Pulmonary Surfactants

Pulmonary Surfactant: Basics - Lung Lubricators

• Composition: Primarily Dipalmitoylphosphatidylcholine (DPPC), Phosphatidylglycerol (PG), cholesterol, and surfactant proteins (SP-A, SP-B, SP-C, SP-D).
  • SP-A, SP-D: Innate immunity, host defense.
  • SP-B: Essential for film spreading & stability.
  • SP-C: Enhances film spreadability. 📌 Mnemonic: "B for Breath" (SP-B vital), "A & D for Defense".
• Synthesis: Produced by Type II pneumocytes; stored in lamellar bodies before secretion.
• Functions:
  • ↓ Alveolar surface tension (Laplace's Law: $P = 2T/r$).
  • Prevents atelectasis (alveolar collapse), especially at end-expiration.
  • ↑ Lung compliance (ease of inflation).
  • Aids host defense.

⭐ SP-B is the most critical protein for surfactant function; its genetic deficiency is lethal.

Surfactant Deficiency: Disorders - When Lungs Lack Lube

• Neonatal Respiratory Distress Syndrome (NRDS) / Hyaline Membrane Disease:
  • Causes: Prematurity (primary), maternal diabetes, C-section (no labor).
  • Pathophysiology: ↓ Surfactant → ↑ alveolar surface tension → diffuse atelectasis → hypoxemia.
  • Clinical: Rapid onset tachypnea (>60/min), grunting, retractions, cyanosis.
  • Diagnosis:
    • CXR: Diffuse reticulogranular ("ground-glass") pattern, air bronchograms.
    • Amniotic fluid $L/S$ ratio < 2:1 (lung immaturity). (📌 Lungs Sad if < 2:1)
• Acute Respiratory Distress Syndrome (ARDS):
  • Surfactant inactivation by plasma proteins due to alveolar injury/inflammation.

⭐ Antenatal corticosteroids (e.g., Betamethasone) for mothers at risk of preterm birth (24-34 wks) reduce NRDS by accelerating fetal lung maturity.

Surfactant Therapy: Preparations - Rescue Agents

• Primarily natural (animal-derived) surfactants are used. Synthetic versions (e.g., Lucinactant, Colfosceril palmitate) are now rarely employed.
• Indications:
  • Prophylaxis: High-risk preterm infants for Neonatal Respiratory Distress Syndrome (NRDS).
  • Rescue: Established NRDS.

Key Natural Surfactant Preparations:

Surfactant Administration: Method & Mishaps - Delivery & Downsides

• Route: Intratracheal instillation via Endotracheal Tube (ETT).
• Techniques:
  • Bolus: Administered in aliquots.
  • INSURE: INtubate-SURfactant-Extubate.
  • LISA/MIST: Less Invasive Surfactant Administration / Minimally Invasive Surfactant Therapy (via thin catheter, spontaneously breathing infant).

    ⭐ LISA/MIST techniques are gaining popularity as they may reduce the need for mechanical ventilation and its complications.

• Dosing Examples:
  • Beractant: 100 mg phospholipid/kg (or $4 \text{ mL/kg}$).
  • Poractant alfa: Initial 100-200 mg/kg (or $1.25-2.5 \text{ mL/kg}$); subsequent 100 mg/kg (or $1.25 \text{ mL/kg}$).
• Adverse Effects:
  • Transient: Hypoxia, bradycardia, airway obstruction (mucous plug), reflux.
  • Potential: Pulmonary hemorrhage, nosocomial infection.
• Monitoring: Post-administration: Oxygenation (SpO₂), ventilation parameters, vital signs.

High‑Yield Points - ⚡ Biggest Takeaways

• Pulmonary surfactants, rich in DPPC (Lecithin), are crucial for reducing alveolar surface tension and preventing atelectasis.
• Their deficiency is the primary cause of Neonatal Respiratory Distress Syndrome (NRDS).
• Administered directly into the lungs via intratracheal instillation.
• Natural surfactants like Beractant, Poractant alfa, and Calfactant generally show better clinical outcomes.
• Monitor for acute adverse effects: transient hypoxia, bradycardia, or airway obstruction.
• A Lecithin/Sphingomyelin (L/S) ratio > 2 indicates fetal lung maturity.