What are the causes of a dilated pulmonary trunk and pulmonary hypertension?

Causes of Dilated Pulmonary Trunk and Pulmonary Hypertension

Pulmonary hypertension (PH) with dilated pulmonary trunk can result from multiple etiologies, with the most common causes being left heart disease, congenital heart defects, and chronic thromboembolic disease.

Classification of Pulmonary Hypertension

Pulmonary hypertension is defined as a mean pulmonary arterial pressure ≥25 mmHg at rest as assessed by right heart catheterization 1. The comprehensive clinical classification includes:

Group 1: Pulmonary Arterial Hypertension (PAH)

• Idiopathic PAH 1
• Heritable PAH (BMPR2 mutations and other genetic causes) 1
• Drug and toxin-induced PAH 1
• Associated PAH (APAH):
  • Connective tissue diseases (e.g., scleroderma) 1
  • HIV infection 1
  • Portal hypertension 1
  • Congenital heart disease 1
  • Schistosomiasis 1

Group 2: PH Due to Left Heart Disease

• Left ventricular systolic dysfunction 1
• Left ventricular diastolic dysfunction 1
• Valvular disease (particularly mitral and aortic) 1
• Congenital/acquired left heart inflow/outflow tract obstruction 1

Group 3: PH Due to Lung Diseases and/or Hypoxia

• Chronic obstructive pulmonary disease 1
• Interstitial lung disease 1
• Sleep-disordered breathing 1
• Alveolar hypoventilation disorders 1
• Chronic exposure to high altitude 1

Group 4: Chronic Thromboembolic PH (CTEPH)

• Chronic thromboembolic pulmonary hypertension 1
• Other pulmonary artery obstructions (tumors, arteritis, congenital stenoses) 1

Group 5: PH with Unclear/Multifactorial Mechanisms

• Hematological disorders (chronic hemolytic anemia, myeloproliferative disorders) 1
• Systemic disorders (sarcoidosis, pulmonary histiocytosis) 1
• Metabolic disorders (glycogen storage disease, Gaucher disease) 1

Specific Causes of Pulmonary Trunk Dilation

The pulmonary trunk typically dilates in response to:

1. Increased Pulmonary Arterial Pressure

• Chronic pulmonary hypertension from any cause leads to vessel wall remodeling and dilation 1
• Massive dilation occurs most commonly with congenital heart disease, particularly with left-to-right shunts 1

2. Congenital Heart Defects

• Atrial septal defects - can cause massive dilation of pulmonary arterial trunk and branches 1
• Ventricular septal defects - particularly with moderate-to-large defects 1
• Eisenmenger syndrome - includes all systemic-to-pulmonary shunts with severe PVR increase 1
• Post-cardiac surgery - PAH may persist or recur after surgical correction 1

3. Valvular Abnormalities

• Pulmonary valve stenosis - paradoxically can lead to post-stenotic dilation of the pulmonary trunk 1
• The pulmonary trunk may dilate with dome-shaped pulmonary valve stenosis due to inherent medial abnormality 1

4. Idiopathic Dilation

• Low-pressure pulmonary artery aneurysms may occur without significant pulmonary hypertension 1
• These can appear as a mediastinal mass on chest x-ray 1

Clinical Implications of Pulmonary Trunk Dilation

Compression of Adjacent Structures

• Left main coronary artery compression - can cause angina, left ventricular ischemia, and sudden death 2, 3
• Left recurrent laryngeal nerve compression - can cause hoarseness 2
• Tracheobronchial tree compression - can cause major airway stenosis 2

Risk Factors for Coronary Compression

• Pulmonary trunk diameter ≥40 mm 4
• Ratio of pulmonary trunk to aortic diameter ≥1.21 4
• Compression was not observed with pulmonary artery diameters <40 mm 4

Diagnostic Approach

Imaging

• Chest radiography may show markedly dilated pulmonary arteries 1
• Echocardiography can measure pulmonary trunk diameter and estimate pulmonary pressures 1
• CT angiography can demonstrate the extent of pulmonary artery dilation and compression of adjacent structures 1, 2

Hemodynamic Assessment

• Right heart catheterization is essential to confirm PH (mean PAP ≥25 mmHg) 1
• Measurement of pulmonary capillary wedge pressure (PCWP) helps differentiate pre-capillary from post-capillary PH 1
• A PCWP >15 mmHg excludes the diagnosis of pre-capillary PAH 1

Management Considerations

Treatment of Underlying Cause

• Optimal treatment of underlying left heart disease is recommended in patients with PH due to left heart disease 1
• PAH-specific therapies should be used only for Group 1 PAH, not for PH due to left heart disease 1
• Patients with coronary compression may require coronary intervention 3, 4

Medication Considerations

• Vasodilator therapies like epoprostenol must be initiated with careful monitoring due to potential hemodynamic effects 5
• Sildenafil and other PDE-5 inhibitors work by increasing cGMP in pulmonary vascular smooth muscle, causing vasodilation 6

Monitoring for Complications

• Regular assessment for signs of right ventricular failure is essential 7
• Patients with pulmonary trunk diameter ≥40 mm should be evaluated for possible coronary compression 4

Pitfalls and Caveats

• Low-pressure pulmonary artery aneurysms rarely rupture, unlike hypertensive pulmonary aneurysms which may rupture 1
• Diastolic dysfunction can be challenging to diagnose as a cause of PH; fluid challenge during catheterization may be needed 1
• Abrupt withdrawal of PAH therapies can lead to rebound pulmonary hypertension with serious consequences 5
• Intubation should be avoided if possible in patients with PH and right ventricular failure, as it may worsen right-sided heart function 7