Surfactant deficient lung disease

Surfactant deficient lung disease (SDLD), also known as neonatal respiratory distress syndrome (NRDS) and previously known as hyaline membrane deficiency, is a condition that occurs due to deficiencies of surfactant in the lungs accompanied by structural immaturity too.

🏃‍♀️ Physiology

The alveoli have 2 types of cells present:

1. Type I pneumocytes - these are thin squamous cells that make up the overwhelming majority of the alveoli. Their thinness allows them to perform their function of gaseous exchange.
2. Type II pneumocytes - these are cuboidal and produce surfactant and also function as progenitors for type I pneumocytes to allow the cell to renew.

Vaporised water sits on top of the cells inside the alveoli, leading to surface tension to rise. Surface tension is the attraction between molecules that tends to pull those molecules together to shrink the surface area as much as possible. The smaller the alveolus, the greater the surface tension is. If the surface tension is unopposed, the alveolus might collapse.

The lung relies on elastic recoil of the alveoli, and more importantly, on surfactant to prevent the alveoli from collapsing. Surfactant coats the inside of the alveoli and reduces the surface tension inside the alveolus; which prevents it from collapsing and maximises the surface area of the alveoli. This in turn reduces the force needed to expand the alveoli during inspiration. This is known as compliance. Therefore, we can simply say surfactant increases lung compliance.

Additionally, as an alveolus expands, the surfactant becomes more thinly spread and therefore the surface tension increases, making it more difficult to expand that alveolus further. This stops one alveolus expanding massively whilst another alveolus only expands a little. Therefore, surfactant promotes equal expansion of all alveoli during inspiration.

Surfactant production begins around 26 weeks gestation and does not reach adequate levels until around 35 weeks. It is made up of phospholipids (lecithin and phosphatidylglycerol) and additional proteins. It has a hydrophilic layer facing the alveolar wall which interacts with the vaporised water on top of the alveoli. It then has a hydrophobic layer facing inwards that prevents the pulling together of the alveolar walls.

Pathophysiology

In certain instances, most commonly in premature neonates whose lungs have not yet developed fully and do not have enough surfactant produced. The ensuing increase in surface tension within the alveoli results in a propensity for collapse, particularly in smaller alveoli. This collapse hinders effective gas exchange, diminishes lung compliance, and triggers respiratory distress marked by rapid and labored breathing.

The risk decreases as the gestational age increases:

• 50% of infants born at 26-28 weeks are at risk of NRDS.
• 25% of infants born at 30-31 weeks are at risk of NRDS.

⚠️ Risk factors

• Premature birth - especially anything less than 34 weeks.
• Maternal diabetes - maternal diabetes, especially if poorly controlled, can affect the production and quality of surfactant in the fetal lungs.
• C-section - during vaginal delivery, the baby undergoes compression as it passes through the birth canal. This compression helps to expel amniotic fluid from the lungs, promoting the clearance of lung fluid and aiding in the initiation of breathing. However, this compression is lost with C-sections and so fluid may not be completely expelled from the airways.
• Multiple pregnancies
• Maternal exposure to medications - for example glucocorticoids. Although glucocorticoids are used promote fetal lung maturation and reduce the risk of NRDS in preterm infants, their potential to inhibit surfactant production may also predispose the baby to NRDS (if used too early).

😷 Presentation

Immediately after birth, signs of SDLD may not be present as the baby used to get their oxygen supply from the umbilical cord which is not affected.

A few hours after birth we may notice the following:

• Tachypnoea
• Dyspnoea
• Tachycardia
• Hypoxaemia

Then, as the baby approaches respiratory failure, they start showing signs of respiratory distress, such as:

• Intercostal and subcostal recessions
• Expiratory grunting
• Nasal flaring
• Cyanosis
• Tracheal tugging
• Abnormal respiratory noises

🔍 Investigations

⭐️ It is predominantly a clinical diagnosis based on the signs and symptoms and the prematurity of the baby.

Supporting investigations may include the following:

• CXR - ground glass appearance with an indistinct heart border.
• Blood gas often shows hypoxemia and hypercapnia (type 2 respiratory failure)

🧰 Management

🧰

Management of SDLD/NRDS:

• 🏆 Intra-tracheal instillation of surfactant + budesonide
• ⭐️ If preterm delivery is suspected → administer glucocorticoids to the mother before delivery can enhance surfactant production in the infant.
• Supplemental oxygen and respiratory support such as CPAP or mechanical ventilation may be required.
• Supportive care - including maintenance of normal body temperature, adequate nutrition, and treatment of concurrent infections, is vital.

🚨 Complications