Acute Respiratory Distress Syndrome (ARDS) | Wet lung | Shock lung

Definition

• Acute Respiratory Distress Syndrome (ARDS) is a severe condition characterized by sudden onset respiratory failure due to widespread inflammation in the lungs. This leads to the leakage of fluid into the alveoli (the tiny air sacs in the lungs), impairing oxygen exchange and causing life-threatening hypoxemia (low blood oxygen levels). Also called  Shock lung and wet lung.

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Pathophysiology

• Alveolar-Capillary Membrane Injury:-

  • The alveolar-capillary membrane is a thin barrier that separates air in the alveoli from blood in the capillaries. In ARDS, this membrane becomes damaged due to direct or indirect lung injury.

  • Rationale:- The damage increases the permeability of the membrane, allowing fluid to leak from the capillaries into the alveoli, resulting in pulmonary edema (fluid in the lungs).

• Inflammatory Response:-

  • The body’s immune response to the injury releases cytokines and other inflammatory mediators that further damage lung tissue.

  • Rationale:- Inflammation exacerbates the permeability of the capillary membrane, causing more fluid to enter the alveoli, which leads to impaired gas exchange.

• Surfactant Dysfunction:-

  • Surfactant is a substance produced by the lungs that reduces surface tension in the alveoli, preventing their collapse.

  • Rationale:- In ARDS, the dilution of surfactant by the leaked fluid leads to alveolar collapse (atelectasis), further impairing oxygen exchange.

• Fibrosis:-

  • Over time, if the inflammation persists, the lung tissue may begin to scar and thicken (fibrosis), reducing lung compliance (the ability of the lung to stretch and expand).

  • Rationale:- Fibrosis makes the lungs stiff and reduces their ability to efficiently exchange gases, leading to chronic respiratory failure.

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Causes 

• Pneumonia:-

  • Definition:- Infection of the lungs caused by bacteria, viruses, or fungi.

  • Rationale:- Pneumonia can cause direct injury to the alveoli and inflammation, leading to increased permeability of the alveolar-capillary membrane and pulmonary edema, a key feature of ARDS.

• Sepsis:-

  • Definition:- A severe systemic response to an infection, leading to widespread inflammation.

  • Rationale:- Sepsis triggers a cascade of inflammatory responses that increase vascular permeability throughout the body, including the lungs. This results in fluid leakage into the alveoli and ARDS.

• Aspiration:-

  • Definition:- Inhalation of foreign materials (e.g., food, vomit) into the lungs.

  • Rationale:- Aspiration can cause direct injury to the lung tissue and introduce bacteria, leading to inflammation, infection, and subsequent ARDS.

• Trauma:-

  • Definition:- Physical injury to the chest or lungs from accidents, falls, or violence.

  • Rationale:- Trauma can damage the lung tissue directly and cause inflammatory responses that increase alveolar-capillary membrane permeability, leading to ARDS.

• Acute Pancreatitis:-

  • Definition:- Inflammation of the pancreas that can lead to systemic inflammatory response syndrome (SIRS).

  • Rationale:- The inflammatory mediators released during acute pancreatitis can lead to widespread inflammation and increased permeability of the alveolar-capillary membrane, causing ARDS.

• Toxic Inhalation:-

  • Definition:- Exposure to harmful substances or chemicals through inhalation.

  • Rationale:- Inhalation of toxic substances can directly damage the respiratory epithelium and increase inflammation, leading to fluid leakage into the alveoli and ARDS.

• Drug Overdose:-

  • Definition:- Excessive intake of medications or substances that can affect respiratory function.

  • Rationale:- Certain drugs can cause direct lung injury or lead to a reduction in respiratory drive, contributing to the development of ARDS through various mechanisms, including aspiration and inflammation.

• COVID-19:-

  • Definition:- A respiratory illness caused by the novel coronavirus SARS-CoV-2.

  • Rationale:- COVID-19 can lead to severe inflammation in the lungs, with an increase in cytokines and immune response leading to ARDS. The viral infection directly injures lung tissue and exacerbates inflammation.

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Symptoms

• Severe Dyspnea:-

  • Definition:- Severe difficulty in breathing or shortness of breath.

  • Rationale:- The fluid-filled alveoli and collapse of lung tissue reduce oxygen exchange, causing the patient to feel breathless even at rest.

• Tachypnea:-

  • Definition:- Abnormally rapid breathing.

  • Rationale:- The body compensates for low oxygen levels by increasing the respiratory rate to deliver more oxygen to the tissues.

• Cyanosis:-

  • Definition:- A bluish discoloration of the skin and mucous membranes due to low oxygen levels in the blood.

  • Rationale:- Insufficient oxygen in the bloodstream causes a blue tint, particularly noticeable in the lips, nail beds, and extremities.

• Bilateral Crackles:-

  • Definition:- Abnormal lung sounds heard through a stethoscope, indicating the presence of fluid in the alveoli.

  • Rationale:- Crackles occur when air passes through fluid-filled alveoli during breathing, reflecting the accumulation of fluid in the lungs.

• Hypoxemia:-

  • Definition:- Abnormally low levels of oxygen in the blood.

  • Rationale:- Due to the fluid-filled alveoli and alveolar collapse, oxygen cannot adequately pass from the lungs into the blood, leading to hypoxemia.

• Mental Confusion:-

  • Definition:- Altered mental status, including confusion, disorientation, or agitation.

  • Rationale:- The brain is highly sensitive to oxygen levels; insufficient oxygenation can impair cognitive function, leading to confusion or even loss of consciousness.

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Diagnosis

Pulmonary Capillary Wedge Pressure (PCWP):-

• Definition:-

  • PCWP is a measurement of the pressure in the left side of the heart, specifically in the left atrium, indirectly assessed by inserting a catheter into a pulmonary artery.

  • Normal Value:- 4-12 mmHg.

  • Rationale:- In ARDS, PCWP typically remains below 18 mmHg, distinguishing it from cardiogenic pulmonary edema, where PCWP is elevated due to left heart failure. A low PCWP in ARDS indicates that the pulmonary edema is due to increased capillary permeability rather than heart failure.

Arterial Blood Gas (ABG) Analysis:-

• Definition:-

  • ABG measures the levels of oxygen (PaO2), carbon dioxide (PaCO2), and blood pH in arterial blood.

  • Normal Values:-

    • PaO2: 75-100 mmHg.

    • PaCO2: 35-45 mmHg.

    • pH: 7.35-7.45.

  • Rationale:- In ARDS, ABG typically shows severe hypoxemia (low PaO2) and respiratory alkalosis (elevated pH) due to hyperventilation. A PaO2/FiO2 ratio of less than 300 mmHg is indicative of ARDS severity.

Chest X-ray:-

• Definition:-

  • A chest X-ray is an imaging test that provides a picture of the lungs and chest.

  • Findings:- Bilateral diffuse infiltrates, which appear as cloudy or white areas on the X-ray, are characteristic of ARDS.

  • Rationale:- These infiltrates represent the fluid and protein accumulation in the alveoli, indicative of pulmonary edema in ARDS.

Computed Tomography (CT) Scan:-

• Definition:-

  • A CT scan uses X-rays to create detailed cross-sectional images of the body.

  • Findings:- Diffuse bilateral infiltrates, areas of consolidation, and ground-glass opacities are commonly seen in ARDS.

  • Rationale:- CT scans provide a more detailed view of lung involvement, showing the extent of alveolar damage and fluid accumulation.

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Stages of ARDS

ARDS is classified into stages based on the severity of hypoxemia, measured by the PaO2/FiO2 ratio (the ratio of arterial oxygen partial pressure to fractional inspired oxygen).

• Normal:- More than 400

• Mild ARDS:-

  • PaO2/FiO2 Ratio:- 200-300 mmHg.

  • Rationale:- Oxygenation is mildly impaired, with moderate hypoxemia.

• Moderate ARDS:-

  • PaO2/FiO2 Ratio:- 100-200 mmHg.

  • Rationale:- Oxygenation is significantly impaired, with more severe hypoxemia.

• Severe ARDS:-

  • PaO2/FiO2 Ratio:- <100 mmHg.

  • Rationale:- Oxygenation is critically impaired, with life-threatening hypoxemia.

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Management

Non-Pharmacological Management

• Mechanical Ventilation:-

  • Low Tidal Volume Ventilation:-

    • Definition:- Ventilation strategy using smaller breaths (tidal volume) to reduce lung injury.

    • Rationale:- Lower tidal volumes (4-8 mL/kg of predicted body weight) prevent overdistension of the alveoli, reducing the risk of ventilator-induced lung injury (VILI).

  • Positive End-Expiratory Pressure (PEEP):-

    • Definition:- A pressure applied by the ventilator to keep the alveoli open during exhalation.

    • Rationale:- PEEP prevents alveolar collapse, improving oxygenation by maintaining open airways and enhancing gas exchange.

• Fluid Management:- 

  • Conservative Fluid Strategy:-

    • Definition:- Limiting the amount of fluids given to the patient to prevent fluid overload.

    • Rationale:- Restricting fluids minimizes pulmonary edema, reducing the volume of fluid that leaks into the alveoli and improving lung function.

Pharmacological Management

• Corticosteroids:-

  • Action:- Reduces inflammation by suppressing the immune response.

  • Example:- Dexamethasone, Methylprednisolone.

  • Side Effects:- Immunosuppression, hyperglycemia, hypertension.

  • Rationale:- Corticosteroids help to reduce the inflammatory response in the lungs, potentially improving outcomes in some ARDS patients.

• Neuromuscular Blockade:-

  • Action:- Induces muscle paralysis to improve mechanical ventilation.

  • Example:- Cisatracurium.

  • Side Effects:- Prolonged paralysis, risk of critical illness polyneuropathy.

  • Rationale:- Neuromuscular blocking agents prevent patient-ventilator dyssynchrony, facilitating more effective ventilation and improving oxygenation in severe ARDS.

• Vasodilators:-

  • Action:- Dilates blood vessels in the lungs to improve blood flow to well-ventilated areas.

  • Example:- Inhaled Nitric Oxide.

  • Side Effects:- Hypotension, rebound hypoxemia.

  • Rationale:- Inhaled nitric oxide selectively dilates pulmonary vessels, improving oxygenation by increasing blood flow to areas of the lung that are better ventilated.

• Antibiotics:-

  • Action:- Treats bacterial infections.

  • Example:- Broad-spectrum antibiotics like Piperacillin-Tazobactam.

  • Side Effects:- Allergic reactions, antibiotic resistance, gastrointestinal disturbances.

  • Rationale:- If ARDS is associated with or caused by a bacterial infection, antibiotics are essential to control the infection and reduce the inflammatory response.

Surgical Management

• Extracorporeal Membrane Oxygenation (ECMO):-

  • Definition:-

    • ECMO is a life-support technique that uses a machine to take over the work of the lungs (and sometimes the heart) by oxygenating the blood outside the body and returning it to the patient’s circulation.

  • Types:-

    • Veno-Venous (VV) ECMO:-Used primarily for respiratory support.

    • Veno-Arterial (VA) ECMO:- Provides both respiratory and cardiac support.

  • Rationale:-

    • VV ECMO:- Bypasses the lungs, allowing them to rest and heal while maintaining oxygenation and carbon dioxide removal.

    • VA ECMO:- Supports both the heart and lungs in cases of combined respiratory and cardiac failure.

      • Rationale:- ECMO serves as a bridge therapy for patients with severe ARDS, particularly when conventional mechanical ventilation fails to provide adequate oxygenation. It buys time for the lungs to recover while ensuring that vital organs receive enough oxygen to function properly.

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