Respiratory distress syndrome (RDS)/ Hyaline membrane disease (HMD)

Question 1

Risk factors

Answer 1

1. Prematurity = incidence inversely related to gestational age and BW
   
   1. 60-80% infants <28 weeks
   2. 15-30% infants 32-36 weeks
   3. Rarely in infants >37 weeks
2. Maternal diabetes
3. Multiple births
4. Cesarean delivery
5. Precipitous delivery
6. Asphyxia
7. Cold stress
8. Maternal history of previously affected infants
9. Note risk reduced = pregnancy-associated hypertension, maternal heroin, PROM, antenatal corticosteroid prophylaxis

Question 2

Pathogenesis

Answer 2

1. Surfactant deficiency = primary cause of RDS
2. Failure to attain an adequate FRC and the tendency of affected lungs to become atelectatic correlate with high surface tension and the absence of pulmonary surfactant
3. With advancing gestational age – increasing amounts phospholipids are synthesized and stored in type II alveolar cells
4. surfactant deficiency
5. Prematurity
6. Genetic disorders - SPB, SPC deficiency, SPD deficiency

Question 3

Pathophysiology

Answer 3

1. Alveolar atelectasis, hyaline membrane formation and interstitial edema à reduced compliance
   
   1. Greater pressure required to expand the alveoli and small airways
   2. Chest wall is highly compliant offers less resistance than that of mature infants to the tendency of the lungs to collapse à at end-expiration, the volume of the thorax and lungs tends to approach residual volume + atelectasis occurs
2. Deficient synthesis/release + small respiratory units + compliant chest wall à atelectasis à VQ mismatch à hypoxia
3. Decrease lung compliance + small tidal volumes + increased physiological dad space + insufficient alveolar ventilation à hypercapnia
4. Hypercapnia + hypoxia + acidosis à pulmonary artery vasoconstriction + increased R to L shunting through patent foramen ovale and ductus arteriosus + within the lung
5. Progressive injury to epithelial + endothelial cells from atelectasis (atelectrauma), volutrauma, ischaemic injury + oxygen toxicity à effusion of proteinaceous material

Question 4

Clinical manifestations

Answer 4

1. Usually occur within minutes of birth
2. Some patients require initial resuscitation at birth
3. Characteristic – tachypnoea, prominent grunting, intercostal and subcostal retractions, nasal flaring and cyanosis
4. Progressive worsening of cyanosis and dyspnoea if not treated
5. If inadequately treated – BP may fall, cyanosis and pallor increase, grunting decreases or disappears
6. Apnoea and irregular respiration are imminent signs of arrest
7. Untreated patients – mixed respiratory-metabolic acidosis, edema, ileus, oliguria
8. Respiratory failure may occur in infants with rapid progression of the disease
9. Signs peak within 3 days – improvement gradual
10. Improvement often heralded by spontaneous diuresis and improved blood gas values at lower inspiratory oxygen levels and/or low ventilation support

Question 5

Investigations

Answer 5

1. CXR
   
   1. Fine reticular granularity of the parenchyma
   2. Air bronchograms – often more prominent early in the left lower lobe
2. Blood gas

Question 6

Treatment

Answer 6

1. Early supportive care – correction of hypoxia, hypotension and acidosis, hypothermia
2. Ventilation
   
   1. Target oxygen saturations 91-95%
   2. CPAP for stabilisation of at-risk premature infants beginning as early as in the delivery room reduces ventilatory needs
   3. Conventional ventilation
   4. HFOV
      
      1. May improve elimination of carbon dioxide and improve oxygenation in patients who show no response to conventional ventilators and those who have severe RDS, interstitial emphysema, recurrent pneumothoraces or MAS
      2. Reduces BPD but may raise the risk of intracranial haemorrhage
   5. HV jet ventilation – facilitates resolution of air leaks
3. Surfactant
   
   1. Immediate effects = improved Aa gradient, reduced ventilatory support, increased pulmonary compliance and improved CXR appearance
   2. Treatment with surfactant initiated immediately after intubation
   3. Repeated dosing every 6-12 hours for total of 2-4 doses (preparation dependent)
4. Other
   
   1. Routine use of systemic corticosteroids NOT recommended – associated with short term AE (hyperglycaemia, hypertension, GI bleeding, GI perforation, HOCM, poor weight gain, poor head growth, higher incidence of periventricular leukomalacia), long-term increase in neurodevelopmental delay and cerebral palsy
   2. Inhaled nitric oxide – decreases need for ECMO in term and near-term infants

Question 7

Prevention

Answer 7

1. Avoidance of unnecessary or poorly planned early cesarean section or induction of labour
2. Appropriate management of high risk pregnancy + labour – including administration of corticosteroids
3. Antenatal steroids
   
   1. Administration to women <34 weeks gestation significantly reduces incidence and mortality of RDS as well as overall neonatal mortality
   2. Also reduce
      
      1. Overall mortality
      2. Need for and duration of ventilation support
      3. Incidence of severe IVH, NEC, neurodevelopmental impairment
   3. NO effect on postnatal growth
   4. Do NOT increase risk of maternal death, chorioamnionitis or puerperal sepsis
   5. Recommended for all women in preterm labour who are likely to deliver within 1 week
   6. Act synergistically with postnatal exogenous surfactant therapy
   7. Betamethasone + dexamethasone both used - betamethasone may be more effective

Question 8

Mortality

Answer 8

1. Severe impairment in gas exchange
2. Alveolar air leaks (interstitial emphysema, pneumothorax)
3. Pulmonary haemorrhage
4. Intraventricular haemorrhage