M04.03.15 Pulmonary Edema

Pulmonary edema is a condition characterized by the accumulation of fluid in the lung’s interstitial and alveolar spaces. This condition disrupts gas exchange and can result in hypoxemia (low blood oxygen levels) and hypercapnia (elevated carbon dioxide levels). The lungs have protective mechanisms, such as low hydrostatic pressure in pulmonary capillaries and effective lymphatic drainage, to prevent fluid accumulation. However, alterations in Starling forces, capillary permeability, or lymphatic drainage can lead to pulmonary edema.

Causes of Pulmonary Edema

Pulmonary edema can be classified into two main types based on its underlying cause:

1. Cardiogenic Pulmonary Edema

This is the more common form and is caused by elevated pulmonary capillary hydrostatic pressure due to cardiac dysfunction.

Mechanism:

• Increased Left Atrial Pressure: Left atrial pressure increases, leading to elevated venous pressure, which subsequently raises capillary pressure.
• Protective Response: Initially, increased lymphatic flow removes interstitial proteins, helping to prevent fluid accumulation.
• Signs and Symptoms:
  • Orthopnea: Difficulty breathing when lying flat, relieved by sitting upright.
  • Elevated Pulmonary Wedge Pressure (PCWP): Used to confirm the diagnosis.
• Treatment: Aimed at reducing left atrial pressure, often through diuretic therapy.

2. Non-Cardiogenic Pulmonary Edema

This type results from increased capillary permeability, often due to direct injury to the alveolar epithelium or secondary to capillary endothelial damage.

Mechanism:

• Direct Injury to Lung Tissue: Injury to the alveolar epithelium or capillary endothelium disrupts normal fluid balance.
• Clinical Presentation:
  • Severe Dyspnea: Rapid onset of difficulty breathing.
  • Hypoxemia: Low oxygen levels in the blood.
  • Diffuse Pulmonary Infiltrates: Fluid accumulation in the lungs leads to respiratory failure.
• Common Causes:
  • Sepsis
  • Bacterial Pneumonia
  • Trauma
  • Gastric Aspirations
• Pathophysiology: Loss of epithelial integrity allows protein-rich fluid to accumulate in the alveoli, which inactivates surfactant and reduces lung compliance. Pulmonary wedge pressure is usually normal or low.

Points to Remember

• Cardiogenic Pulmonary Edema: Related to increased left atrial pressure, leading to high capillary pressure and fluid accumulation.
• Non-Cardiogenic Pulmonary Edema: Caused by increased capillary permeability due to direct lung injury or systemic conditions like sepsis.
• Pulmonary Wedge Pressure: Elevated in cardiogenic edema and normal or low in non-cardiogenic edema.
• Treatment: Cardiogenic edema responds to therapies reducing left atrial pressure, such as diuretics. Non-cardiogenic edema requires treatment of the underlying cause, such as infection control in cases of sepsis.

Bibliography

• American Thoracic Society – Pulmonary Edema
• Mayo Clinic – Pulmonary Edema
• Medscape – Pulmonary Edema Overview