Acute Respiratory Distress Syndrome (ARDS) 

1. Definition

ARDS (Acute Respiratory Distress Syndrome) is a diffuse inflammatory lung injury characterized by:

• Increased alveolar-capillary permeability
• Non-cardiogenic pulmonary edema
• Severe hypoxemia
• Reduced lung compliance

Leading to acute respiratory failure requiring oxygen or ventilatory support.

2. Berlin Definition (2012) — Diagnostic Criteria

ARDS is defined by four criteria.

Severity Classification

(PaO₂ measured with PEEP ≥5 cmH₂O)

2. New Global Definition of ARDS (2023)

Developed by international critical care experts led by Luciano Gattinoni and the European Society of Intensive Care Medicine task force.

Why Was a New Definition Needed?

Limitations of the Berlin Definition:

1. Excluded patients on HFNC/NIV
2. Required PEEP ≥5 cmH₂O
3. Required arterial blood gas (PaO₂)
4. Required bilateral infiltrates on chest imaging
5. Poor applicability in low-resource settings
6. Did not recognize “early ARDS” on noninvasive support

If arterial blood gas unavailable:

SpO₂/FiO₂ (S/F ratio) may be used

3. Epidemiology

• Incidence: ~10% of ICU admissions
• ~23% of mechanically ventilated patients
• Mortality:
• Mild: ~27%
• Moderate: ~32%
• Severe: ~45–50%

Major causes:

4. Etiology

Direct Lung Injury (Pulmonary ARDS)

Indirect Lung Injury (Extrapulmonary ARDS)

5. Pathophysiology

ARDS progresses through three overlapping phases.

Exudative Phase (Day 1–7)

Histological Hallmark

Diffuse Alveolar Damage (DAD)

Features:

• Hyaline membranes
• Alveolar edema
• Neutrophilic infiltration

Proliferative Phase (Day 7–21)

Repair phase involving:

• Type II pneumocyte proliferation
• Fibroblast activation
• Partial resolution of edema

Fibrotic Phase (Late ARDS)

Occurs in ~30–40% of patients.

Features:

• Interstitial fibrosis
• Pulmonary hypertension
• Severe reduction in lung compliance

6. Pathophysiologic Abnormalities

Shunt Physiology

The dominant mechanism of hypoxemia.

Mechanism:

• Alveoli filled with fluid but Perfusion continues—-No ventilation

→ True intrapulmonary shunt

V/Q Mismatch

Some regions:

• Poor ventilation
• Preserved perfusion

Reduced Compliance

ARDS lung becomes stiff.

Reasons:—-Edema—Atelectasis—Fibrosis

Pulmonary Hypertension

Mechanisms:—Hypoxic vasoconstriction—Microthrombi—Vascular remodeling

7. ARDS Lung Mechanics — “Baby Lung Concept”

Introduced by Luciano Gattinoni

Key idea:Only a small portion of lung remains aerated.

Thus:Normal tidal volume overdistends remaining lung—Causes ventilator-induced lung injury

8. Clinical Features

Symptoms

• Dyspnea
• Tachypnea
• Hypoxemia

Signs

9. Investigations

 Arterial Blood Gas

Early:Respiratory alkalosis

Late:Severe hypoxemia—Possible respiratory acidosis

 Chest X-ray

• Bilateral diffuse infiltrates
• “White lung”

 CT Scan

Gold standard imaging. Findings:

Lung Ultrasound

 Hemodynamic Assessment

To exclude cardiogenic edema.Methods:

• Echocardiography
• Pulmonary artery catheter (rarely used)

10. Differential Diagnosis

11. ARDS Management

 Lung Protective Ventilation 

Established by the ARDSNet ARMA Trial.

Predicted Body Weight Formula

Male:

PBW = 50 + 0.91(height cm − 152.4)

Female:

PBW = 45.5 + 0.91(height − 152.4)

 PEEP Strategy

Purpose:Prevent alveolar collapse —Reduce atelectrauma—Reduce FiO₂ requirement—Achieve lung recruitment safely—Improve oxygenation—Reduce shunt fraction

Two approaches:

ARDSNet Low PEEP-FiO₂ Table

High PEEP Strategy

Higher PEEP tables use:

• More aggressive recruitment
• Higher mean airway pressure

Potential benefits:

• Better oxygenation
• Reduced collapse

Potential harms:

• Overdistension
• Hemodynamic instability

Evidence for High vs Low PEEP

Major trials:

• ALVEOLI
• LOVS
• EXPRESS

Findings:

• No major mortality benefit overall
• Moderate-severe ARDS may benefit from higher PEEP
• Better oxygenation and fewer rescue therapies

Meta-analysis:

• Severe ARDS likely benefits more from higher PEEP

Methods to Set Optimal PEEP

1. ARDSNet PEEP-FiO₂ Table

Most common bedside method.

Advantages:

• Simple—Evidence-based—Safe

Disadvantages:

• Not individualized

2. Best Compliance Method

Increase PEEP gradually and identify best compliance.

Compliance improves with recruitment but worsens with overdistension.

Optimal PEEP:Highest static compliance

Static compliance formula: Cstat =Pplat −PEEPVT

Higher compliance suggests better recruitment without excessive overdistension.

3. Driving Pressure Guided PEEP

Important modern strategy.

Driving pressure:ΔP=Pplat −PEEP

Goal:Keep driving pressure < 15 cm H₂O

Lower driving pressure associated with improved survival.

If increasing PEEP:

• Decreases driving pressure → beneficial recruitment
• Increases driving pressure → overdistension likely

4. Pressure-Volume Curve Method

Identify:Lower inflection point (LIP) and Upper inflection point

Set PEEP: Slightly above LIP

Limitations: Complex,Rarely used clinically now

5. Esophageal Pressure Guided PEEP

Uses esophageal balloon to estimate pleural pressure.

Transpulmonary pressure:PL =Paw −Ppl

Useful in:

• Obesity
• Elevated abdominal pressure
• Severe ARDS

Goal:End-expiratory transpulmonary pressure around 0–5 cm H₂O

Permissive Hypercapnia

Allowed in ARDS ventilation.

Reason:Low tidal volume leads to CO₂ retention.

Acceptable:pH ≥ 7.20

Contraindications:

• Raised ICP
• Severe pulmonary hypertension

Prone Positioning

Supported by the PROSEVA Trial

Indication:PaO₂/FiO₂ <150

Protocol:≥16 hours/day

Benefits:

Neuromuscular Blockade

Used early in severe ARDS.during the first 48 hours 

Evidence:ACURASYS Trial

Drug:Cisatracurium infusion (48 hrs)

Benefits:

• Reduced ventilator asynchrony
• Improved oxygenation

Conservative Fluid Strategy

Trial: FACTT Trial

Approach:

• Restrict fluids
• Use diuretics

Target:-CVP <4 mmHg or PAOP <8 mmHg.

Benefits:Shorter ventilation duration

ECMO

Used in refractory hypoxemia.

Evidence: EOLIA Trial

Indications:

Type:VV ECMO

Recruitment Maneuvers

Transient increase in airway pressure.

Examples:

• Sustained inflation
• Staircase recruitment

Evidence: uncertain benefit

Corticosteroids

Evidence evolving.

Recent guidelines suggest:

Dexamethasone or methylprednisolone

Benefits:

• Reduced ventilation duration
• Possible mortality reduction

12. Adjunctive Therapies

13. Complications

• Secondary infections
• ICU myopathy
• Delirium
• Fibrosis

 Prognostic Factors

Poor prognosis:

• Age >65
• Severe ARDS
• Sepsis
• Multi-organ failure

14. ARDS Mortality Causes

Most deaths due to:

• Sepsis
• Multi-organ failure

NOT hypoxemia alone.

15. Emerging Concepts

ARDS Phenotypes

Two biological phenotypes identified:

Precision Medicine

Future therapy may involve:

• Biomarker-guided treatment
• Personalized ventilation

16. Key Trials in ARDS 

Phenotype-Directed ARDS Therapy

1. Concept of ARDS Phenotypes

ARDS is not a single disease but a heterogeneous syndrome.

Patients with ARDS differ in:

• Inflammatory response
• Lung recruitability
• Response to ventilatory strategies
• Response to drugs

Because of this heterogeneity, uniform treatment may not work for all patients.

This led to the concept of phenotype-directed therapy, where treatment is tailored according to biological or physiological phenotype.

Major work on ARDS phenotypes was performed by Carolyn S. Calfee and Luciano Gattinoni.

2. Major ARDS Phenotypes

Two biological phenotypes have been identified using biomarker analysis.

3. Hyperinflammatory ARDS

Clinical Features

• Severe sepsis
• Vasopressor requirement
• Higher lactate
• Metabolic acidosis
• Severe hypoxemia

Laboratory Features

Elevated inflammatory biomarkers:

Pathophysiology

• Severe endothelial injury
• Cytokine storm
• Capillary leak
• Multi-organ failure

Clinical Characteristics

4. Hypoinflammatory ARDS

Clinical Features

• Less systemic inflammation
• Lower vasopressor requirement
• Better lung compliance

Biomarker Profile

Lower levels of:

• IL-6
• IL-8
• TNF-α

Clinical Characteristics

5. Evidence for Phenotype-Specific Treatment

Reanalysis of several ARDS trials showed different responses to therapies depending on phenotype.

Important trials analyzed include:

• ARDSNet ARMA Trial
• ALVEOLI Trial
• FACTT Trial
• HARP‑2 Trial

6. Treatment Differences Between Phenotypes

7. Ventilation Phenotypes

ARDS can also be classified based on lung mechanics and recruitability.

7.1 Recruitable vs Non-Recruitable ARDS

Recruitable ARDS

Characteristics:

• Extensive atelectasis
• Dependent lung collapse
• Good response to high PEEP

Common causes:

• Extrapulmonary ARDS
• Abdominal sepsis
• Trauma

Management:

• Higher PEEP
• Recruitment maneuvers
• Prone positioning

Non-Recruitable ARDS

Characteristics:

• Consolidated lung
• Minimal recruitable tissue

Common causes:

• Pneumonia
• Pulmonary ARDS

Management:

• Moderate PEEP
• Avoid aggressive recruitment

8. Morphological Phenotypes (CT-Based)

Based on CT imaging.

Treatment Implications

9. COVID-19 ARDS Phenotypes

During the pandemic, Luciano Gattinoni proposed two phenotypes.

Management

10. Precision Medicine in ARDS

Future ARDS therapy aims to personalize treatment based on:

• Biomarkers
• Imaging
• Lung mechanics
• Genetics

Possible targeted therapies:

11. Clinical Application in ICU

Currently phenotype-directed therapy is not yet routine clinical practice, but certain principles are used.

Practical ICU Approach

12. Limitations

Challenges include:

• Biomarkers not routinely available
• Complex classification systems
• Overlapping phenotypes

Therefore standard lung-protective ventilation remains cornerstone therapy.