Acute Respiratory Distress Syndrome

Acute Respiratory Distress Syndrome (ARDS) – High‑Yield Study Guide

Definition

Acute Respiratory Distress Syndrome (ARDS) is a form of diffuse, non-cardiogenic acute hypoxemic respiratory failure caused by inflammatory injury to the alveolar–capillary membrane, leading to increased pulmonary capillary permeability, pulmonary edema, reduced lung compliance, and severe V/Q mismatch.

The current standard definition is the Berlin Definition, which requires:

• Timing: Respiratory symptoms beginning within 1 week of a known clinical insult or new/worsening symptoms during that time.
• Chest imaging: Bilateral opacities on CXR or CT not fully explained by effusions, lobar/lung collapse, or nodules.
• Origin of edema: Respiratory failure not fully explained by cardiac failure or fluid overload; objective assessment (e.g., echocardiography) is recommended when no risk factor is present.
• Oxygenation: Hypoxemia quantified by PaO₂/FiO₂ ratio with PEEP ≥ 5 cm H₂O:

• Mild ARDS: PaO₂/FiO₂ 200–300 mmHg
• Moderate ARDS: PaO₂/FiO₂ 100–200 mmHg
• Severe ARDS: PaO₂/FiO₂ < 100 mmHg

Epidemiology

ARDS is a common cause of respiratory failure in the ICU and is associated with high morbidity and mortality.

• Incidence in developed ICUs is estimated at approximately 10% of all ICU admissions and around 23% of mechanically ventilated patients.
• Mortality varies by severity: roughly 27–35% in mild, 32–40% in moderate, and up to 45–50% or higher in severe ARDS, depending on comorbidities and resources.
• Most cases occur in the setting of an identifiable risk factor, such as sepsis, pneumonia, aspiration, or trauma.
• Older age, multiple organ failure, and higher severity scores (e.g., SOFA, APACHE II) are associated with worse outcomes.

Etiology and Risk Factors

ARDS is usually triggered by a direct pulmonary insult or an indirect systemic insult.

• Direct lung injury:
  • Severe pneumonia (bacterial, viral such as influenza or COVID‑19, fungal)
  • Aspiration of gastric contents
  • Pulmonary contusion (blunt or penetrating chest trauma)
  • Near‑drowning
  • Inhalational injury (toxic gases, smoke)
• Indirect (systemic) insults:
  • Sepsis and septic shock (most common overall cause)
  • Major trauma and multiple transfusions (TRALI, massive transfusion)
  • Acute pancreatitis
  • Severe burns
  • Drug overdose and some toxins
  • Cardiopulmonary bypass

Pathophysiology

ARDS is fundamentally an acute inflammatory injury of the alveolar–capillary barrier leading to increased permeability, edema, and loss of aerated lung units.

Phases of ARDS

• Exudative phase (first 7 days):
  • Injury to type I and type II pneumocytes and pulmonary endothelium.
  • Release of inflammatory mediators (TNF‑α, IL‑1, IL‑6, IL‑8) and neutrophil activation.
  • Increased capillary permeability → protein‑rich fluid leaks into interstitium and alveoli → non‑cardiogenic pulmonary edema.
  • Formation of hyaline membranes (fibrin, necrotic epithelial cells, proteins) lining alveoli.
  • Surfactant depletion and dysfunction → alveolar collapse (atelectasis) and decreased compliance.
  • Result: severe V/Q mismatch, shunt physiology, refractory hypoxemia.
• Proliferative phase (days 7–21):
  • Some resolution of edema and beginning of repair.
  • Proliferation of type II pneumocytes to re‑epithelialize the alveolar surface.
  • Organization of intra‑alveolar exudate; early fibroblast proliferation in interstitium.
  • Improvement in oxygenation and lung compliance in many patients.
• Fibrotic phase (after ~3 weeks, in a subset of patients):
  • Excessive collagen deposition and fibrosis of alveolar walls and interstitium.
  • Development of cysts and emphysema‑like changes.
  • Persistent low compliance (stiff lungs), pulmonary hypertension, and prolonged ventilator dependence.

Pathophysiologic Consequences

• Hypoxemia: Due to shunt and severe V/Q mismatch; often refractory to supplemental oxygen alone.
• Decreased lung compliance: "Stiff" lungs require higher pressures for ventilation.
• Increased work of breathing: Leads to respiratory muscle fatigue and need for mechanical ventilation.
• Pulmonary hypertension: Due to hypoxic vasoconstriction, microthrombi, and vascular remodeling.
• Right ventricular strain: Can progress to acute cor pulmonale in severe cases.

Clinical Presentation

ARDS typically presents in the context of a known risk factor (e.g., sepsis, pneumonia) with rapid progression to severe respiratory failure.

Symptoms

• Acute onset of dyspnea and tachypnea.
• Cough, sometimes with frothy sputum.
• Signs related to underlying cause (e.g., fever and productive cough in pneumonia; abdominal pain in pancreatitis; hypotension in sepsis).
• Increasing oxygen requirement over hours to days.

Signs

• Respiratory: Tachypnea, use of accessory muscles, nasal flaring, paradoxical breathing; diffuse crackles on auscultation.
• Hypoxemia: Low SpO₂ despite high FiO₂; may show cyanosis.
• Hemodynamic: Tachycardia, hypotension if associated with sepsis or shock.
• General: Agitation, confusion, or decreased level of consciousness in severe hypoxemia or hypercapnia.

Diagnostic Evaluation

The diagnosis of ARDS is clinical and radiologic, integrating the Berlin criteria and exclusion of cardiogenic pulmonary edema. Workup also aims to identify and manage the underlying cause.

Arterial Blood Gas (ABG)

• Hypoxemia with PaO₂ < 80 mmHg and low PaO₂/FiO₂ ratio.
• Initially respiratory alkalosis from tachypnea, progressing to respiratory acidosis as fatigue occurs and ventilation worsens.

Imaging

• Chest X‑ray:
  • Bilateral, diffuse alveolar infiltrates described as "white‑out" in severe cases.
  • No marked cardiomegaly or prominent Kerley B lines typical of cardiogenic pulmonary edema.
• Chest CT (if obtained):
  • Patchy or dependent consolidations and ground‑glass opacities.
  • Air bronchograms, relative sparing of nondependent lung in early phases.

Laboratory Studies

• Complete blood count, metabolic panel, lactate, coagulation profile.
• Markers of infection and sepsis (e.g., blood cultures, sputum cultures, procalcitonin where used).
• Tests targeting specific causes: lipase for pancreatitis, viral panels, COVID‑19 testing, etc.

Cardiac Evaluation

• Echocardiography: Often used to assess LV function and rule out cardiogenic pulmonary edema; especially important if no clear ARDS risk factor or in older patients with cardiac disease.
• BNP or NT‑proBNP may support but cannot definitively distinguish cardiogenic vs non‑cardiogenic edema.

Berlin Definition – Oxygenation Severity

• Ensure PEEP ≥ 5 cm H₂O for PaO₂/FiO₂ calculation.
• Mild: PaO₂/FiO₂ 200–300 mmHg.
• Moderate: PaO₂/FiO₂ 100–200 mmHg.
• Severe: PaO₂/FiO₂ < 100 mmHg.

Differential Diagnosis

Key differentials focus on causes of acute bilateral pulmonary infiltrates and hypoxemia:

• Cardiogenic pulmonary edema: Due to left ventricular failure or volume overload; usually with cardiomegaly, elevated JVP, S3 gallop, and elevated BNP; echocardiogram reveals impaired LV function.
• Acute interstitial pneumonia / diffuse alveolar hemorrhage: May resemble ARDS; consider hemoptysis, anemia, or autoimmune disease.
• Pulmonary embolism: May cause hypoxemia but typically does not cause diffuse bilateral alveolar infiltrates (more often normal CXR or wedge-shaped infarcts).
• Exacerbation of chronic ILD: May be difficult to distinguish; history of chronic lung disease, characteristic imaging pattern.
• Volume overload from renal failure: Clinical context, response to diuresis, and echocardiography help differentiate.

Management

Management of ARDS focuses on supportive care, lung-protective ventilation, and treatment of the underlying cause. Early recognition and a bundle of evidence-based strategies improve outcomes.

General Supportive Care

• Identify and treat the underlying cause:
  • Early broad-spectrum antibiotics for sepsis or pneumonia, then de-escalate based on culture results.
  • Source control (drain abscesses, remove infected lines, treat pancreatitis, etc.).
• Hemodynamic support:
  • Aim for adequate perfusion using cautious fluid resuscitation and vasopressors when needed.
  • After initial resuscitation, use a conservative fluid strategy (minimize positive fluid balance) to reduce lung edema and ventilator days.
• Nutritional support: Early enteral nutrition is preferred to maintain gut integrity and reduce infections.
• Thromboprophylaxis: Pharmacologic VTE prophylaxis unless contraindicated.
• Stress ulcer prophylaxis in high-risk ICU patients.

Lung-Protective Mechanical Ventilation

Lung-protective ventilation is the cornerstone of ARDS management and significantly reduces mortality.

• Low tidal volume ventilation (LTVV):
  • Tidal volume (V_T) ≈ 6 mL/kg predicted body weight (not actual weight).
  • Allow permissive hypercapnia (PaCO₂ 50–70 mmHg) if needed, provided pH remains ≥ 7.15–7.20.
• Plateau pressure (P_plat) < 30 cm H₂O:
  • Measured during an inspiratory hold; limits barotrauma and volutrauma.
  • Decrease V_T further (4–5 mL/kg) if P_plat exceeds 30 cm H₂O.
• Appropriate PEEP and FiO₂:
  • Use PEEP to prevent alveolar collapse and improve oxygenation.
  • Follow ARDSNet-type tables to titrate PEEP and FiO₂ to maintain SpO₂ 88–95% or PaO₂ 55–80 mmHg, avoiding excessive FiO₂ when possible.
• Respiratory rate: Increase to maintain adequate minute ventilation when using low tidal volumes, avoiding auto‑PEEP.

Prone Positioning

• Indication: Moderate to severe ARDS (typically PaO₂/FiO₂ < 150 mmHg) despite optimal conventional ventilation.
• Mechanism: Improves V/Q matching, reduces dorsal lung compression by the heart and abdominal contents, and improves secretion clearance.
• Implementation: Prolonged sessions (e.g., 12–16 hours/day) under close monitoring; has been shown to reduce mortality when applied early and systematically.

Neuromuscular Blockade and Sedation

• Sedation:
  • Provide adequate analgesia and sedation to improve ventilator synchrony and reduce oxygen consumption.
  • Use the lowest effective doses and consider daily sedation interruptions when feasible.
• Neuromuscular blocking agents (NMBAs):
  • Short course (e.g., 24–48 hours) of continuous infusion (e.g., cisatracurium) may be considered in early severe ARDS to improve oxygenation and synchrony.
  • Use cautiously and weigh risk of ICU-acquired weakness.

Fluid Management

• After initial resuscitation, a conservative fluid strategy (restrictive fluids, diuretics when appropriate) is favored.
• Goal is to avoid fluid overload and pulmonary edema while maintaining adequate organ perfusion.

Adjunctive Therapies

• Corticosteroids:
  • Evidence is evolving; moderate-dose systemic corticosteroids (e.g., IV methylprednisolone) may shorten duration of mechanical ventilation and possibly improve outcomes in some ARDS phenotypes, particularly when given early and in moderate to severe ARDS.
  • They are also indicated when ARDS is associated with specific conditions (e.g., COVID‑19 ARDS, certain vasculitides).
• Inhaled pulmonary vasodilators:
  • Inhaled nitric oxide or prostacyclin can transiently improve oxygenation in refractory hypoxemia by improving V/Q mismatch.
  • No proven mortality benefit; used as rescue therapy or bridge to more definitive interventions.
• Extracorporeal membrane oxygenation (ECMO):
  • Venovenous ECMO is considered in selected cases of severe ARDS with refractory hypoxemia or hypercapnia despite optimal conventional and rescue ventilatory strategies.
  • Requires specialized centers and expertise; used as a bridge to recovery.

Weaning and Long-Term Outcomes

• As lung function improves, gradually reduce FiO₂, PEEP, and ventilator support while monitoring gas exchange and respiratory effort.
• Use spontaneous breathing trials to assess readiness for extubation.
• Post‑ARDS, patients may experience prolonged physical debility, cognitive impairment, and psychological symptoms; early mobilization and multidisciplinary rehabilitation are important.

Complications

• Ventilator-associated lung injury (VALI): Barotrauma (pneumothorax, pneumomediastinum), volutrauma, atelectrauma, and biotrauma; minimized by lung-protective strategies.
• Nosocomial pneumonia / VAP: Due to prolonged mechanical ventilation and impaired host defense.
• ICU-acquired weakness: From critical illness polyneuropathy/myopathy, prolonged immobility, steroids, and neuromuscular blockade.
• Delirium and long-term cognitive dysfunction.
• Psychosocial sequelae: PTSD, anxiety, and depression post-discharge.
• Chronic lung disease: In some survivors, persistent restriction, reduced DLCO, and exertional dyspnea, though many recover near-normal lung function over months.

Prognosis

• Mortality correlates with ARDS severity, age, number of failing organs, and comorbidities.
• Early application of lung-protective ventilation, prone positioning in severe cases, and meticulous supportive care have improved outcomes.
• Functional recovery may be slow, with many survivors having persistent physical and psychological impairments at 6–12 months.

Key Clinical Pearls for Exams and Practice

• Think ARDS in any ICU patient with acute onset hypoxemia, bilateral infiltrates, and a known risk factor (especially sepsis or pneumonia) within 1 week of symptom onset.
• The Berlin Definition emphasizes timing, bilateral opacities, non-cardiogenic edema, and PaO₂/FiO₂ threshold with PEEP ≥ 5 cm H₂O.
• Non-cardiogenic pulmonary edema is a hallmark; echocardiography is key to exclude LV failure when the cause is unclear.
• Lung-protective ventilation (V_T ≈ 6 mL/kg predicted body weight and P_plat < 30 cm H₂O) is the single most important intervention to reduce mortality.
• Permissive hypercapnia is acceptable if pH is maintained ≥ 7.15–7.20; focus on limiting pressures and volumes.
• Prone positioning for 12–16 hours/day should be considered early in moderate–severe ARDS with PaO₂/FiO₂ < 150 despite optimal ventilation.
• Use a conservative fluid strategy after initial resuscitation to reduce duration of mechanical ventilation and ICU stay.
• Corticosteroids may be beneficial in some ARDS patients, especially when initiated early and in COVID‑19 ARDS, but indications and doses vary.
• ECMO is a rescue therapy for refractory severe ARDS in experienced centers.
• Long-term sequelae are common; think beyond survival to multidisciplinary rehabilitation and follow-up.

High-Yield Exam Tips

• On exams, ARDS is often described as a patient with sepsis or trauma who develops acute dyspnea, bilateral diffuse opacities on CXR, normal heart size, and refractory hypoxemia.
• Distinguish ARDS from cardiogenic pulmonary edema by PCWP (if measured), echocardiography, and clinical context.
• Know the key management triad: low tidal volumes, limit plateau pressure, adequate PEEP.
• Remember that surfactant therapy helps neonatal RDS but is not standard of care in adult ARDS.
• Be able to calculate and interpret the PaO₂/FiO₂ ratio for severity grading.