Overview Respiratory distress syndrome (RDS) also known as hyaline membrane disease is a pulmonary disease resulting from immature lung development and surfactant deficiency. Most frequently seen in preterm infants, the clinical presentation is characterized by signs of respiratory distress including nasal flaring, tachypnea, and grunting (an expiratory sound that is heard as the infant exhales against a partially closed glottis). Symptoms often present shortly after birth and can increase in severity over the first 48 to 72 hours of life. The radiologic findings of RDS consist of a homogenous ground glass appearance of the lung fields with visible air bronchograms.

Transition from foetal life to neonatal life requires:

• Fetal lungs are filled with fluid from pulmonary secretions
• Most fluid moves out into amniotic fluid with breathing movements
• Labour onset causes release of catecholamines, fluid in lungs is largely squeezed out with passage of the chest through the birth canal
• With normal chest recoil, the infant’s lungs fill with air.
• Surfactant is released from type II pneumocytes to lower surface tension and establish residual lung volume
• Onset of regular breathing
• ↑pulmonary blood flow as a result of increased systemic vascular resistance and ↓pulmonary vascular resistance

Pulmonary Surfactant

• Pulmonary surfactant is a complex mixture of lipids and proteins
• The main function of surfactant is to decrease surface tension of the alveoli allowing for maintenance of functional residual capacity preventing atelectasis and lung injury.
• The surfactant proteins also contribute to natural immunologic defences and assist with the spreading of surfactant throughout the alveoli and the recycling of surfactant between cells and the airspaces.

Usually occur at or soon after birth (first 4-6 hours)

• Tachypnoea
• Laboured breathing
• Cyanosis
• Grunting
• Nasal flaring
• Intercostal and subcostal retraction

A chest x-ray is the investigation to perform in infants with respiratory distress. Findings for RDS are white lungs with the presence of air bronchograms and a ground glass appearence. Air bronchograms are so called because the small air-filled bronchi are seen outlined against the opacified fluid-filled alveoli. Ground glass is because of alveoli collapse.

Diagnosis is made on the basis of the combination of clinical and radiological features

• The primary cause of RDS is deficiency of pulmonary surfactant, which is developmentally regulated.
• The fetal lung is filled with fluid and provides no respiratory function until birth
• Fetal alveolar development is occurring in utero. Week 20 of gestation, surfactant is slowly expressed.
  • Pre-term delivery → immature lungs/surfactant → RDS
  • Gene mutations for proteins in surfactant  → surfactant deficiency and/or dysfunction → RDS
• Insufficient surfactant stores leads to:
  • Decreased compliance of the lungs
  • Inability of the infant to maintain air in the lung at the end of expiration resulting in a tiny area of collapsed lung (microatelectasis).
    • ↑ effort on inhalation with microatelectasis → Ventilation and perfusion mismatch → Hypoxaemia, Tachypnoea, laboured breathing
  • Lung inflammation and respiratory epithelial injury, which may result in pulmonary oedema and increased airway resistance

Management

Prognosis 

• Dependent on general appearance at birth and severity of underlying lung disease. Long-term risks of chronic lung disease.
• Classically worsens for 2-3days, then diuresis occurs recovery over 3-4days. Subsequent improvement is coincident with increased production of endogenous surfactant with resolution of symptoms by one week of age.

Complications 

• Prolonged ventilation
• Increased risk of bronchopulmonary dysplasia
• Sudden deterioration
  • Pneumothorax
  • Endotacheal tube blockage or displacement
  • Mechanical failure with the ventilator
  • Increase in the severity of the underlying lung disease
  • Massive intraventricular haemorrhage
  • Necrotizing enterocolitis
  • Patent ductus arteriosus

Overview ARDS is an acute, diffuse, inflammatory lung injury that leads to increased pulmonary vascular permeability, increased lung weight, and a loss of aerated tissue. Clinical hallmarks of ARDS are hypoxemia and bilateral radiographic opacities, while the pathological hallmark is diffuse alveolar damage.

Clinical Presentation

• 6 to 72 hours of an inciting event and worsen rapidly
• Tachypnoea
• Tachycardia
• Diaphoresis
• Accessory muscle use
• Dyspnoea
• Cyanosis (hypoxemia)
• Diffuse crackles
• +/- Cough and chest pain

Clinical Course

• The first several days of ARDS are characterized by hypoxemia requiring a moderate to high concentration of inspired oxygen
• Most patients who survive this initial course begin to exhibit better oxygenation and decreasing alveolar infiltrates over the next several days.
• Some patients, however, have persistent, severe hypoxemia and remain ventilator-dependent. Pulmonary proliferative changes and fibrosis may progressively replace the pathological findings of diffuse alveolar damage as early as ten days after the onset of the respiratory failure.

Diagnostic Criteria - Berlin Definition

• Acute onset of symptoms of a known clinical insult
• Bilateral opacities consistent with pulmonary edema on Chest X-ray or CT. Other causes ruled out:
  • Pulmonary nodules
  • Lung collapse
  • Pleural effusion
• Cardiac failure and fluid overload ruled out
• Decreased/impaired oxygenation must be present
  • Mild
  • Moderate
  • Severe

Aetiology

• Indirect lung injury: sepsis, shock, trauma, pancreatitis, drug OD, intracranial haemorrhage, blood-transfusion related lung injury
• Direct lung injury: aspiration, inhalational injury, embolism

Pathophysiology

• Insult → release of inflammatory mediators promoting neutrophil accumulation in microcirculation of lung
• Neutrophils damage vascular endothelium + alveolar epithelium
• Leads to pulmonary oedema, hyaline membrane formation & difficult gas exchange

Complications of ARDS

• Barotrauma
• Nosocomial infection
• Deep Vein Thrombosis
• Delirium
• Gastrointestinal ulcer
• Poor nutrition

UpToDate
Best Practice
Victoria Health - Australia