Acute Respiratory Distress Syndrome (ARDS) is a life-threatening condition characterised by severe respiratory failure and pulmonary inflammation. It is a complex clinical syndrome that can develop as a result of direct lung injury or as a consequence of systemic inflammation.
Pathophysiology
ARDS is characterised by a complex cascade of pathological events that lead to severe respiratory dysfunction. These include:
- Alveolar Epithelial Injury: The initial insult in ARDS often involves direct or indirect damage to the alveolar epithelium. This can occur due to infections, aspiration of gastric contents, inhalation of toxic substances or systemic inflammation. The injury disrupts the integrity of the alveolar-capillary barrier, leading to increased permeability and leakage of fluid into the alveolar space.
- Inflammatory Response: Alveolar epithelial injury triggers an intense inflammatory response. Alveolar macrophages and immune cells are activated, releasing pro-inflammatory cytokines and chemokines. This inflammatory cascade attracts more immune cells to the site of injury, exacerbating the lung inflammation. The influx of neutrophils and other immune cells into the alveoli further damages the alveolar-capillary membrane and contributes to the development of diffuse alveolar damage.
- Alveolar Oedema Formation: Increased permeability of the alveolar-capillary barrier leads to the accumulation of protein-rich fluid in the alveoli. The leakage of plasma proteins and inflammatory mediators results in the formation of pulmonary oedema. Alveolar oedema impairs gas exchange, reduces lung compliance and compromises oxygenation.
- Interstitial Inflammation and Fibrosis: In addition to alveolar damage, ARDS involves interstitial inflammation and fibrotic changes. Activated fibroblasts produce excessive collagen, leading to the deposition of fibrotic tissue within the lung interstitium. This fibrotic remodelling can further impair lung function and contribute to long-term complications in some ARDS patients.
- Impaired Surfactant Function: ARDS disrupts the production and function of pulmonary surfactant, a substance that reduces surface tension within the alveoli. Reduced surfactant levels and altered surfactant composition lead to alveolar collapse and atelectasis, further compromising gas exchange.
- Pulmonary Vasoconstriction and Hypoxic Pulmonary Vasoconstriction: In response to hypoxaemia and local inflammation, pulmonary arterioles constrict. This vasoconstriction redirects blood flow from poorly ventilated regions of the lung to well-ventilated areas, optimising ventilation-perfusion matching. However, persistent vasoconstriction can increase pulmonary vascular resistance and worsen pulmonary hypertension.
- Systemic Effects: The pathophysiological changes in ARDS extend beyond the lungs and affect various organ systems. The release of inflammatory mediators into the systemic circulation can lead to endothelial dysfunction, coagulation abnormalities, multi-organ dysfunction syndrome, and impaired oxygen delivery to vital organs.
ARDS Signs and Symptoms
The signs and symptoms of ARDS are primarily related to the severe respiratory compromise and impaired gas exchange that occurs. Prompt recognition of these manifestations is crucial for early intervention and management.
- Respiratory Distress and Increased Work of Breathing: Patients with ARDS often present with rapid, shallow breathing and an increased respiratory rate. They may exhibit retractions (visible inward movement of the soft tissues between the ribs or at the base of the neck) and use accessory respiratory muscles to aid in breathing. This increased work of breathing is a result of the compromised lung function.
- Hypoxaemia and Cyanosis: ARDS is characterised by profound hypoxaemia, which is a low level of oxygen in the blood. Despite high concentrations of supplemental oxygen, patients may still exhibit low oxygen saturation levels. Cyanosis, a bluish discolouration of the skin and mucous membranes, may be present and is indicative of severe hypoxaemia.
- Changes in Lung Sounds and Auscultation Findings: Lung auscultation may reveal abnormal findings. Diffuse crackles, also known as rales, may be heard throughout the lung fields due to the presence of fluid-filled alveoli. In more severe cases, breath sounds may be diminished or even absent, suggesting alveolar collapse and decreased air entry.
- Associated Systemic Manifestations: In addition to respiratory symptoms, patients with ARDS may experience systemic manifestations related to the underlying cause or the inflammatory response. These may include fever, tachycardia (rapid heart rate), hypotension (low blood pressure), altered mental status, and signs of organ dysfunction.
Risk Factors for ARDS
Several risk factors have been identified that increase the likelihood of developing ARDS. Understanding these risk factors can help in identifying patients who may be at higher risk and enable early intervention and preventive measures.
Direct Lung Injury
- Pneumonia: Bacterial, viral, or fungal pneumonia can lead to severe lung inflammation and injury, predisposing individuals to ARDS.
- Aspiration: Inhaling gastric contents or foreign substances into the lungs can trigger a local inflammatory response, potentially resulting in ARDS.
- Near-drowning: Submersion in water or other fluids can cause aspiration and subsequent lung injury, increasing the risk of ARDS.
- Inhalation Injury: Exposure to toxic gases, smoke, or chemicals can damage the airways and lung tissue, increasing the likelihood of developing ARDS.
Indirect Lung Injury
- Sepsis: Systemic infection, particularly severe cases of sepsis, can lead to an overwhelming inflammatory response that affects the lungs and contributes to ARDS development.
- Trauma: Severe physical injuries, such as those resulting from accidents or major surgeries, can cause systemic inflammation and lung injury, increasing the risk of ARDS.
- Pancreatitis: Severe inflammation of the pancreas can trigger a systemic inflammatory response that affects multiple organs, including the lungs, leading to ARDS.
- Transfusion-related Lung Injury: In some cases, blood transfusions can result in an immune-mediated reaction that causes lung injury and contributes to ARDS development.
ARDS Treatment
The management of ARDS requires a comprehensive approach aimed at addressing the underlying causes, optimising oxygenation and providing supportive care to improve patient outcomes. The following treatments are commonly utilised in the management of ARDS:
- Mechanical Ventilation: Mechanical ventilation is a crucial intervention in ARDS to provide respiratory support and maintain adequate oxygenation. The use of positive end-expiratory pressure (PEEP) helps keep the alveoli open, improves lung compliance, and facilitates gas exchange. Ventilatory strategies such as low tidal volume ventilation and lung-protective ventilation techniques are commonly employed to minimise ventilator-induced lung injury.
- Oxygen Therapy: Supplemental oxygen is administered to maintain adequate oxygen levels in the blood. High-flow oxygen delivery systems, such as nasal cannula or non-rebreather masks, may be used to optimise oxygenation. In severe cases, patients may require mechanical ventilation with high concentrations of inspired oxygen.
- Fluid Management: Fluid management plays a crucial role in the management of ARDS. Optimal fluid balance should be maintained to prevent fluid overload, which can worsen lung function. Diuretics may be used to promote fluid removal in cases of fluid overload.
- Prone Positioning: Proning, or placing the patient in a prone position, can improve oxygenation in ARDS. This maneuver helps to redistribute blood flow and reduce the compression of lung tissue, enhancing ventilation-perfusion matching.
- Pharmacological Interventions: Various medications may be utilized in the treatment of ARDS, depending on the underlying cause and patient’s condition. These may include:
- Antibiotics: If an infection is identified as the cause of ARDS, appropriate antibiotics are administered.
- Anti-inflammatory Agents: Corticosteroids or other anti-inflammatory medications may be prescribed in certain cases to modulate the immune response and reduce lung inflammation.
- Vasopressors: In cases of sepsis-related ARDS, vasopressor medications may be required to support blood pressure and organ perfusion.
- Supportive Care: Alongside specific treatments, supportive care is essential in managing ARDS. This includes adequate pain control, nutrition support, prevention of complications (such as deep vein thrombosis and pressure ulcers) and regular monitoring of vital signs and oxygenation levels.