Lung surfactant is a lipoprotein substance that lines the inner surface of the alveoli in the lungs.
Composition:
• Dipalmitoylphosphatidylcholine (DPPC) / Lecithin – Main component
• Surfactant proteins: SP-A, SP-B, SP-C, SP-D
• Cholesterol
• Phosphatidyl glycerol
Surfactant protein B (SP-B):
• SP-B helps DPPC spread evenly over the alveolar surface.
• Deficiency of SP-B leads to neonatal pulmonary alveolar proteinosis (PAP).
Functions:
• Reduces surface tension in alveoli → prevents alveolar collapse (atelectasis) during expiration.
• Increases lung compliance → makes breathing easier.
• Prevents pulmonary edema → by keeping alveoli dry.
• Plays a role in host defense → surfactant proteins (especially SP-A, SP-D) help in innate immunity.
Type | Condition | Effect on Compliance |
↓ Decreased Compliance | • Pulmonary fibrosis • Edema • Pneumonia | Lungs stiff — hard to expand |
↑ Increased Compliance | Emphysema | Lungs lose elastic recoil — too easy to expand |
Clinical significance:
• Produced by: Type II alveolar cells (Type II pneumocytes)
• Synthesis begins in fetal lungs at: 20 weeks of gestation
• Begins to appear in amniotic fluid at: 28-32 weeks of gestation
• Mature surfactant is present in adequate amount at: >35 weeks of gestation
• Deficiency causes: Respiratory Distress Syndrome (RDS) / Hyaline membrane disease in premature infants.
Management:
• Exogenous surfactant therapy → Intra tracheal surfactant (INSURE) administered to premature babies after birth.
Prevention of NRDS:
• Antenatal corticosteroids (e.g., Dexamethasone, betamethasone) → accelerate surfactant production.
Note: Antenatal corticosteroids is contraindicated in chorio amnionitis.
Dexamethasone | Feature | Betamethasone |
6 mg IM every 12 hours × 4 doses | Dose | 12 mg IM every 24 hours × 2 doses Mnemonic: Beta means 12mg (बारह) |
24 mg (6 x 4) | Total Dose | 24 mg (12 x 2) |
48 hours | Course Duration | 48 hours |
Intramuscular (IM) | Route | Intramuscular (IM) |
