Acute Respiratory Distress Syndrome (ARDS) Overview

ARDS (Acute Respiratory Distress Syndrome) Study Notes

Definition and Overview

  • ARDS is a life-threatening inflammatory lung injury.
  • It is neither a primary disease nor a single entity.
  • ARDS is part of a multi-organ illness.
  • Clinical manifestations typically occur within 6 to 72 hours following an inciting event and worsen rapidly.

Etiology of ARDS

Common Causes
  • Severe pneumonia
  • Aspiration (inhalation of food or liquid into the lungs)
  • Chest or abdominal trauma
  • Sepsis: This is the most common cause.
  • Massive blood transfusions
  • Oxygen toxicity
  • Congestive heart failure (CHF)
  • Near drowning
  • Radiation-induced lung injury
  • Inhalation of toxins and irritants
  • Drug overdose
  • Infections
  • Burns

Historical Terms for ARDS

Other Names Used in the Past
  • Adult hyaline membrane disease
  • Adult respiratory distress syndrome
  • Capillary leak syndrome
  • Congestion atelectasis
  • Da Nang lung (noted for high instances in the Vietnam War)
  • Hemorrhagic pulmonary edema
  • Noncardiac pulmonary edema
  • Oxygen pneumonitis
  • Oxygen toxicity
  • Postnontraumatic pulmonary insufficiency
  • Postperfusion lung
  • Postpump lung
  • Posttraumatic pulmonary insufficiency
  • Shock lung syndrome
  • Stiff lung syndrome
  • Wet lung
  • White lung syndrome

Anatomic Alterations of the Lungs

  • Similar alterations occur regardless of the cause of ARDS.
  • Pulmonary capillaries become engorged in response to injury.
  • Permeability of the alveolar-capillary (A/C) membrane increases, leading to edema and hemorrhage.
  • Reduced pulmonary surfactant results in atelectasis (lung collapse).
  • As the condition progresses, a hyaline membrane develops, consisting of fibrin and cellular debris.
  • The lungs may appear red, resembling beef or liver-like.

Clinical Manifestations

  • Increased respiratory rate (RR)
  • Increased heart rate (HR) and blood pressure (BP)
  • Substernal and intercostal retractions
  • Use of accessory muscles for breathing
  • Chest pain
  • Cyanosis (bluish discoloration of skin due to lack of oxygen)
  • Dyspnea (difficulty breathing)
  • Diaphoresis (excessive sweating)
  • Refractory hypoxemia (persistent low oxygen levels)
  • Cough with blood-tinged sputum
  • Chest assessment findings:
    • Dull percussion note
    • Bronchial breath sounds
    • Bilateral crackles

Refractory Hypoxemia

  • Definition: Occurs when PaO2 cannot be maintained above 50-60 mmHg with FiO2 of 0.50 or greater.
  • Indication for PEEP (Positive End-Expiratory Pressure) or CPAP (Continuous Positive Airway Pressure) as increased end-expiratory pressure improves oxygenation by stabilizing the lung and decreasing physiological shunting.

Pulmonary Function Test Findings

Forced Expiratory Volume and Flow Rate:
  • FVC: Normal or decreased
  • FEV1: Normal or decreased
  • FEV1/FVC ratio: Normal or increased
  • FEF25%-75%: Normal or decreased
  • FEF50%: Normal or decreased
  • FEF200-1200: Normal or decreased
  • PEFR: Normal or decreased
  • MVV: Normal or decreased
Lung Volume and Capacity:
  • VT (Tidal Volume): Normal or decreased
  • IRV (Inspiratory Reserve Volume): Decreased
  • ERV (Expiratory Reserve Volume): Decreased
  • RV (Residual Volume): Decreased
  • VC (Vital Capacity): Decreased
  • IC (Inspiratory Capacity): Decreased
  • FRC (Functional Residual Capacity): Decreased
  • TLC (Total Lung Capacity): Normal
  • RV/TLC ratio: Decreased
Diffusion Capacity (DLco)
  • Generally decreased.

Arterial Blood Gases (ABG) Findings

Mild to Moderate ARDS:
  • Acute Alveolar Hyperventilation with Hypoxemia (Acute Respiratory Alkalosis)
  • pH: Increased
  • PaCO2: Decreased
  • HCO3-: Normal or decreased
  • PaO2: Decreased
  • SaO2/SpO2: Decreased but normally mild.
Severe ARDS:
  • Acute Ventilatory Failure with Hypoxemia (Acute Respiratory Acidosis)
  • pH: Decreased
  • PaCO2: Increased
  • HCO3-: Decreased
  • PaO2: Decreased
  • SaO2/SpO2: Decreased but normally mild.

Chest X-ray Findings

  • Characterized by increased opacity: more severe cases appear as whiter areas on the X-ray, often referred to as ground glass appearance.

Causes of Hypoxemia in ARDS Patients

  • Causes include:
    • Widespread alveolar consolidation
    • Atelectasis
    • Increased A/C membrane thickening
    • Refractory hypoxemia due to pulmonary capillary shunting

Berlin Definition of ARDS

  • A set of criteria for diagnosing ARDS that includes:
    • Respiratory symptoms manifesting within 1 week of a known clinical event or new/worsening symptoms over the past 7 days
    • Bilateral opacities observed on chest X-ray or CT resembling pulmonary edema
    • Respiratory failure not explained by fluid overload or heart failure
    • Moderate to severe impairment of oxygenation must be present, assessed through the PaO2/FiO2 ratio.

PaO2/FiO2 Ratio

  • Used to estimate oxygenation impairment:
    • Normal: 500-600

Treatment and Respiratory Care Protocol

  • Oxygen Therapy
  • Lung Expansion Therapy
  • PEEP/CPAP: To offset alveolar consolidation and atelectasis.
  • Mechanical Ventilation: Immediate intervention; do not trial Noninvasive Positive Pressure Ventilation (NPPV).
  • Medications:
    • Antibiotics
    • Corticosteroids
    • Diuretics: Often have poor effects; do not reduce inflammation.

Ventilation Strategy for ARDS

  • Low tidal volumes and high respiratory rates to decrease barotrauma.
  • Initial Tidal Volume (VtV_t): 4-6 mL/kg (compared with 6-8 mL/kg for patients without ARDS).
  • Ventilatory rates of up to 35 bpm.
  • PEEP/CPAP: Set plateau pressure between 25-30 cmH2O.
  • Permissive hypercapnia: This is a tradeoff to protect lungs from high airway pressures, ensuring that pH does not fall below 7.2.

References

  • Des Jardin, Terry R. (2016) Clinical Manifestations and Assessment of Respiratory Disease. Maryland Heights, MO: Mosby Elsevier.