## Pathophysiology of RDS **Key Point:** Respiratory Distress Syndrome (RDS), also known as Hyaline Membrane Disease (HMD), is fundamentally caused by deficiency of pulmonary surfactant in the newborn lungs. ### Surfactant Function and Deficiency Surfactant is a lipoprotein complex synthesized by type II alveolar cells. It serves two critical functions: 1. **Reduces surface tension** at the air-liquid interface in alveoli 2. **Prevents alveolar collapse** during expiration (maintains functional residual capacity) In RDS, insufficient surfactant leads to: - High surface tension in alveoli - Alveolar collapse at end-expiration (atelectasis) - Increased work of breathing - Ventilation-perfusion mismatch - Hypoxemia and hypercapnia ### Surfactant Composition | Component | Percentage | Function | | --- | --- | --- | | Lipids (DPPC, PG, PI) | 90% | Surface tension reduction | | Proteins (SP-A, SP-B, SP-C, SP-D) | 10% | Immune defense, spreading | **High-Yield:** Dipalmitoylphosphatidylcholine (DPPC) is the most important surface-active lipid in surfactant. ### Risk Factors for RDS - **Prematurity** (< 34 weeks gestation) — most important risk factor - Maternal diabetes - Cesarean delivery without labor - Perinatal asphyxia - Intrauterine growth restriction **Clinical Pearl:** Surfactant production begins around 24–25 weeks gestation but reaches adequate levels only after 34–35 weeks. This is why prematurity is the strongest predictor of RDS. **Mnemonic:** **SURFACTANT** = **S**urface tension reduction, **U**nder-production in prematurity, **R**esulatory distress, **F**unctional residual capacity loss, **A**lveolar collapse, **C**ritical in < 34 weeks, **T**ype II cells produce it, **A**cute respiratory failure, **N**eed for mechanical support, **T**reatment with exogenous surfactant. 
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