Fetal Circulation

Fallot’ Project 2sd December 2009

Anatomical aspect of the fetal circulation

Role of the shunts and their regulation

Ductus arteriosusForamen ovaleDuctus veinosus

Fetal cardiac output / particularities of the fetal heart

Modification at birth of the fetal circulation

References

• Rudolph AM. Circ. Res. 1985; 57; 811-821

• Kiserud T and Rasmussen S.

Ultrasound Obstet Gynecol 2001; 17: 119–124

• Jouannic J.-M , Fermont L, Brodaty G, Bonnet D, Daffos F.

J Gynecol Obstet Biol Reprod 2004 ; 33 : 291-296.

FetalCirculation

lungs

liver

placenta

aorta

Ductus arteriosus

Foramen ovale

Umbilical vein

Umbilical arteries

Ductus veinosus(Arantius canal)

Umbilical cord

Anatomic aspect

Placenta serves as the site for gas exchange

Role of the shunts

Orientate oxygenated blood flow to the supra-aortic parts (brain / heart)

Umbilical Vein -> DV -> IVC -> RA -> FO -> LA -> LV -> ascending aorta

Orientate deoxygenated blood flow to the infra-aortic parts toward the placenta

IVC -> RA -> RV -> PA -> DA -> descending aorta

Role of the foramen ovale

Preferential flow from the IVC and from the right hepatic vein (anterior part of the IVC) to the right ventricle(less or deoxygenated blood flow)

Preferential flow from the ductus veinosus (U.V.) and from the left hepatic vein (posterior part of the IVC) to the foramen ovale then to the left ventricle (oxygenated blood flow)

Orientate oxygenated blood flow to the supra-aortic parts (brain / heart)

Hypothesis : the streamlining of flows in the inferior vena cava

« Anatomical canal » into the RA (Eustachian valve)helps direct the flow into the LA via FO

Difference of the velocities between the two flows. Kiserud T. Fetal venous circulation — an update on hemodynamics. J Perinat Med 2000; 28: 90-6.

Role of the ductus arteriosus

High pulmonary vascular resistancesShunt from RV and PA to the descending aorta

Regulation:Vasodilatation

Prostaglandin (PGE)Low PO2

VasoconstrictionIndomethacinEndothelin 1 (<= smooth muscular cells / endothelium)High PO2 (at birth)

Role of the ductus veinosus

50% of the blood flow coming from the umbilical vein bypasses the liver and goes directly to the left ventricle through the foramen ovale (70% in case of hypoxemia or hypovolemia)

The O2 extraction by the liver is weak: only 15%

Importance of the flow’s regulation in case of decreasing of the pressure into the umbilical vein: prostagladins, CO, adrenergic system,…i.e.: when umbilical venous return is progressively reduced the percentage of umbilical venous blood passing through the ductus venosus increases progressively

40%

5%

Fetal cardiac output (425ml/mn/kg)

• The factors that influence cardiac output are heart rate, filling pressure or preload, compliance of the ventricles, resistance against which the ventricles eject, or afterload, and myocardial contractility.

• Fetal myocardial compliance– Lower possibility to increase the stroke volume after increasing

of the preload than in an adult heart (less compliant)

• Fetal myocardial contractility– Difficulty to support stroke volume after increasing of the

postload

Percentages of combined

ventricular output ejected

by each ventricle

from Rudolph / Circ Res 1985

70%

20%

10%

SVC

IVC

Modification of the fetal circulation

Pulmonary output=> Only 10% of the combined output

At birth :Importante decreasing of the pulmonary resistances:Mechanical factorsVasoactive substances: NO / PO2

Increasing of:pulmonary outflowleft venous return=> closure of the foramen ovale (Vieussens valve)

High PO2 => vasoconstriction of the D.A.

Oxygen saturations

65%50%

70%

35%