Fetal Lung Development and Maternal-Fetal Considerations

RC 141: WEEK 1 - Walsh Book: Chapters 1, 2, & 3

Introduction

  • Course Welcome: Energetic introduction to class, emphasizing a supportive learning environment.

Fetal Lung Development Overview

  • Gas Exchange Necessity: Fetal lung development is incomplete until alveoli have adequate surface area for gas exchange.
  • Development Phases: Lung development occurs in five phases.

Five Phases of Fetal Lung Development

  • 1. Embryonal Phase

    • Timeline: Day 26 to 52.
    • Development Starts: Lung begins as a bud from the pharynx, appearing 26 days after conception.
    • Structures Formed: Formation of trachea, with distinct bronchial buds developing.

  • 2. Pseudoglandular Phase

    • Timeline: Day 52 to 16 weeks.
    • Characteristics: Extensive subdivision of conducting airways; acini may appear; lymphatics develop initially at hilar region.
    • Cilia and Goblet Cells: Appearance of cilia, goblet cells, and submucosal glands, indicating developing airways.
    • Fetal Breathing Movements: Initiation of movements begins here.

  • 3. Canalicular Phase

    • Timeline: 17 to 26 weeks.
    • Vascular Growth: Significant growth of vascular beds by development of capillaries by 20 weeks and increased numbers by 22 weeks.
    • Differentiation: Type I and Type II alveolar epithelial cells differentiate.
    • Surfactant Production: Appearance of surfactant, marking the development of pulmonary acinar units (respiratory bronchioles, alveolar ducts, and sacs).
    • Viability: Extrauterine viability is possible from 22 to 24 weeks with gas exchange becoming feasible by the end of this phase.

  • 4. Saccular Phase

    • Timeline: 26 to 35/36 weeks.
    • Structures: Terminal structures known as saccules, which begin development of mature alveoli.
    • Gas Exchange Potential: Increased maturation leads to greater gas exchange surface area by around 32 weeks.

  • 5. Alveolar Stage

    • Timeline: Approximately 36 weeks to 18 months of age.
    • Maturation: Postnatal principally, and by birth, alveoli range from 20 to 150 million (with 50 million being an accepted mean).
    • Alveoli Count Post Birth: Number grows to about 300 million by age 8 to 12, primarily through size increases.

Postnatal Lung Growth

  • Growth Duration: Lung growth continues through infancy and childhood, with more than 80% of total alveoli formed after birth.
  • Oxygen Uptake: Increases proportionally as lungs grow larger.

Factors Affecting Lung Growth

  • Initial Structures: Development of initial structures occurs in embryonal stage.
  • Pulmonary Hypoplasia: Incomplete lung development can manifest in the pseudoglandular phase. Conditions like atresia, stenosis, and various genetic disorders are involved.

Causes of Diminished Lung Growth

  • Categories:
    • Chest Wall Compression: Example - diaphragmatic hernia or hydrops fetalis.
    • Diminished Respiration: Lack of lung stretch due to hormonal or metabolic disorders (e.g., leprechaunism, diabetes).

Pulmonary Hypoplasia

  • Definition: Incomplete lung development leading to a reduced number of airways and alveoli with a prevalence of 10-25% diagnosed post-mortem.
  • Associated Conditions: Congenital diaphragmatic hernia (CDH) incidence is 1 in every 4000 births affected largely by compression factors like chest wall abnormalities and oligohydramnios.

Alveolar Cell Development and Surfactant Production

  • Cell Types:

    • Type I Pneumocytes: Make up 97% of alveolar surface, maintaining structure.
    • Type II Pneumocytes: Critical for surfactant production and secretion.

  • Surfactant Functionality: Surfactant reduces surface tension within alveoli and prevents collapse.

  • Fetal Lung Maturity Assessment: Analyzed through the amniotic fluid for phosphatidylglycerol (PG) and Lecithin/Sphingomyelin (L/S) ratio.

Lecithin/Sphingomyelin Ratio (L/S Ratio)

  • Definition: Measures surfactant presence; normal ratio is 2:1. Testing involves mixing fluid with ethanol to check surfactant maturity and RDS (Respiratory Distress Syndrome) risk.

Phosphatidylglycerol (PG)

  • Significance: Appears around week 35, indicating mature surfactant presence; absence in immature surfactant.
  • Utility in Diabetes: Testing for PG gives better insight into fetal lung maturity, especially in diabetic pregnancies.

Fetal Lung Liquid

  • Functionality: Essential during development as fetal lungs don't serve a respiratory function; lung fluid prevents alveolar collapse, secreted at a rate of 250-300 mL/day.
  • Importance of Outflow: Proper clearance is crucial for neonatal respiratory function, as outflow obstruction could linguistically impair type II cell growth.

Embryological Overview of Development

  • Dependence on Maternal Circulation: Fetus relies on mother for gas exchange; neither shares blood.
  • Timeline: Heart begins beating day 22, following implantation of zygote and blastocyst formation.

Embryonic Layers

  • Ectoderm: Develops into the CNS, PNS, skin, and sensory structures (eyes, ears).
  • Mesoderm: Forms cardiovascular, lymphatic systems, connective tissues, muscle types, and reproductive structures.
  • Endoderm: Gives rise to respiratory and digestive systems.

Maternal–Fetal Gas Exchange: The Placenta

  • Function: Serves as the organ for gas exchange in pregnant women, with muscles receiving nutrients from maternal blood and removing waste products.
  • Hormonal Production: Produces crucial hormones (e.g., progesterone) for pregnancy maintenance.
  • Structure: Approximately 6-8 inches diameter, 1 inch thick, weighing about 1 pound.

Placental Barriers

  • Functionality: Filters harmful substances from maternal to fetal blood; has metabolic activity that supports fetal needs.

Umbilical Cord Structure and Functionality

  • Composition: Consists of 2 arteries (deoxygenated blood) and 1 vein (oxygenated blood), typically around 20 inches in length.
  • Protection and Support: Wharton’s jelly protects vessels from compression, aiding in blood flow regulation and related stem cell presence.

Cardiovascular Development Overview

  • Heart Formation: Begins in the third week of gestation and is the first organ to become fully functional by the eighth week.
  • Development Phases: Starts as a tubular structure that evolves into a standard heart shape.

Fetal Circulation and Shunts

  • Circulation System: Unique to fetuses, redirects blood away from lungs and liver through specific structures:
    • Ductus Venosus
    • Foramen Ovale
    • Ductus Arteriosus

Ductus Venosus

  • Location: Abdominal cavity, connecting umbilical vein to IVC, shunting majorly oxygenated blood directly to the IVC.

Foramen Ovale

  • Functionality: Allows blood passage from the right to the left atrium with a valve action from the septum primum, critical in utero given the high pulmonary vascular resistance.

Ductus Arteriosus

  • Connection: Links pulmonary artery to aorta, enabling maximal blood passage to the placenta as lung perfusion is reduced during fetal life.

Transitions to Extrauterine Life

  1. Umbilical Vessel Clamping: This shifts fetal circulation and reduces PVR, impacting heart pressures.
  2. Foramen Ovale Closure: Occurs when pressure ratios reverse.
  3. Ductus Arteriosus Closure: Post-delivery environmental changes lead to its necessary closure within 96 hours.

Maternal and Perinatal Disorders

  • Diabetes Mellitus in Pregnancy:
    • Types: Pregestational (before pregnancy) and gestational diabetes.
    • Effects on Mother and Fetus: Risks include ketoacidosis, fetal structural malformations, macrosomia, and others.
  • Infectious Diseases in Pregnancy: GBS, HSV, HIV, and HBV significantly affect both mother and fetus.

High-Risk Conditions During Pregnancy

  1. Hypertension: Impacts a significant percentage; presents risks like placental abruption and fetal death.
  2. Preeclampsia: Occurs during pregnancy, linked to severe complications and can necessitate immediate delivery.
  3. Preterm Birth: Presents the greatest risk for newborns and is classified through multiple signs and medical indicators.

Antenatal Assessment Techniques

  • Ultrasound: The primary noninvasive assessment tool.
  • Chorionic Villus Sampling (CVS) and Amniocentesis: Invasive tests for genetic information and congenital abnormalities.

Delivery Methods

  • Mode of Delivery Types: Vaginal, Cesarean, and assisted deliveries using instruments like forceps or vacuums.
  • Induction of Labor: Commonly performed with oxytocin, assessing fetal health before delivery.

Preparation for High-Risk Delivery**

  • Neonatal Resuscitation: Guidelines during transition from intrauterine to extrauterine life include monitoring, intubation, and care team notifications.

  • Fetal Resuscitation Techniques: Various methods like self-inflating bags and T-piece resuscitators used based on the infant's condition.