Fetal Circulation Anatomical aspect of the fetal circulation Role of the shunts and their regulation Ductus arteriosus Foramen ovale Ductus veinosus Fetal cardiac output / particularities of the fetal heart Modification at birth of the fetal circulationFallot’ Project 2sd December 2009
Ductus arteriosus lungs Fetal Circulation Foramen ovale liver aorta Anatomic aspectDuctus veinosus(Arantius canal) Umbilical veinplacenta Placenta serves as the site for gas exchange Umbilical arteries Umbilical cord
Role of the shuntsOrientate oxygenated blood flow to thesupra-aortic parts (brain / heart)Umbilical Vein -> DV -> IVC -> RA -> FO ->LA -> LV -> ascending aortaOrientate deoxygenated blood flow tothe infra-aortic parts toward the placentaIVC -> RA -> RV -> PA -> DA -> descendingaorta
Role of the foramen ovalePreferential 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 fromthe left hepatic vein (posterior part of the IVC) to the foramenovale 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 arteriosusHigh pulmonary vascular resistances Shunt from RV and PA to the descending aortaRegulation:Vasodilatation Prostaglandin (PGE) Low PO2Vasoconstriction Indomethacin Endothelin 1 (<= smooth muscular cells / endothelium) High PO2 (at birth)
Role of the ductus veinosus50% of the blood flow coming from theumbilical vein bypasses the liver and goesdirectly to the left ventricle through theforamen ovale (70% in case of hypoxemia orhypovolemia)The O2 extraction by the liver is weak: only15%Importance of the flow’s regulation in caseof decreasing of the pressure into theumbilical vein: prostagladins, CO, adrenergicsystem,…i.e.: when umbilical venous return is progressively 40%reduced the percentage of umbilical venous bloodpassing through the ductus venosus increases 5%progressively
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 combinedventricular output ejected by each ventricle 20% SVC 70% 10% IVC from Rudolph / Circ Res 1985
Modification of the fetal circulationPulmonary output=> Only 10% of the combined outputAt birth :Importante decreasing of the pulmonaryresistances:Mechanical factorsVasoactive substances: NO / PO2Increasing of:pulmonary outflowleft venous return=> closure of the foramen ovale (Vieussens valve)High PO2 => vasoconstriction of the D.A.