Section 6.4: Pulmonary Vascular & Pleural Diseases

Pulmonary Vascular and Pleural Diseases: Acute Management and Diagnostic Analysis

Pulmonary vascular and pleural diseases frequently manifest as acute cardiopulmonary compromise. For the USMLE Step 3, the clinician must master the diagnostic workup and risk-stratification of acute pulmonary embolism, the classification and hemodynamic profiles of pulmonary hypertension, and the diagnostic interpretation of pleural fluid to guide thoracentesis and subsequent management.

Acute Pulmonary Embolism (PE) Management

The clinical approach to suspected pulmonary embolism begins with risk assessment using the Wells Criteria for PE (which scores clinical signs of deep vein thrombosis [DVT], heart rate > 100/min, immobilization/surgery in the past 4 weeks, history of DVT/PE, hemoptysis, active malignancy, and whether PE is the most likely diagnosis).

Diagnostic Algorithm:

• Wells Score <= 4 (PE Unlikely): Perform a high-sensitivity D-dimer assay. If the D-dimer is normal (< 500 ng/mL, or age-adjusted: age x 10 for patients > 50 years), PE is ruled out. If the D-dimer is elevated, proceed to computed tomography pulmonary angiography (CTPA).
• Wells Score > 4 (PE Likely): Skip the D-dimer and proceed directly to CTPA.
• CTPA Contraindications (e.g., severe renal impairment [GFR < 30 mL/min/1.73 m2], severe contrast allergy, or pregnancy): A ventilation-perfusion (V/Q) scan is the preferred alternative diagnostic study, provided the patient has a normal baseline chest radiograph.
• Hemodynamically Unstable Patient (Systolic BP < 90 mmHg): Perform a bedside transthoracic echocardiogram (TTE) to look for signs of right ventricular (RV) strain (RV dilation, McConnell’s sign [hypokinesis of the RV free wall with sparing of the apex], or tricuspid regurgitation). If RV dysfunction is present and the patient is too unstable for transport to CTPA, presumptive therapy for massive PE is indicated.

Management Based on Risk Stratification:

1. Massive PE (Hemodynamically Unstable: SBP < 90 mmHg for > 15 minutes, or requiring vasopressor support): Requires immediate reperfusion therapy. Systemic thrombolysis (e.g., alteplase) is first-line in the absence of absolute contraindications (recent intracranial hemorrhage, major active bleeding, stroke within 3 months, or recent major surgery/trauma). Intravenous unfractionated heparin (UFH) should be initiated. If thrombolysis is contraindicated or fails, emergent catheter-directed embolectomy or surgical embolectomy is indicated.
2. Submassive PE (Hemodynamically Stable with evidence of RV strain on TTE/CT, or elevated cardiac biomarkers [troponin or BNP]): Requires therapeutic anticoagulation (low-molecular-weight heparin [LMWH], fondaparinux, or UFH) and close monitoring in an inpatient unit. Systemic thrombolysis is not routinely indicated for submassive PE but can be rescue therapy if clinical deterioration occurs.
3. Low-Risk PE (Hemodynamically Stable without RV strain or biomarker elevation): Treated with oral direct oral anticoagulants (DOACs; apixaban or rivaroxaban) as first-line monotherapy. These patients may be managed as outpatients if they meet low-risk criteria (e.g., low Pulmonary Embolism Severity Index [PESI] score), have adequate social support, and can access medication.

Duration of Anticoagulation:

• Provoked PE (due to a transient, reversible risk factor like surgery or trauma): 3 months of anticoagulation.
• Unprovoked PE or persistent risk factors (active cancer, antiphospholipid syndrome): Indefinite anticoagulation.
• Inferior Vena Cava (IVC) Filter: Indicated only if the patient has an active, life-threatening hemorrhage making anticoagulation contraindicated, or recurrent PE despite documented therapeutic anticoagulation. Filters must be retrieved once anticoagulation is safe.

Pulmonary Hypertension: Classification and Hemodynamics

Pulmonary hypertension (PH) is hemodynamically defined as a mean pulmonary arterial pressure (mPAP) > 20 mmHg at rest, measured via right heart catheterization. Clinicians must classify patients into one of five World Health Organization (WHO) groups, as treatment is group-specific and inappropriate therapy can be harmful:

1. Group 1: Pulmonary Arterial Hypertension (PAH): Includes idiopathic, heritable (BMPR2 mutations), drug-induced (aminorex, methamphetamines), or connective tissue disease-associated (scleroderma) PAH. The primary pathophysiology involves endothelial dysfunction, smooth muscle proliferation, and plexiform lesions in small pulmonary arteries, leading to a precapillary hemodynamic profile: mPAP > 20 mmHg, pulmonary capillary wedge pressure (PCWP) <= 15 mmHg, and pulmonary vascular resistance (PVR) >= 2 Wood units.
   • Vasoreactivity Testing: Performed during right heart catheterization using inhaled nitric oxide. If positive (mPAP decreases by >= 10 mmHg to an absolute value <= 40 mmHg with stable cardiac output), the patient is treated with high-dose calcium channel blockers (nifedipine, diltiazem).
   • Advanced Therapies: For vasoreactivity-negative patients, advanced therapies include endothelin receptor antagonists (bosentan, ambrisentan, macitentan), phosphodiesterase-5 (PDE-5) inhibitors (sildenafil, tadalafil), soluble guanylate cyclase stimulators (riociguat), and prostacyclin analogs (epoprostenol, treprostinil).
2. Group 2: PH due to Left Heart Disease: The most common form of PH, caused by HFrEF, HFpEF, or valvular heart disease. It represents postcapillary PH: mPAP > 20 mmHg and PCWP > 15 mmHg. Treatment is directed at optimizing the underlying left heart disease (diuretics, beta-blockers, ACE inhibitors). Advanced PAH-specific therapies (e.g., sildenafil, bosentan) are contraindicated, as they can cause severe pulmonary edema and increase mortality.
3. Group 3: PH due to Chronic Lung Disease and/or Hypoxia: Caused by COPD, interstitial lung disease, or obstructive sleep apnea. Hypoxia triggers pulmonary vasoconstriction. Treatment involves supplemental oxygen (if criteria are met) and optimization of the underlying disease. Advanced PAH-specific therapies are generally not recommended.
4. Group 4: Chronic Thromboembolic Pulmonary Hypertension (CTEPH): Caused by chronic obstruction of the pulmonary arteries by organized thrombi. It is a precapillary PH profile. Surgical pulmonary thromboendarterectomy (PTE) is potentially curative and is the treatment of choice. Inoperable patients are treated with riociguat and lifelong anticoagulation.
5. Group 5: PH with Unclear or Multifactorial Mechanisms: Associated with sarcoidosis, Langerhans cell histiocytosis, or hematologic disorders. Treatment targets the underlying cause.

Pleural Effusions: Transudative vs. Exudative

The initial step in evaluating a pleural effusion is determining whether a diagnostic thoracentesis is indicated. Thoracentesis should be performed for any new, unexplained pleural effusion that is > 1 cm in depth on lateral decubitus chest radiography. The exception is a patient with clinical features highly characteristic of congestive heart failure (symmetric bilateral effusions, no fever, and no pleuritic chest pain); in these cases, a trial of loop diuretics is initiated first, and thoracentesis is deferred unless the effusion fails to resolve after 3 days of therapy.

Light’s Criteria: To distinguish between transudative and exudative effusions, pleural fluid and serum levels of protein and lactate dehydrogenase (LDH) are analyzed. An effusion is classified as an exudate if it meets at least one of the following criteria:

1. Pleural fluid protein / Serum protein ratio > 0.5
2. Pleural fluid LDH / Serum LDH ratio > 0.6
3. Pleural fluid LDH > 2/3 the upper limit of normal for serum LDH

Transudates are caused by systemic imbalances in hydrostatic or oncotic pressure (e.g., congestive heart failure, hepatic hydrothorax, nephrotic syndrome) and require treatment of the underlying systemic disease. Exudates are caused by local inflammatory, infectious, or neoplastic processes (e.g., pneumonia, malignancy, pulmonary embolism, tuberculosis) and require directed diagnostic workup of the pleural space.

Pleural Fluid Analysis:

• pH < 7.2: Indicates a complicated parapneumonic effusion, empyema, rheumatoid arthritis effusion, esophageal rupture, or tuberculosis. A pH < 7.2 in a parapneumonic effusion indicates that the fluid is unlikely to resolve with antibiotics alone and requires tube thoracostomy (chest tube drainage).
• Glucose < 60 mg/dL: Associated with rheumatoid arthritis (typically very low, < 30 mg/dL), empyema, malignancy, and tuberculosis.
• Amylase Elevated: Suggests pancreatitis, pancreatic pseudocyst, esophageal rupture, or malignancy.
• Triglycerides > 110 mg/dL: Diagnostic of a chylothorax, resulting from thoracic duct disruption (usually due to trauma, cardiothoracic surgery, or lymphoma).
• Parapneumonic Effusions:
  • Uncomplicated: Sterile pleural fluid associated with adjacent pneumonia; pH > 7.3, glucose > 60 mg/dL, and LDH < 1000 U/L. Resolves with antibiotics alone.
  • Complicated: Bacterial invasion of the pleural space; pH < 7.2, glucose < 60 mg/dL, and LDH > 1000 U/L. Requires chest tube drainage.
  • Empyema: Frank pus in the pleural cavity or positive Gram stain/culture. Requires urgent chest tube drainage. Refractory empyema with loculations may require intrapleural fibrinolytics (tPA and DNase) or surgical decortication via video-assisted thoracoscopic surgery (VATS).