What is Acute Respiratory Distress Syndrome (ARDS)?

Overview:

• Previously called Adult Respiratory Distress Syndrome; now termed “Acute” because it can affect any age.
• It mimics drowning in the lungs, characterized by severe inflammation and fluid buildup in alveoli leading to impaired gas exchange and hypoxemia.
• Common causes include severe viral infections (influenza, SARS-CoV-1, SARS-CoV-2), pneumonia, burns, trauma, choking, or corrosive inhalation (acid spill).

Pathophysiology:

• Damage to alveolar walls and capillaries triggers an intense inflammatory response.
• Release of pro-inflammatory cytokines increases capillary permeability, allowing protein-rich exudate to flood alveoli.
• Formation of hyaline fibrous membranes: proteinaceous material coats alveolar walls, blocking gas diffusion, stiffening lungs, decreasing compliance.
• Damage affects alveolar Type 1 cells (gas exchange) and Type 2 cells (surfactant production); reduced surfactant causes alveolar collapse and fibrosis, resulting in “shock lung.”
• Microthrombi can develop, impairing blood flow in pulmonary capillaries and causing tissue hypoxia systemically.

Consequences:

• Reduced alveolar compliance leads to “stiff lung.”
• Hypoxemia causes rapid, shallow breathing, increased heart rate, and signs of brain hypoxia (restlessness, confusion, decreased consciousness).
• Acid-base imbalance: metabolic acidosis from lactic acid (anaerobic respiration) or respiratory acidosis from CO₂ retention.

Signs & Symptoms:

• Rapid onset pulmonary edema visible on chest X-ray.
• Severe dyspnea, rales, productive cough, cyanosis.
• Hypoxemia with increased respiratory rate, shallow breaths, decreased tidal volume.
• Systemic hypoxia may cause neurological symptoms and risk of multi-organ failure.

Progression if untreated:

• Worsening gas exchange can lead to respiratory failure (ABGs below 50 mm Hg for O₂ or above 50 mm Hg for CO₂).
• Can progress to respiratory arrest, cardiac arrest, shock.

Summary of ARDS Steps of Development (Pathogenesis):

1. Initial Injury (trauma, infection, etc.)
2. Inflammatory Response — increased permeability, exudate.
3. Alveolar Damage — hyaline membrane formation, surfactant loss, fibrosis.
4. Decreased Compliance — labored breathing, hypoxemia.
5. Progression to Failure — hypoxemic respiratory failure.
6. Potential Outcomes: Organ hypoxia, confusion, arrhythmias, shock, death.

Infant Respiratory Distress Syndrome (IRDS):

• Mostly affects premature infants due to immature lungs.
• Surfactant deficiency causes alveolar collapse and atelectasis.
• Treatment: Synthetic surfactant spray into the lungs, often effective in stabilizing the newborn.

Summary:

ARDS is a critical condition with rapid progression if untreated, resulting from alveolar injury and inflammation. Early diagnosis, oxygen therapy, and addressing the underlying cause are crucial to prevent irreversible lung damage and multi-organ failure.