CHD Module 2 - Fetal Circulation and Changes at Birth

Oxygen saturation.

The percentage of O2 carried in the blood.

Natal

Relating to birth

List the key structures of fetal circulation.

Placenta

Umbilical vein

Ductus venosus

Ductus Arteriosus

Foramen Ovale

Describe the importance of the placenta to fetal blood flow.

• low resistance for gas exchange

• secretes prostaglandins

  • keeps the ductus arteriosus open

Describe the importance of the umbilical vein to fetal blood flow.

• brings oxygenated blood from the placenta to fetus

• `80% O2 saturation

Describe the importance of the ductus venosus to fetal blood flow.

• vessel between the umbilical vein and IVC

• allows O2 saturated blood to bypass the liver and enter the heart

Describe the pathway(s) of blood in the fetal heart starting from umbilical vein and ending at the descending aorta.

UV → DV → IVC → RA → FO → LA → LV → Ao Arch → Desc Ao

UV → DV → IVC → RA → RV → MPA → DA → Desc Ao

UV → DV → IVC → RA → RV → MPA → lungs → PV → LA → LV → Ao Arch → Desc Ao

2/3 of blood goes from RA to _____.

RV

What differences in ventricular size are expected prenatal vs neonatal.

Prenatal - RV is larger and more dominant

Postneo - LV is larger and more dominant

Describe the fetal oxygenation of blood in the:

• LA

• RA

• LV

• RV

• MPA

• Ascending Ao

• Descending Ao

• SVC

• IVC

High oxygenation:

• IVC

• LA

• LV

• Ascending Ao

Medium oxygenation:

• RV

• MPA

• Descending Ao

Low oxygenation:

• SVC

• RA

How is the ascending Ao well oxygenated and descending Ao is medium oxygenated? Why is this necessary?

Ascending Ao receives blood from the high oxygenated IAS shunt (FO)

Isthmus connects to the descending Ao and mixes medium oxygenated blood from the RV to high oxygenated blood from the ascending Ao

This is necessary to ensure the head and neck vessels receive well oxygenated blood

Describe the isthmus.

Level of the Ao that the DA connects to, important in shunting blood from the MPA to descending Ao

Most of the blood from the RV goes to the ______ because it is _____ resistance.

DA, low

What is the embryological origin of the DA?

Left 6th aortic arch

Umbilical arteries are derived from the _____ and carry ______ oxygenated blood.

internal iliac arteries, medium

O2 saturation of the fetus is highest in the ______ and lowest in the ______

placenta, organs

Define series and parallel circulation and describe where each is seen.

Series circulation: one pathway for blood through the heart

• normal post natal physiology

Parallel circulation: more than one pathway for blood through the heart

• fetal circulation

• septal defects

Contrast how gas exchange occurs in fetal vs neonatal

Fetal - in placenta

Neonatal - in lungs

Describe the intracardiac shunts in fetal vs neonatal.

Fetal - FO and DA

Neonatal - none

Explain how pulmonary vascular resistance changes at birth and how this change alters circulation.

Fetal pulmonary vascular resistance is high - only 10% of blood from the RV enters the lungs

Postnatal pulmonary vascular resistance is low - 100% of blood from the RV enters the lungs

Explain why pulmonary vascular resistance is extremely high in the fetus.

• lungs are filled with fluid

• alveoli are collapsed

• vessels are constricted

The ductus venosus closes ~1 week after birth because…

It collapses without flow (not being supplied by the placenta)

Why does RAP greatly decrease after birth?

Decrease in VR → ↓ RAP

Explain the importance of the first breath for circulation and what changes occur to result in IAS closure.

Important for blood oxygenation as the placenta no longer supplies oxygenated blood through the umbilical vein.

Fills the lungs with air → increase in pulmonary vasodilation → decrease in pulmonary vascular resistance → increase in pulmonary blood flow → increase in pulmonary VR → increase in LAP → closure of FO → closure of the IAS

Explain the importance of placenta removal for IAS closure.

Umbilical vein cut → ↓DV flow → ↓IVC flow → ↓RAP → closure of FO → closure of IAS

To close the FO, the ____ needs to be greater than ____.

LAP, RAP

When the foramen ovale closes permanently, it is called the _____ and can be seen in echo as the _____.

fossa ovalis, thin walled portion of IAS

At birth, there is still flow through the DA because there is flow supplying it. Describe the physiology of closing the DA and what it is called after closure.

• removal of placenta

  • ↓prostaglandins (not supplied by placenta) → closure of DA

• increase in oxygenation

  • oxygen causes the release of bradykinin → stimulates muscle contraction → closure of DA

Define ductal dependent lesions. What treatment would a newborn receive and why is this effective?

Any lesion that severely obstructs flow to body or lungs.

Treatment: prostaglandins

• keeps the DA open to ensure blood is supplied to the body

Give some examples of ductal dependent lesions.

• aortic atresia (closure of AoV)

• pulmonary atresia (closure of PV)

• severe coarctation (narrow Ao)

• severe arterial stenosis or pulmonary stenosis (stiffness)

• hypoplastic left heart syndrome

Describe hypoplastic left heart syndrome and what complications arise.

Blocked or closed MV and AoV, meaning the LV is underdeveloped

• LV is very small and underdeveloped

• no blood flow from descending Ao from LV

• requires the DA to remain open for blood flow to descending Ao 

Functional closure of the DA occurs in _____ days while permanent closure occurs in ______ days.

hours to, ~21

Describe patent ductus arteriosus.

DA remains open more than 3 weeks after birth.

Name 2 structures in the heart that collapse within the first week of birth.

• umbilical arteries - becomes umbilical ligaments

• umbilical veins - becomes ligamentum teres