Pathophysiology of HMD
Hyaline Membrane Disease(HMD) arises due to a deficiency in pulmonary surfactant production, which leads to increased surface tension in the alveoli. The key steps in its pathophysiology include:
- Surfactant Deficiency:
- Surfactant is a lipoprotein complex produced by alveolar type II cells starting from 24 weeks of gestation, with sufficient production by 34-36 weeks.
- Premature newborns (<34 weeks) have immature lungs and insufficient surfactant synthesis.
- Increased Alveolar Surface Tension:
- In the absence of adequate surfactant, alveoli collapse during exhalation (atelectasis), leading to a reduction in functional residual capacity (FRC).
- This increases the work of breathing and oxygen consumption.
- Impaired Gas Exchange:
- Collapsed alveoli impair ventilation and perfusion matching, causing hypoxemia and hypercarbia (respiratory acidosis).
- Hypoxemia triggers pulmonary vasoconstriction, further reducing oxygenation.
- Inflammatory Response and Hyaline Membrane Formation:
- Atelectasis and lung injury cause epithelial and endothelial cell damage.
- Proteinaceous exudate from damaged capillaries combines with necrotic epithelial cells to form “hyaline membranes,” lining alveoli and further impairing gas exchange.
- Cycle of Worsening Respiratory Failure:
- Without intervention, the combination of atelectasis, surfactant deficiency, and inflammation perpetuates respiratory failure.
Strategies to Prevent HMD
Antenatal Strategies:
- Antenatal Corticosteroids:
- Administration of corticosteroids (e.g., betamethasone) to mothers at risk of preterm labor (24-34 weeks) enhances fetal lung maturity.
- These steroids stimulate surfactant production and improve alveolar stability.
- Prevention of Preterm Birth:
- Tocolytic agents can be used to delay labor in high-risk pregnancies to allow time for corticosteroids to take effect.
- Optimal prenatal care and management of maternal conditions like hypertension and infections reduce preterm birth risk.
Postnatal Strategies:
- Surfactant Replacement Therapy:
- Exogenous surfactant, delivered via endotracheal tube, reduces surface tension, improves oxygenation, and decreases mortality. It can be used prophylactically or therapeutically.
- Examples: Poractant alfa, beractant.
- Respiratory Support:
- Continuous Positive Airway Pressure (CPAP): Prevents alveolar collapse and improves oxygenation.
- Mechanical Ventilation: Required in severe cases but should be used judiciously to minimize lung injury (barotrauma/volutrauma).
- Oxygen Therapy:
- Careful titration of oxygen to avoid hypoxemia and hyperoxia. Pulse oximetry and blood gas monitoring are essential.
- Minimizing Ventilation-Associated Injury:
- Strategies such as permissive hypercapnia and use of non-invasive ventilation are preferred.
- Nutritional Support:
- Early initiation of parenteral or enteral nutrition helps optimize overall recovery and growth.
References:
- Nelson Textbook of Pediatrics, 21st Edition: Discusses the development of the respiratory system and pathophysiology of HMD.
- Cloherty and Stark’s Manual of Neonatal Care, 8th Edition: Provides detailed preventive strategies and management of HMD