The Physiology of the Fetal Cardiovascular System Jeff Vergales, MD, MS Department of Pediatrics Division of Pediatric Cardiology jvergales@virginia.edu
Disclosures I serve as the medical director for Locus Health, a home monitoring company in a non-paid, non-employed position I have research grant support from Merck, unrelated to this topic
Objectives 1. Provide a simple overview of why the fetal circulation needs to, and does, differ from post-natal circulation 2. Evaluate the origination and function of the shunts present in fetal circulation 3. Examine the basic physiologic principles that govern blood flow and oxygenation in the fetus 4. Briefly demonstrate specific changes at the time of birth that are important in managing congenital cardiac lesions
Normal Postnatal Circulation Normal post-natal circulation has near complete separation of de-oxygenated blood (blue) and oxygenated blood (red) The end result of this is the delivery of fully saturated blood to the head and brain as well as to the remainder of the body Gas exchange occurs in the capillary beds of the lungs as well as the beds of all other end organs Lim and Everett, Congenital Heart Disease and Repair, 2 nd ed, 2004
Building the Perfect Fetal Circulatory System The Fetus does not inflate the lungs and consequently does not perform significant gas exchange across the pulmonary bed The Fetus is connected to a transient structure (Placenta) that provides oxygen, nutrition, waste removal and other metabolic functions. Upon birth and cord clamping, the circulatory system must have an ability to immediately transition to a system that does not rely on the placenta Lim and Everett, Congenital Heart Disease and Repair, 2 nd ed, 2004
Lim and Everett, Congenital Heart Disease and Repair, 2 nd ed, 2004 Fetal Circulation
Fetal Circulation The Ductus Venosus is designed to be a vascular connection between the placenta and the heart, itself Lim and Everett, Congenital Heart Disease and Repair, 2 nd ed, 2004
Fetal Circulation The Foramen Ovale is a normal connection between the two atria designed to allow appropriate shunting Lim and Everett, Congenital Heart Disease and Repair, 2 nd ed, 2004
Fetal Circulation Lim and Everett, Congenital Heart Disease and Repair, 2 nd ed, 2004 The Ductus Arteriosus is a vascular structure between the PA confluence and the aorta designed to shunt blood away from the lungs
Shunting in Fetal Circulation 1. Ductus Venosus Disappears frequently a few hours after birth but responds to similar hormonal effects as the DA 2. Foramen Ovale Pressure closes it but it will persist in 25% of the population 3. Ductus Arteriosus Frequently has completely closed to blood flow within 24 hours in the term infant Lim and Everett, Congenital Heart Disease and Repair, 2 nd ed, 2004
Blood is oxygenated in the placenta and returns via the umbilical vein. Gives off branches to the left lobe of the liver in addition to arcuate branch to the portal system before giving off the ductus venosus. The ductus venosus then courses through the liver before joining the IVC below the diaphragm. The Ductus Venosus The proportion of blood that passes through the ductus venosus varies greatly (20-90%) Rudolph, Congenital Diseases of the Heart, 3 rd ed, 2009.
Venous Return Via the SVC SVC blood is at very low oxygen tension (near 20%) It gets preferentially directed across the tricuspid valve and into the right ventricle Less than 5% will go across the foramen ovale to the left heart Lim and Everett, Congenital Heart Disease and Repair, 2 nd ed, 2004
Lim and Everett, Congenital Heart Disease and Repair, 2 nd ed, 2004 The Foramen Ovale Formed as a communication when the septum primum only partially occludes the fenestration in the rightward septum secundum. Tends to close soon after birth but is still present in nearly 25% of adults. Preferentially shunts oxygenated blood from the ductus venosus to the left heart for delivery to the ascending aorta. The direction of shunting is important in diagnosing and managing a variety of congenital lesions
Streaming at the Foramen Ovale Oxygenated umbilical blood and left hepatic vein blood is preferentially streamed across the foramen ovale to the left heart Poorly oxygenated blood from abdominal IVC and right hepatic veins gets sent across the tricuspid valve. Rudolph, Congenital Diseases of the Heart, 3 rd ed, 2009. Again, setting up a system that preferentially shunts oxygenated blood to areas that need it the most
The Eustachian Valve Most commonly, it is a crescentic fold of endocardium arising from the anterior rim of the IVC orifice Also termed the Valve of the IVC, it further exists to direct oxygen-rich blood from the ductus venosus across the foramen ovale and into the left heart Frequently persists into adulthood, but has a huge variability in its size, thickness, origins and insertions.
The Coronary Sinus The coronary sinus receives blood from the myocardium directly initially part of the embryonic cardinal system. It is very deoxygenated and thus is designed to go to the RV through the tricuspid valve along with blood from the SVC
The Ductus Ateriosus Normal structure that measures about the same as descending aorta (near 9 mm at term) that goes from the PA confluence to an area 5 mm distal to the origin of the left subclavian Carries a significant amount of blood flow from the RV (80-90% of its cardiac output) meaning minimal blood will actually go the lungs. Further, the RV pumps about 60% of the total cardiac output meaning close to half of the fetus circulating blood volume will traverse the ductus.
The Ductus Ateriosus Can be affected by the amount of blood flow that traverses it. Further, the response of the ductal tissue to varying PaO2 s in CHD could have a theoretical effect on ductal size, similar to how the duct responds post-natally Rudolph, Congenital Diseases of the Heart, 3 rd ed, 2009.
Shunting in Fetal Circulation Once again, we have set up 3 specific areas of shunting in the fetal circulation 1. Ductus Venosus 2. Foramen Ovale 3. Ductus Arteriosus Lim and Everett, Congenital Heart Disease and Repair, 2 nd ed, 2004 The overall oxygen tension of fetal blood is lower than that in the post-natal infant and this whole system is now set up to deliver the most-oxygenated blood to the tissue that needs it the most.
Varying Saturations POSTNATAL FETAL
Cardiac Output Percentages of the combined ventricular output flowing through the major veins and arteries, and the proportions of blood ejected by each ventricle, are shown for the fetal lamb. 55% of blood flow goes through the ductus venosus meaning the 2 transient structures (DV and DA) are receiving a large portion of the cardiac output.
Changes at Birth The placenta is a very low resistor in the circuit, so with its removal systemic vascular resistance (SVR) then rises. Expansion of the lungs then leads to a nearimmediate decrease in pulmonary vascular resistance End result is a significant increase in pulmonary blood flow, and increased pulmonary venous return to the left atrium The increase in left atrial pressure will often then cause the forman flap to close (though not immediately). Lim and Everett, Congenital Heart Disease and Repair, 2 nd ed, 2004
Teitel et al., Pediatr Res 1987;22(5):557 66 Changes at Birth
Changes at Birth While pulmonary vascular resistance drops precipitously even in the first few hours from birth, it remains 2-3 times normal for up to 4 weeks postnatally. Further, there is a direct correlation between vascular resistance and overall pulmonary blood flow Rudolph Congenital Diseases of the Heart, Clinical-Physiological Considerations, 2009.
Changes at Birth CONTROL CORD CLAMPING 1. Significantly increased amount to and from the lungs. 2. Diminished amount across PDA 3. Significantly less going to body than head (now no longer have placenta) 4. Minimal traversing of the PFO. All 3 shunts from the fetal circulation start to go away with cord clamping. Rudolph Congenital Diseases of the Heart, Clinical-Physiological Considerations, 2009.
Fate of the Ductus Arteriosus The decrease in PVR followed by the increase in SVR causes reversal of flow in the ductus arteriosus from all right-to-left to all left-toright. Clip of a PDA Preterm infants will further have a higher incidence of PDA s secondary to a poor response of the ductal tissue to normal physiologic changes
The Physiology of the Fetal Cardiovascular System Jeff Vergales, MD, MS Department of Pediatrics Division of Pediatric Cardiology jvergales@virginia.edu