ARDS
This summary covers the first half of the lecture on Acute Respiratory Distress Syndrome (ARDS), a clinical syndrome marked by acute hypoxemic respiratory failure from widespread lung inflammation and edema.
Definition and Key Features
ARDS is a severe, rapid-onset inflammatory reaction in the lungs triggered by an insult. It is characterized by :[1]
- Widespread Inflammation: Involves both lungs.
- Increased Permeability: Pulmonary capillary permeability is increased, leading to non-cardiogenic pulmonary edema.
- Hypoxemia: Life-threateningly low blood oxygen levels that do not respond well to oxygen therapy.
- Diffuse Infiltration: Widespread opacities are visible on chest imaging.
- Pathological Changes: Key features include capillary endothelial injury, diffuse alveolar damage, the formation of hyaline membranes in the alveoli, and arterial vasoconstriction leading to pulmonary hypertension.
Diagnostic Criteria
Berlin Definition (2012)
The 2012 Berlin Definition is the standard for diagnosing ARDS and requires all four of the following criteria to be met :[1]
| Criteria | Description |
|---|---|
| Timing | Respiratory symptoms must have started within one week of a known clinical insult, or new/worsening symptoms appeared within the past week. |
| Chest Imaging | Bilateral opacities (infiltrates) are visible on a chest X-ray or CT scan. These opacities must be consistent with pulmonary edema and not be fully explained by other causes like pleural effusions or lung collapse. |
| Origin of Edema | The respiratory failure cannot be fully explained by cardiac failure or fluid overload. If no risk factor for ARDS is present, an objective evaluation (like an echocardiography) is needed to rule out hydrostatic pulmonary edema. |
| Oxygenation | Oxygenation is impaired, as measured by the PaO₂/FiO₂ ratio. The severity is graded as:
|
2023 Update and Kigali Modification
A 2023 conference proposed an updated, broader definition of ARDS. This new definition :[1]
- Includes patients receiving High-Flow Nasal Oxygen (HFNO).
- Allows diagnosis using thoracic ultrasonography.
- Eliminates the need for arterial blood gas analysis.
- Incorporates the Kigali modification for resource-limited settings, which does not require PEEP and can use CPAP, absence of cardiac failure, bilateral opacities, and hypoxemic respiratory failure for diagnosis.
Etiology and Pathophysiology
The causes of ARDS can be divided into two main categories: pulmonary (direct lung injury) and extrapulmonary (indirect lung injury).[1]
| Cause Type | Examples & Pathophysiology |
|---|---|
| Pulmonary (Direct) | Examples: Pulmonary infection, aspiration. Pathophysiology: Injury to the alveolar epithelium triggers an inflammatory and pro-coagulant cascade. This leads to the loss of type II alveolar cells, causing surfactant dysfunction and increasing the risk of atelectasis (lung collapse) and superinfection. Impaired alveolar fluid clearance is linked to the severity of ARDS. |
| Extrapulmonary (Indirect) | Examples: Sepsis, trauma, massive transfusion, pancreatitis, drug overdose, drowning, fat embolism, inhalation of toxic fumes. Pathophysiology: Inflammatory mediators damage the pulmonary vascular endothelium. This disrupts cell junctions, leading to the leakage of protein-rich fluid into the lung tissue. Endothelial injury also promotes the adhesion of neutrophils and platelets, resulting in microthrombi formation and increased dead space ventilation. |
Biological Phenotypes
Two distinct biological phenotypes of ARDS have been identified :[1]
- Hyper-inflammatory: Associated with higher levels of cytokines and higher mortality.
- Hypo-inflammatory: A less severe inflammatory response.
These phenotypes respond differently to treatments such as fluid management, PEEP, simvastatin, and corticosteroids.[1]
Risk Factors
Several factors increase the risk of developing ARDS :[1]
- Advanced age
- Female sex
- Smoking
- Alcohol use
- Traumatic brain injury
- Aortic or cardiovascular surgery
Epidemiology
ARDS has a significant impact on public health :[1]
- Incidence: Approximately 86 cases per 100,000 person-years in the U.S., which translates to about 190,000 cases annually.
- Mortality: The 28-day mortality rate is approximately 34.8%, with an overall in-hospital mortality rate of 40.0%. Mortality rates vary by severity: 27% for mild, 32% for moderate, and 45% for severe ARDS.
Histopathology
ARDS progresses through three distinct histological phases :[1]
| Phase | Timeframe | Key Histological Changes |
|---|---|---|
| Exudative Phase | 0–3 days | The initial phase is characterized by alveolocapillary injury. This includes damage to type I pneumocytes and the vascular endothelium, leading to alveolar edema, hemorrhage, and congestion. Proteinaceous fluid leaks into the alveolar space, forming classic hyaline membranes. |
| Fibroproliferative Phase | 4–10 days | This proliferative phase involves initial lung repair efforts. |
| Fibrosis Phase | >10 days | The final stage, where fibrotic tissue begins to form in the lungs. |
Clinical Presentation and Phases
The clinical course of ARDS typically begins with an acute onset over hours, with symptoms progressively worsening over days.[1]
Initial Findings:
- Severe dyspnea (shortness of breath) developing within 12–24 hours.
- Tachypnea (rapid breathing) and increased work of breathing.
- Central or peripheral cyanosis (bluish discoloration of the skin).
- Bilateral widespread crackles heard on chest auscultation.
The clinical course can be broken down into three phases :[1]
| Phase | Timeframe | Clinical Findings |
|---|---|---|
| 1. Acute Phase | First 3–5 days | Within 12–24 hours, patients develop tachycardia, tachypnea, and respiratory alkalosis. Breathing is rapid and shallow. Respiratory failure increases, with hypoxemia that persists despite oxygen therapy. |
| 2. Subacute Phase | 5–7 days | Respiratory insufficiency and hypoxemia become severe. Hypercapnia (elevated CO₂ levels) develops. Chest imaging shows bilateral infiltrates, and lung compliance decreases, requiring high FiO₂ and PEEP for oxygenation. |
| 3. Chronic Phase | After 2 weeks | Hypoxemia may begin to improve, but lung compliance remains low, and the dead space ratio is high. Fibrosis starts to develop in the lungs. |
Evaluation
Diagnosis relies on the criteria outlined in the Berlin definition. Imaging and other tests are crucial for evaluation.[1]
CT Scan Findings:
- Exudative & Early Proliferative Phase: Irregular, patchy ground-glass opacities, interlobular septal thickening, and reticular patterns in both lungs. Mild dilatation of peripheral bronchioles may be an early sign.
- Late Proliferative Phase: As the disease progresses, signs include bronchial wall thickening, luminal dilatation, and reduced lung volume.
- Fibrosis: When fibrosis develops, large net-like and small cystic structures may appear, along with traction bronchiectasis.
Other Evaluations:
- Left ventricular function is assessed to rule out cardiogenic causes.
- Bronchoscopy with culture can identify underlying pneumonia.
- Laboratory and radiographic tests are used to identify the root cause of ARDS.
Do the 2nd half now
This summary covers the second half of the lecture on Acute Respiratory Distress Syndrome (ARDS), focusing on treatment, prognosis, complications, and future classifications.[1]
Treatment and Management
The cornerstone of ARDS management is supportive care aimed at maintaining oxygenation while preventing further lung injury. There is no specific curative therapy for ARDS.[1]
Primary Goals of Management:
- Reduction of intrapulmonary shunting
- Increased oxygen delivery to tissues
- Reduced overall oxygen consumption
- Prevention of ventilator-induced lung injury
Lung-Protective Mechanical Ventilation
This is a critical component of ARDS care. The strategy involves specific ventilator settings to minimize lung damage.[1]
| Parameter | Target Value |
|---|---|
| Tidal Volume (VT) | 4–8 mL/kg (based on predicted body weight) |
| Respiratory Rate | Up to 35 breaths/minute |
| Oxygen Saturation (SpO₂) | 88–95% |
| Plateau Pressure (Pplat) | Less than 30 cm H₂O |
| Arterial pH | 7.30–7.45 (allowing for permissive hypercapnia) |
| Inspiratory/Expiratory (I:E) Ratio | Less than 1 |
Additional Therapeutic Strategies
Other interventions are employed to manage ARDS, particularly in severe cases :[1]
- Prone Positioning: Placing the patient face-down improves oxygenation by recruiting collapsed lung areas.
- Refractory Hypoxemia Management: For patients who do not respond to conventional therapy, Extracorporeal Membrane Oxygenation (ECMO) may be used to oxygenate the blood outside the body.
- Fluid Management: Diuretics are used to prevent fluid overload, which can worsen pulmonary edema.
- Nutritional Support: Enteral nutrition is recommended. A diet high in fat and low in carbohydrates, containing gamma-linolenic acid (GLA) and eicosapentaenoic acid (EPA), may be beneficial.
Differential Diagnosis
It is crucial to distinguish ARDS from other conditions that present with similar clinical features. The main differential diagnoses include :[1]
- Cardiogenic edema
- Exacerbation of interstitial lung disease
- Acute interstitial pneumonia
- Alveolar hemorrhage
- Acute eosinophilic lung disease
- Pneumonia
Prognosis and Complications
While ARDS remains a serious condition, mortality rates have declined due to improved care practices.[1]
- Prognosis: Mortality was around 30–40% until the 1990s but has since decreased to approximately 20%.
- Complications: Patients are at risk for numerous complications, including:
- Ventilator-Related: Barotrauma from high PEEP, need for prolonged mechanical ventilation (tracheostomy), laryngeal edema, and subglottic stenosis after extubation.
- Infections: Hospital-acquired infections like pneumonia and sepsis are common.
- Systemic Issues: Deep vein thrombosis, antibiotic resistance, renal failure, and severe muscle weakness.
- Long-Term: Survivors may experience Post-Traumatic Stress Disorder (PTSD).
Conclusion
ARDS is a syndrome with high morbidity and mortality, and it is difficult to prevent despite known risk factors. Early diagnosis of hypoxemia is critical for improving outcomes. Survivors often face a reduced quality of life, a prolonged rehabilitation period, and persistent dyspnea even with mild exertion.[1]
Proposed New Classification of ARDS
A new classification has been proposed to provide a more detailed assessment of ARDS severity.[1]
| Criteria | Mild | Moderate | Severe |
|---|---|---|---|
| Timing | Acute onset (symptoms within 1 week of a clinical insult or new/worsening symptoms) | Same as mild | Same as mild |
| Hypoxemia | PaO₂/FiO₂ = 201–300 with PEEP/CPAP ≥ 5 | PaO₂/FiO₂ ≤ 200 with PEEP ≥ 5 | PaO₂/FiO₂ ≤ 100 with PEEP ≥ 10 |
| Cause of Edema | Respiratory failure not fully explained by cardiac failure or fluid overload | Same as mild | Same as mild |
| Radiologic Abnormalities | Bilateral opacities | Bilateral opacities | Opacities involving at least three quadrants |
| Additional Physiological Disorders | N/A | N/A | Static Lung Compliance (CRS) < 40 mL/cmH₂O or Corrected Minute Ventilation (VEcorr) > 10 L/min |