Perkembangan normal terjadi pada 4 tahap :.

Periode Embryonic

Sekitar 26 hari gestasi, tahap embrionik mulai pertama kali terlihat pada paru fetus, terlihat sebagai sebuah protrusi foregut. Mulainya percabangan terjadi pada hari ke 33 gestasi membentuk bronkus utama prospektif, yang mulai meluas kedalam mesekim. Kedepan, bentuk percabangan membentuk bronkus segmental karena paru memasuki tahap selanjutnya perkembangan.

Tahap Pseudoglandular

Minggu ke 7 s.d 16 gestasi, sebanyak 15 -20 generasi jalan nafas terjadi 7-16 minggu gestasi, 15- 20 generasi percabangan jalan nafas terjadi mulai dari bronkus segmental utama dan akhir bronkhiolus terminal.

Akhir dari fase pseudoglandular, jalan nafas airways are dikelilingi oleh mesenkim longgar, termasuk sedikit pembuluh darah dan disertai dengan kaya glikogen dan morfologi sel epitel tidak terdiferensiasi dengan bentuk columnar ke cuboidal. Secara umum, diferensiasi epitel adalah sentrifugal maka tubulus paling distal digarisi dengan sel tidak terdiferensiasi dengan diferensiasi epitel progresif dari jalan nafas lebih proksimal. Fase Canalicular

16 dan 25 minggu gestasi, transisi dari sebelum berjalan previable to a potential viable lung occurs as the respiratory bronchioles and alveolar ducts of the gas exchange region of the lung are formed.

The surrounding mesenchyme becomes more vascular and condenses around the airways.

The closer vascular proximity ultimately results in fusion of the capillary and epithelial basement membranes.

After 20 weeks gestation, cuboidal epithelial cells begin to differentiate into alveolar type II cells with formation of cytoplasmic lamellar bodies [2]. The glycogen in these cells is used for surfactant production, which is stored in the lamellar bodies.

Saccular stage

About 24 weeks gestation, there is potential for viability because gas exchange is possible due to the presence of large and primitive forms of the future alveoli.

In this stage, formation of alveoli (ie, alveolarization) occurs by the outgrowth of septae that subdivide terminal saccules into anatomic alveoli, where air exchange occurs.

The number of alveoli in each lung increases from zero at 32 week gestation to between 50 and 150 million alveoli in term infants and 300 million in adults.

Alveolar growth continues for at least two years after birth at term. Metabolism surfaktan

Komponen surfaktan disintesis dari prekusor RE dan ditransport melalui apparatus golgi oleh multivesicular bodies.

Komponen ini akhirnya dibungkus dalam lamellar bodies yang disimpan dalam granul intraseluler sebelum disekresikan.

After secretion (exocytosis) into the liquid lining of the alveolus, surfactant phospholipids are organized into a complex lattice called tubular myelin. Tubular myelin is believed to generate the phospholipid that provides material for a monolayer at the air-liquid interface in the alveolus, which lowers surface tension.

Surfactant phospholipids and proteins are subsequently taken back into type II cells, in the form of small vesicles, apparently by a specific pathway that involves endosomes, and then are transported for storage into lamellar bodies for recycling.

Alveolar macrophages also take up some surfactant in the liquid layer.

A single transit of the phospholipid components of surfactant through the alveolar lumen normally requires a few hours.

The phospholipid in the lumen is taken back into type II cell and is reused 10 times before being degraded.

Surfactant proteins are synthesized in polyribosomes and extensively modified in the endoplasmic reticulum, Golgi apparatus, and multivesicular bodies.

Surfactant proteins are detected in lamellar bodies or secretory vesicles closely associated with lamellar bodies before they are secreted into the alveolus.

L/S ratio

Amniotic fluid L/S ratio increases progressively with gestational age. L/S ratio greater than two signifies maturity of surfactant system of lung

Patofisiologi- Surfactant deficiency

- Inflammation and Lung injury

- Pulmonary edema

- Surfactant inactivation

- Pulmonary function and gas exchange

Impaired surfactant synthesis and secretion ( atelectasis, V/Q inequality, hypoventilation ( hypoxemia and hypercarbia

Respiratory / metabolic acidosis ( pulmonary vasoconstriction ( impaired endothelial and epithelial integrity ( leakage of proteinaceous exudate and formation of hyaline membranes

Deficiency of surfactant ( decreases lung compliance and FRC, with increased dead space

Impair surfactant production and/or secretion

Hypoxia, acidosis, hypothermia, hypotension

Oxygen toxicity ( influx of inflammatory cell ( exacerbates vascular injury ( BPD

Antioxidant deficiency and free-radical injury worsen injury

Pemenuhan paru

Lungs with HMD require far more pressure than to achieve a given volume of inflation than do lungs obtained from an infant dying of a nonrespiratory cause.

Arrows indicate inspiratory and expiratory limbs of the pressure-volume curves.

Note the decreased lung compliance and increased critical opening and closing pressures, respectively, in the premature infant with HMD

Membrane hialin

line the alveoli (see the image below) may form within a half hour after birth.

In larger premature infants, the epithelium begins to heal at 36-72 hours after birth, and endogenous surfactant synthesis begins.

The recovery phase is characterized by regeneration of alveolar cells, including type II cells, with a resultant increase in surfactant activity.

CXR

low lung volume and the classic diffuse reticulogranular ground-glass appearance with air bronchograms

pencegahan two doses of betamethasone administered 24 hours apart is currently the recommended steroid for antenatal use

Antenatal steroid administration has been shown to be beneficial if provided fewer than 24 hours before delivery

Furthermore, a reduction in RDS has been seen in infants born up to 7 days after the first dose of antenatal steroids was administered. (1) No benefit is seen in infants who receive the first dose of steroids more than 7 days before birth.

They recommend repeat doses of corticosteroids in women at risk for preterm birth when the first course of steroids was administered more than 7 days previously because of the short-term benefits to the fetal lungs. They do, however, warn about the possibility of decreased birthweight and head circumference at birth, which has been reported.