Fetal Circulation Cliffnotes

Question 1

What is the goal of fetal circulation?

Answer 1

To bypass the nonfunctioning fetal lungs and oxygenate via placenta

Question 2

3 main shunts?

Answer 2

1. Ductus venosus
2. Foramen ovale
3. Ductus arteriosus

Question 3

Purpose of the ductus venosus?

Answer 3

Bypass the liver

• umbilical vein -> IVC

Question 4

Purpose of the ductus arteriosus?

Answer 4

Bypass the lungs

• pulmonary artery -> aorta

Question 5

Purpose of the foramen ovale?

Answer 5

Bypass the lungs

• RA -> LA

Question 6

Umbilical artery function?

Answer 6

2 umbilical arteries that direct flow of blood from the fetus to the placenta.

• carries deoxygenated blood + waste
• Remove CO2, urea, and waste to placenta for elimination

Question 7

Umbilical vein function?

Answer 7

1 vein that directs flow from the placenta to the fetus.

• carries oxygenated blood + nutrients
• connects placenta to fetal liver via ductus venosus and then IVC

Question 8

Direction of flow of umbilical arteries vs veins?

Answer 8

“Arteries Away, Vein to the Baby”

– Umbilical arteries carry blood away from the fetus

– Umbilical vein carries blood to the fetus

• opposite of adults

Question 9

What does knees to chest (or squatting) do for fetal circulation?

Answer 9

Increases SVR which reduces right to left shunting

• the position increases pressure in teh abdomen and legs which in turn increases SVR

Question 10

Why does increased SVR reduce R->L shunting in cyanotic heart defects

Answer 10

Increased SVR pushes more blood into pulmonary circulation. The increased pressure makes it harder for deoxygenated blood to bypass the lungs via defects like VSDs.

• Remember that blood will always flow where it is easier. when SVR exceeds PVR it will go into the lungs.

Question 11

What are tet spells?

Answer 11

Sudden episodes of cyanosis and breathing difficulties in babes with heart defects.

• rapid drop in O2 in blood triggered by crying, feeding, or waking up
• Becomes harder for the heart to pump blood into the lungs

Question 12

What effect does oxygen have on PVR and shunting?

Answer 12

Decreases PVR -> Improves pulmonary blood flow -> decreases (R->L) shunting

Question 13

What effect does PEEP have on PVR and shunting?

Answer 13

PEEP can decrease PVR by recruiting alveoli

• too much can lead to over distention which increases PVR

Question 14

What effect does iNO have on PVR and shunting?

Answer 14

Decreases PVR which reduces (R->L) shunting.

• effect is huge in PPHN

Question 15

What is the role of prostaglandins and indomethacin in managing a patent ductus arteriosus (PDA)?

Answer 15

1. Prostaglandin E1 (PGE1): Keeps PDA open – used in congenital heart defects like TGA or TOF to allow mixing of blood
2. Indomethacin (NSAID): Closes PDA – used in preterm infants with symptomatic PDA to reduce L→R shunt

Question 16

A newborn presents with cyanosis at birth that worsens with crying. Which heart defects should you suspect?

Answer 16

Think cyanotic congenital heart defects (R→L shunt):

• TGA (Transposition of the Great Arteries)
• TOF (Tetralogy of Fallot)
• Both result in poorly oxygenated systemic blood; require PDA or other shunt to allow oxygen mixing

Question 17

What affect does CO2 have on PVR and Shunting?

Answer 17

• High CO2 increases PVR -> Increases (R->L) shunting worsening hypoxemia
• Low CO2 decreases PVR -> Decreases (R->L) shunting improving oxygenation

Question 18

Why would increased CO2 levels be bad?

Answer 18

In PPHN or congential heart diseases, hypercapnia can exacerbate shunting by keeping PVR high