Defective surfactant aggravated by high oxygen concentration, poor airway drainage, asphyxia, hypoxia, hypovolemia leading

surfactant in alveolar lining. Defective surfactant aggravated by high oxygen concentration, poor airway drainage, asphyxia, hypoxia, hypovolemia leading to pulmonary ischemia and cold stress.(2). Small alveoli which inflate with difficulty and tend to collapse on expiration. (3). Weak and compliant chest wall. PATHOPHYSIOLOGY: Surfactant (dipalmityl phosphatidyl choline) deficiency leads to collapse owing to increased surface tension, so there is a failure to maintain functional residual capacity Surfactant synthesis by Type II alveolar cells increases with foetal maturity. Diameters of airways upto bronchioles are smaller in prematures, requiring greater force for inflation and larger trans-pulmonary pressure to keep them from deflating. Also highly compliant chest wall at this time compounds tendency to atelectasis, offering poor resistance. These factors produce near-atelectasis, dyspnoea and tachypnoea, increased airway resistance and work of breathing, and eventually hypoxia, hypercapnia and acidosis. These events cause pulmonary vasoconstriction, maintenance of ductal blood flow and foramen ovale flow in an attempt to perfuse the lungs Decreased lung flow causes ischemic necrosis of surfactant cells and the vasculature causing effusion of protein-like material into alveolar spaces, producing the hyaline membrane. Lungs appear liver-like. Microscopically there is extensive atelectasis, engorgement of vessels and lining of alveoli by acidophilic homogenous membrane, membranes are seen only 6-8 hours after birth. Clinical features: Signs appear minutes after birth; may require resuscitation. Late tachypnoea is very unusual. (1) Rapid shallow respirations increasing to 60 or more per minute. (2) Audible grunting (ominous sign). (3) Intercostal and subcostal retractions. (4) Nasal flare. (5) Duskiness with increasing cyanosis poorly responsive to oxygen. (6) Breath sounds normal or diminished with harsh, tubular quality, fine crepitations on deep inspiration at lung bases. Worsening is characterised by increased dyspnoea, air hunger, progressive cyanosis, decrease and then absence of grunting, absent breath sounds despite chest movement, hypothermia, hypotension, irregular breathing, acidosis and apnoea, progresses to death in a few hours Death is rare after 3 days if an infant survives severity of newborn respiratory distress judged by Anderson Silvermans score (see p 619). Complications: A Of the disease - (1) Hypoxia, hypercapnia, acidosis, respiratory failure. (2) Persistence of ductus arteriosus - delayed closure due to hypoxia, acidosis, increased pulmonary pressure, immaturity of the infant and local release of ductal dilators such as prostaglandin E1 and E2 causes persistent apnoea, systolic or continuous murmur, increases oxygen dependancy, aggravates hypercapnia, cardiomegaly and cardiac failure (3) Intraventricular and pulmonary haemorrhage. B. Of intensive care - (1) Tracheal intubation - Obstruction of tube, cardiac arrest during suction, bleeding and ulceration of nose and throat from trauma, vocal cord avulsion, subglottic stenosis, laryngeal oedema and stridor (2) Umbilical catheterization - Vascular embolism, thrombosis, perforation, ischemic necrosis of viscera, gangrene of legs, haemorrhage after removal of heparinised catheter. (3) Oxygen toxicity - Retrolental fibroplasia, broncho-pulmonary dysplasia. (4) Pneumothorax and pneumoperitoneum (with intubation, respirator). (5) Secondary infection. (6) Anemia due to frequent blood collections. Diagnosis: (1) Low pO2 <> 50 mm Hg and acidosis signal respiratory failure. (2) X-ray chest -Fine reticular granularity of lung fields with air bronchogram seen within 6-12 hours characteristic but not pathognomonic other causes of neonatal respiratory distress (a) Respiratory causes - i. Prenatal, natal meconium aspiration. 2 Intrauterine or postnatal pneumonia, esp group B streptococcal pneumonia. 3 Pneumothorax, pneumomediastinum. 4. Pulmonary haemorrhage 5 Transient tachypnoea of the full-term newborn (persistence of lung fluid, recovers fully). 8. Wilson-Mikity syndrome (late onset dyspnoea. X-ray shows 'bubbly1 lungs, cause unknown). 7. Congenital malformations - Choanal atresia, Pierre Robin syndrome (small chin, cleft palate, tendency for tongue to fall back), laryngeal webs, stenosis, atresia, vascular rings, ectopic goitre, T-O fistula, congenital lobar emphysema, pulmonary agenesis, diaphragmatic hernia, congenital tumours, e g cysts, teratomas. (b) Cardiovascular - CCF from any cause, congenital heart disease, severe anemia, polycythemia, diabetic offspring (c) Neurological - Birth asphyxia or injury, intracranial haemorrhage, plexus injuries in breech deliveries (d) Metabolic -Acidosis, uremia, inborn errors of metabolism (hyperammonemia). Treatment: Course can be altered by intensive care and supportive therapy 1 Treatment of inadequate gas exchange - Warm humidified oxygen to keep arterial blood levels between 50 to 70 mm Hg with stable vital signs, while minimising risks of toxicity. Inspired concentration safely kept at 40-70% If > 50 mm Hg O2 cannot be achieved, use of continuous positive airway pressure (CPAP) by nasal prongs or head box is used Persistent apnoea, blood pH<72,>60 mm Hg, pO2<50>