Write a short note on pulmonary surfactant.

Definition

Composition

• Phospholipids — 90%; chief active constituent is dipalmitoylphosphatidylcholine (DPPC)
• Proteins — 10%

– SP-A, SP-D → innate immune defence (opsonins)

– SP-B, SP-C → aid spreading of the phospholipid film

• Neutral lipids and Ca2+

Functions

1. ↓ alveolar surface tension → ↑ compliance → ↓ work of breathing

2. Prevents atelectasis — by Laplace law, P = 2T/r, a small alveolus would collapse into a large one. As the alveolus shrinks, surfactant molecules crowd together → T falls more in small alveoli → stabilises alveoli of unequal size

3. Keeps alveoli dry — opposes surface-tension forces that would suck transudate from capillaries

4. Alveolar defence — SP-A and SP-D act as opsonins

Law of Laplace

• Without surfactant, T is constant → smaller alveolus has higher collapsing pressure → empties into the larger one
• Surfactant makes T variable with radius → abolishes this instability

Development

• Secretion begins at 24 weeks of gestation
• Adequate by 34–35 weeks
• Fetal lung maturity assessed by L:S ratio in amniotic fluid — > 2 = mature

Causes of Surfactant Deficiency

• Prematurity — commonest
• Infant of a diabetic mother (insulin antagonises cortisol)
• Caesarean section without labour
• Perinatal asphyxia, acidosis, hypothermia
• Prolonged pulmonary ischaemia (cardiopulmonary bypass)
• Smoking, ARDS in the adult

Applied Aspects

Preterm birth → Surfactant deficiency → ↑ surface tension → ↓ compliance + atelectasis → Respiratory Distress Syndrome

• Features — tachypnoea, grunting, intercostal recession, cyanosis; ground-glass chest X-ray
• Prevention — antenatal corticosteroids to the mother
• Treatment — exogenous surfactant, CPAP