ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS)

By the end of this session, students will be able to:

Understand the causes, pathophysiology, diagnosis, and initial management of Acute Respiratory Distress Syndrome (ARDS).

Acute Respiratory Distress Syndrome (ARDS):
- Characterized as hypoxemic respiratory failure with acute abnormalities affecting both lungs.
- Historical context:
  - Initially termed 'shock lung' observed in military hospitals during the Vietnam War in the 1960s.
- Represents acute, diffuse inflammatory lung injury primarily affecting alveolar function, leading to diffuse alveolar damage.
- The condition can affect individuals of all ages and was initially termed 'adult respiratory distress syndrome'.

Mechanisms of Normal Lung Function:
- Alveoli are patent and positioned closely to perfused capillaries.
- The normal capillary endothelium exhibits selective permeability; fluid movement is controlled by hydrostatic and oncotic forces, while serum proteins remain intravascular.
- The oncotic gradient is maintained to favor fluid reabsorption, with interstitial lymphatics assisting in returning fluids to circulation.
- Tight junctions among alveolar epithelial cells prevent extravasation of fluid into the air spaces.

Consequences of Alveolar Injury:
- Injury induces the release of inflammatory cytokines, such as IL-1, IL-6, IL-8, and tumor necrosis factor-alpha (TNF-α).
- Recruitment of neutrophils leads to the production of reactive oxygen species and proteolytic enzymes, damaging both capillary endothelium and alveolar epithelium.
Pathologic Changes in ARDS:
1. Epithelial Changes:
- Loss of tight junctions and disruption of the alveolar epithelium.
- Flooding of air spaces with bloody, proteinaceous edema and cellular debris.
- Loss of surfactant leads to alveolar collapse.
1. Endothelial Damage:
- Serum proteins leak from the vascular space, altering oncotic gradients and causing fluid accumulation in the interstitium, exceeding lymphatic removal capabilities leading to pulmonary edema.

Clinical Manifestations of ARDS:
- Impaired gas exchange resulting in:
- Hypoxemia: Refers to inadequate oxygenation.
  - Mechanism involves ventilation-perfusion (V/Q) mismatch (low ventilation relative to perfusion).
  - Presence of physiologic shunting with non-ventilated but perfused alveoli due to alveolar flooding.
- Hypercapnia: Inadequate ventilation resulting in elevated carbon dioxide levels.
Imaging Findings:
- Initial Chest XR: Bilateral, homogeneous infiltrates are typically observed as changes in lung opacity indicative of ARDS.

Exudative Stage:
- The initial phase is characterized by pulmonary edema due to alveolar-capillary barrier disruption.
Proliferative Stage:
- Notable for the formation of hyaline membranes, interstitial inflammation, and the beginning of fibrosis.
Fibrotic Phase:
- Extensive interstitial fibrosis and fibroblastic proliferation may occur.

Extrinsic (Systemic) Causes:
- Sepsis, trauma, transfusions (more than 15 units), hematopoietic stem cell transplantation (HSCT), thoracic surgery, medications such as chemotherapy or aspirin, and pancreatitis.
Intrinsic Causes:
- Includes pneumonia (viral or bacterial), aspiration pneumonitis, pulmonary contusion, burn or smoke inhalation, lung transplant failures, and genetic factors.

Clinical Diagnosis Criteria:
- Symptoms of cough, dyspnea, and progressive respiratory failure developing 6-72 hours post-insult.
- Presence of bilateral infiltrates on chest imaging, decreased PaO2/FiO2 ratio of less than 300, and an increased A-a gradient.
- Exclusion of cardiogenic pulmonary edema is essential for ARDS diagnosis.
Global Definition of ARDS Under Berlin Definition:
- Acute onset of respiratory failure, primarily not caused by fluid overload or cardiogenic causes, with bilateral opacities on imaging, presenting criteria differentiated by oxygenation levels.

Mechanical Ventilation:
- Utilizing low tidal volume ventilation ranging from $4-8 ext{ ml/kg PBW}$ , with an increase in PEEP to aid in alveolar recruitment and prevent collapse.
- Avoid alternate ventilator modes; keep plateau pressures below to prevent further damage.
Supportive Management:
- Treat underlying causes of ARDS while maintaining appropriate fluid management (CVP <4).
- Employ conservative fluid management and positions such as proning (16 hours) may yield improved outcomes.
- Employ patients with severe ARDS on paralysis under specific conditions (doesn't always guarantee benefit).

Possible roles of inhaled pulmonary vasodilators (e.g., Nitric Oxide) have been demonstrated to improve oxygenation without mortality benefit, and ECMO may be beneficial within a week of onset.
Glucocorticoids (e.g., dexamethasone for COVID-related ARDS) are responsive, with corticosteroids improving outcomes in acute settings.

Oxygenation Goal: Maintain PaO2 levels between $55-80 ext{ mmHg}$
Criteria for Inclusion: The presence of bilateral infiltrates and the absence of left atrial hypertension are necessary to accept the diagnosis of ARDS.
The importance of calculating predicted body weight for ventilator settings and adjusting tidal volume based on plateau pressure goals can improve therapeutic outcomes in ARDS management.

Case Scenario: A 53-year-old male with acute alcoholic pancreatitis develops hypoxia. His condition aligns with ARDS, characterized by a P/F ratio of 72.5, and requires intubation and mechanical ventilation.
- Diagnostic Considerations: Strategies include maintaining adequate oxygen saturation and minimizing both volutrauma and barotrauma through ventilator management.

Understanding the multifaceted aspects of ARDS, including its pathophysiology, clinical manifestations, imaging, and management protocols, is vital.
A thorough grasp of ventilator strategies, supportive therapies, and innovative interventions can drastically influence the mortality and morbidity associated with ARDS.