mismatched perfusion defects, which has near 100% sensitivity for CTEPH and is preferred over CTA for screening purposes. Definitive diagnosis requires right heart catheterization and pulmonary angiography. The treatment of choice for operable CTEPH is pulmonary thromboendarterectomy (PTE), a potentially curative surgical procedure. For inoperable or persistent PH after surgery, riociguat (a soluble guanylate cyclase stimulator) and balloon pulmonary angioplasty are options.

SECTION 6

Worked Example — Interpreting Right Heart Catheterization Data

Case: A 42-year-old woman with progressive dyspnea on exertion

Step 1 — Gather Clinical ContextA 42-year-old woman with a history of scleroderma (limited cutaneous systemic sclerosis) presents with 6 months of progressive dyspnea on exertion and lower extremity edema. Echocardiography reveals an estimated RV systolic pressure of 65 mmHg with RV dilation. She is referred for right heart catheterization.

Step 2 — Review Catheterization DataRight heart catheterization reveals the following: mean pulmonary artery pressure (mPAP) = 48 mmHg, pulmonary arterial wedge pressure (PAWP) = 10 mmHg, cardiac output (CO) = 4.0 L/min, right atrial pressure (RAP) = 12 mmHg. Normal mPAP is ≤ 20 mmHg; PAWP ≤ 15 mmHg is normal.

Step 3 — Calculate PVRUsing the PVR formula: PVR = (mPAP − PAWP) / CO = (48 − 10) / 4.0 = 38 / 4.0 = 9.5 Wood units.

PVR = 9.5 Wood units (markedly elevated; normal < 2 WU)

Step 4 — Classify the Pulmonary HypertensionThe hemodynamic profile shows: mPAP 48 mmHg (> 20, confirming PH), PAWP 10 mmHg (≤ 15, ruling out post-capillary cause), and PVR 9.5 WU (≥ 3, confirming pre-capillary PH). This is consistent with pre-capillary pulmonary hypertension. Given her scleroderma, this is most likely WHO Group 1 PAH — connective tissue disease-associated.

Diagnosis: WHO Group 1 PAH secondary to systemic sclerosis

Step 5 — Determine ManagementThis patient has severe PAH with evidence of right heart failure (elevated RAP, peripheral edema). Initial management includes: (1) acute vasoreactivity testing during catheterization—if positive, a trial of calcium channel blockers; if negative (as in the majority of scleroderma-PAH patients), initiate combination PAH-specific therapy. Current guidelines support initial combination therapy with an endothelin receptor antagonist (e.g., ambrisentan) plus a PDE-5 inhibitor (e.g., tadalafil). Diuretics for volume management and referral to a pulmonary hypertension center are also indicated. Lung transplantation evaluation should be considered given her severity.

Plan: Vasoreactivity testing → likely upfront combination therapy (ERA + PDE5i) + diuretics + PH center referral

SECTION 7

Comparing PAH-Specific Therapies

Three principal molecular pathways are targeted by PAH-specific therapies, each corresponding to a component of endothelial dysfunction. These agents are approved specifically for WHO Group 1 PAH and, in certain cases, Group 4 CTEPH. It is critical to recognize that these medications are generally contraindicated in Group 2 (left heart disease) because pulmonary vasodilation in the setting of elevated left atrial pressure can precipitate pulmonary edema.

PAH-Specific Therapies by Molecular Pathway

Pathway	Drug Classes	Examples	Key Side Effects / Notes
Nitric Oxide	PDE-5 inhibitors; sGC stimulators	Sildenafil, tadalafil; riociguat	Headache, flushing, hypotension. PDE-5i and riociguat are contraindicated together. Riociguat also approved for CTEPH.
Endothelin	Endothelin receptor antagonists (ERAs)	Bosentan, ambrisentan, macitentan	Hepatotoxicity (bosentan > others); teratogenic—mandatory pregnancy testing. Monitor LFTs with bosentan.
Prostacyclin	Prostacyclin analogues; IP receptor agonists	Epoprostenol (IV), treprostinil (SC/IV/inhaled), iloprost (inhaled), selexipag (oral)	IV epoprostenol requires continuous infusion (half-life ~6 min); line infections are a major complication. Jaw pain is a class effect.
Calcium Channel Blockers	Dihydropyridines (high dose)	Nifedipine, diltiazem, amlodipine	Only for vasoreactive patients (~10% of IPAH). Avoid verapamil (negative inotropy). Long-term responders are rare but have excellent prognosis.

✦ KEY TAKEAWAY

Think of PAH treatment as working three different relief valves on a backed-up pipeline: the nitric oxide pathway relaxes smooth muscle via cGMP, the prostacyclin pathway provides vasodilation and antiproliferation via cAMP, and endothelin receptor antagonists block the signal that is actively constricting and remodeling the vessel. Modern evidence—particularly the AMBITION trial—supports opening two valves simultaneously (upfront combination therapy with an ERA plus a PDE-5 inhibitor) for most patients with FC II–III PAH. Epoprostenol remains the only agent with a demonstrated mortality benefit and is reserved for FC IV or rapidly deteriorating patients.

SECTION 8

Connection to Advanced Topics & Emerging Concepts

Pulmonary vascular disease intersects with numerous advanced clinical topics that extend beyond the core classification and treatment algorithms. Understanding these connections deepens clinical reasoning and prepares you for complex integrative questions.

Step 2 Foundations and Their Advanced Extensions

Core Concept (Step 2)	Advanced / Emerging Topic
WHO Group 1 PAH: endothelial dysfunction triad	BMPR2 mutations and TGF-β signaling in heritable PAH; sotatercept (activin signaling inhibitor) as a novel fourth pathway targeting vascular remodeling
RV failure as the final common pathway in PH	RV-pulmonary artery coupling assessed by pressure-volume loops; cardiac MRI for RV strain quantification; mechanical circulatory support (RVAD) as bridge to transplant
Acute PE risk stratification (massive vs. submassive)	PERT (Pulmonary Embolism Response Teams); catheter-directed therapy (CDT) as intermediate between systemic lysis and surgery; PE severity index (PESI) for outpatient management
CTEPH diagnosis with V/Q scanning	Dual-energy CT for perfusion mapping; balloon pulmonary angioplasty (BPA) for distal or inoperable CTEPH; combined PTE + BPA strategies
Group 2 PH: treat the left heart	Combined pre- and post-capillary PH (CpcPH) with elevated DPG; sacubitril/valsartan effects on pulmonary hemodynamics; role of SGLT2 inhibitors in HFpEF-related PH

The FDA approval of sotatercept (2024) represents a paradigm shift in PAH treatment by targeting the proliferative component of vascular remodeling through activin signaling inhibition—a mechanism distinct from the three classic pathways. This "fourth pathway" approach reflects a broader trend in pulmonary vascular medicine toward anti-remodeling strategies rather than purely vasodilatory ones. While the details of sotatercept pharmacology are unlikely to appear on Step 2, understanding that the field is evolving beyond simple vasodilation toward disease modification contextualizes the current therapeutic landscape and may inform future clinical practice.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

A patient undergoes right heart catheterization that reveals mPAP = 32 mmHg, PAWP = 22 mmHg, and CO = 5.0 L/min. Which WHO group of pulmonary hypertension is most consistent with these findings, and why?

PROBLEM 2 — BASIC CALCULATION

Calculate the pulmonary vascular resistance for a patient with mPAP = 55 mmHg, PAWP = 12 mmHg, and CO = 3.5 L/min. Classify the hemodynamic profile.

PROBLEM 3 — INTERMEDIATE

A 58-year-old man with a history of DVT two years ago presents with progressive exertional dyspnea. Echocardiography shows RV dilation and estimated RVSP of 70 mmHg. CTA of the chest shows no acute embolus but reveals chronic-appearing filling defects in the lobar pulmonary arteries. What is the most likely diagnosis, what additional test confirms it, and what is the treatment of choice?

PROBLEM 4 — APPLIED

A 30-year-old woman on oral contraceptives presents to the emergency department with acute-onset pleuritic chest pain, tachycardia (HR 118), tachypnea, and SpO₂ of 91% on room air. Her Wells score is 7.5 (PE likely). D-dimer is 2,400 ng/mL. BP is 128/78 mmHg. CTA reveals bilateral pulmonary emboli with RV/LV ratio of 1.3 on CT. Troponin I is mildly elevated. How do you classify this PE, and what is the appropriate management strategy?

PROBLEM 5 — CRITICAL THINKING

A 65-year-old man with severe COPD (FEV₁ 35% predicted) is found to have mPAP of 38 mmHg, PAWP of 8 mmHg, and CO of 5.5 L/min on right heart catheterization performed during evaluation for lung transplantation. His pulmonologist is considering starting sildenafil for his pulmonary hypertension. Discuss the classification of his PH, the potential risks of PAH-specific therapy in this setting, and the most appropriate management.

SUMMARY

Pulmonary Vascular Disease — Summary

Pulmonary vascular disease encompasses conditions that impair blood flow through the pulmonary vasculature, with pulmonary hypertension (mPAP > 20 mmHg) as the central hemodynamic finding. The WHO classification divides PH into five groups based on etiology: Group 1 (pulmonary arterial hypertension), Group 2 (left heart disease—the most common cause overall), Group 3 (lung disease/hypoxia), Group 4 (chronic thromboembolic), and Group 5 (multifactorial). The pivotal diagnostic distinction is between pre-capillary PH (PAWP ≤ 15, PVR ≥ 3 WU) and post-capillary PH (PAWP > 15), determined by right heart catheterization.

For Group 1 PAH, targeted therapies address three pathways: the nitric oxide pathway (PDE-5 inhibitors, riociguat), the endothelin pathway (ERAs like bosentan and ambrisentan), and the prostacyclin pathway (epoprostenol, selexipag). Upfront combination therapy is now standard for most patients. Acute pulmonary embolism is risk-stratified into massive (hemodynamic instability → systemic thrombolysis), submassive (RV dysfunction → anticoagulation with close monitoring), and low-risk (anticoagulation). CTEPH is screened with V/Q scanning and treated with pulmonary thromboendarterectomy when operable. The right ventricle's response to chronic afterload—its capacity for compensatory hypertrophy and eventual failure—remains the principal determinant of prognosis across all forms of pulmonary vascular disease.