Fetal Circulation and Congenital Heart Diseases

Fetal Circulation and Congenital Heart Diseases

Fetal Circulation

Fetal circulation differs significantly from postnatal circulation. Key differences include:

1. Placenta: The placenta provides oxygen and nutrients, bypassing the non-functional fetal lungs.
2. High Pulmonary Vascular Resistance: Fetal lungs are non-functional, leading to high resistance in pulmonary blood vessels.
3. Special Structures: These include ductus venosus, foramen ovale, and ductus arteriosus.

Special Structures and Their Functions

• Ductus Venosus: A bypass channel across the liver, connecting the umbilical vein to the inferior vena cava (IVC). The umbilical vein carries oxygenated blood (approximately 80% saturation) from the placenta to the fetus. Unlike typical veins, which carry deoxygenated blood, the umbilical vein is an exception.

• Foramen Ovale: An opening between the right and left atria, allowing oxygenated blood to be shunted from the right atrium to the left atrium, bypassing the lungs.

• Ductus Arteriosus: A connection between the pulmonary artery and the descending aorta, shunting blood away from the non-functional lungs into the systemic circulation. Deoxygenated blood from the pulmonary artery mixes with the blood in the descending aorta, reducing oxygen saturation in that region.

The umbilical artery carries deoxygenated blood from the baby to the placenta for oxygenation.

Blood Flow and Oxygen Saturation Levels

1. Umbilical Vein: Oxygen saturation of approximately 80%.
2. Ductus Venosus: Oxygen saturation around 75%.
3. Inferior Vena Cava (IVC): Oxygen saturation of about 70% due to mixing with deoxygenated blood.
4. Left Ventricle: Receives selectively channelized oxygenated blood with approximately 70% saturation.
5. Right Ventricle: Receives a mixture of oxygenated and deoxygenated blood, resulting in an oxygen saturation of about 55%.
6. Descending Aorta: Carries blood from the pulmonary artery via the ductus arteriosus, resulting in an oxygen saturation of 50-60%.
7. Umbilical Artery: Carries deoxygenated blood back to the placenta with an oxygen saturation of 50-60%.

Circulatory Changes at Birth

Immediate Changes

1. Placental Cut-Off: Umbilical vessels close due to the separation of the baby from the placenta.
2. Closure of Ductus Venosus: Occurs because the umbilical vein is no longer functional.
3. First Breath: Oxygen entering the lungs decreases pulmonary vascular resistance. This reduces pressure in the pulmonary artery and the right heart chambers (right atrium and right ventricle).
4. Umbilical Cord Clamping: Increases systemic vascular resistance.
5. Removal of Placenta: Removal of the low-pressure placental circulation also contributes to the increase in systemic vascular resistance, leading to increased pressure in the aorta and left heart chambers (left atrium and left ventricle).

Closure of Fetal Structures

• Ductus Arteriosus Closure: Results from the change in pressure difference between pulmonary and systemic (aortic) circulation. Oxygen entering the lungs is a strong factor associated with its closure.
• Foramen Ovale Closure: Results from the change in right atrial pressure and left atrial pressure.

Timeline for Closure of Structures After Birth

• Functional Closure: Physiological closure where blood flow ceases.
• Structural Closure: Anatomical closure of the structure.

• The last structure to close functionally is the ductus arteriosus (10-14 hours after birth).
• The last structure to close anatomically is the foramen ovale (up to 3 months).
• Patent Foramen Ovale (PFO) is the structural patency of the foramen ovale and is generally not a cause for concern.

Remnants of Closed Structures

• Umbilical Vein: Becomes ligamentum teres.
• Umbilical Artery: Becomes medial umbilical ligament.
• Ductus Venosus: Becomes ligamentum venosum.
• Foramen Ovale: Becomes fossa ovalis.
• Ductus Arteriosus: Becomes ligamentum arteriosum.

Congenital Heart Diseases (CHDs)

Prevalence

• CHDs occur in 6-8 per 1,000 live births, making it one of the more common congenital malformations.

Inheritance

• Polygenic or multifactorial inheritance, implying multiple genetic associations.

Common Types

• Most Common Overall: Ventricular Septal Defect (VSD), which is acyanotic.
• Most Common Cyanotic CHD: Tetralogy of Fallot (TOF).

Genetic Syndromes Associated with CHDs

• Down Syndrome (Trisomy 21): Endocardial cushion defect, involving atrial and ventricular septal defects, and atrioventricular valve defects (mitral and tricuspid regurgitation).

• Turner Syndrome: Bicuspid aortic valve and coarctation of the aorta.

• Noonan Syndrome: Pulmonary stenosis, atrial septal defect (ASD), and hypertrophic obstructive cardiomyopathy (HOCM).

• Edwards Syndrome (Trisomy 18) and Patau Syndrome (Trisomy 13): Ventricular septal defect (VSD).

• DiGeorge Syndrome: Conotruncal defects.

• Williams Syndrome: Supravalvular aortic stenosis.

• Holt-Oram Syndrome: Autosomal dominant, with skeletal anomalies (absent radius or thumb) and atrial septal defect (ASD). It has a strong genetic association with CHDs.

• Alagille Syndrome: Pulmonic stenosis (PS).

Maternal Associations with CHDs

• Maternal Diabetes (Overt): Increased risk of VSD. Gestational diabetes isn't associated with increased risk.

• Congenital Rubella Syndrome: Patent ductus arteriosus (PDA), pulmonary stenosis (PS), and VSD.

• Maternal Lupus (SLE): Anti-Ro (SSA) or anti-La (SSB) antibodies can cross the placenta and cause congenital heart blocks in the fetus.

Teratogen Exposure

• Alcohol and Phenytoin: Increased risk of VSD.
• Lithium: Epstein's anomaly.

NADA's Criteria for Congenital Heart Disease

• Major Criteria:
  • Systolic murmur, grade 3 or more.
  • Diastolic murmur, any grade.
  • Cyanosis.
  • Features of congestive cardiac failure.
• Minor Criteria:
  • Systolic murmur less than grade 3.
  • Abnormal S2.
  • Abnormal chest X-ray.
  • Abnormal blood pressure.
  • Abnormal ECG.
• CHD is possible if one major criterion or two minor criteria are present.

Diagnostic Considerations

• Systolic Murmurs:
  • Pansystolic Murmurs: Heard throughout systole, high grade (grade 3 or more), and always pathological.
    • Examples: VSD, mitral regurgitation, tricuspid regurgitation.
  • Ejection Systolic Murmurs: Can be normal or pathological.
    • Pathological: Grade 3 or more, examples include aortic stenosis and pulmonary stenosis.
    • Normal: Less than grade 3, short systolic murmur, and associated with high fever or anemia.
• Cyanosis:
  • Hypoxia: SPO2 less than 94% in children.
  • Cyanosis is noted only in severe cases of hypoxia, SPO2 less than 85%.
  • Peripheral Cyanosis: Noted only in the peripheries or in the extremities
  • Central Cyanosis: Noted in the extremities and mucous membranes
  • Mixing from the right side of the circulation to the left or obstruction to the pulmonary blood flow can cause cyanosis.