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Fetal circulationDr Shreetal Rajan Nair Fetal circulationFetal circulation : Physiological and morphological aspectsPost natal and transitional circulation: Changes at birth and thereafterPathophysiological considerations in CHDsAll our present day knowledge about fetal circulation is based on continuing research of more than 40 years.

Most of the work has been done in fetal lambs whose circulation closely resembles to that of human fetal circulation.Fetal circulation Fetal circulationCentral circulation Arteries Veins Shunts Peripheral components Various regional vascular beds

Salient featuresPresence of shuntsParallel arrangement of two main arterial systems and their respective ventricles.Preferential streaming of blood High impedance and low flow of pulmonary circulation.Low impedance and high flow of placental circulation.Placenta is the site of gas exchange

Shunts in the fetal circulation The fetal circulation is characterised by four shunts.

first, within the placentasecond, across the ductus venosusthird, through the foramen ovalefourth, across the ductus arteriosus The PlacentaFacilitates gas and nutrient exchange between maternal and fetal blood.The blood itself does not mix.

Umbilical CirculationPair of umbilical arteries carry deoxygenated blood & wastes to placenta.

Umbilical vein carries oxygenated blood and nutrients from the placenta.

Arrangement of blood vessels in placenta

Oxygen exchange functionHigher hemoglobin level in fetus as compared to mother facilitates oxygen uptake by the fetus in the placenta.Oxygen dissociation curve of fetal red cells is shifted to left as compared to adult red cells.HbF has less affinity towards organic phosphates like 2,3 DPG and ATP.

Oxygen exchange functionThese phosphates that are present in red cells compete with oxygen for binding to hemoglobin.Affinity of reduced hemoglobin to 2,3 DPG is higher than that of oxyhemoglobin and this facilitates oxygen delivery at tissue site.

Oxygen exchange functionAs CO2 crosses placenta from fetus to mother, it creates a local acidosis.In the face of decreasing pH, mothers hemoglobin shows less affinity towards Hb and oxygen release is enhanced[Bohr effect]This supports diffusion of more oxygen across the diffusion membrane to fetus. Umbilical vein to portal circulationSome blood from the umbilical vein enters the portal circulation allowing the liver to process nutrients.The majority of the blood enters the ductus venosus, a shunt which bypasses the liver and puts blood into the hepatic veins.

Foramen ovaleBlood is shunted from right atrium to left atrium, skipping the lungs.

Is a valve with two flaps that prevent back-flow.

Foramen ovale

Ductus arteriosusThe blood pumped from the right ventricle enters the pulmonary trunk. Most of this blood is shunted into the aortic arch through the ductus arteriosus.

Fetal circulation physiologyMost of the unsaturated blood reaches the right ventricle and this blood is channelled via the ductus arteriosus and descending aorta to the placenta for oxygenation

Most of the saturated blood reaches the left ventricle which is driven into the ascending aorta to the heart and brain

Ref : Moss and Adams textbook of congenital heart diseasesRef : Moss and Adams textbook of congenital heart diseases21How this is achieved? By the appropriate mixing of the venous blood returning to the heart

And its preferential streaming Central venous circulation5 sources: upper body - SVC myocardium coronary sinus the lungs pulmonary veins the lower body - IVC placenta umbilical vein ductus venosus IVC

Least saturated blood

From upper body via SVC

From myocardium via coronary sinus

This blood is directed to the RV via the tricuspid valveVenous return from lungsNot well saturated Preferential flow to RV is not possible because of the normal drainage of pulmonary veins to LABut pulmonary blood flow only 8% of combined ventricular outputSo no appreciable effect on oxygen delivery

Inferior venacaval returnLower body

Placenta

How this blood is preferentially directed to the RVSVC blood is directed away from the foramen ovale and through the tricuspid valve due to the leftward and superior course of the eustachian valve The location of the coronary sinus caudad to the foramen causes venous blood from the myocardium to flow through the tricuspid valve to the RV How preferential flow occursLower body flow except that from the liver ascends the distal IVC and this stream of relatively desaturated blood enters the lateral margin of the right atrium and directed to the RV via TVUmbilical venous return to the LVVenous flow from the liver from right lobe to the RV and the left lobe to the LV via foramen ovale

Venous return to heartUmbilical vein gives branches to left lobe of liver and then divides into DV and arcuate vein.Arcuate vein joins the portal vein and then gives of branches to right lobe of liver.Left hepatic vein joins the DV at its entry to IVC and Right hepatic vein joins the IVC directly.

Venous return to heartRight lobe of liver receives poorly oxygenated portal venous blood and left lobe receives well oxygenated umbilical venous blood.Both lobes receive small contribution of blood from hepatic artery.Saturation of RHV is lower than that of LHV.Venous return to heartPosterior and left stream of IVC blood carries oxygenated blood while anterior and right stream carries poorly oxygenated blood.Preferential streaming of DV and LHV blood across the foramen ovale and abdominal IVC and RHV blood across the TV.

Venous return to heartEustachian valve helps to direct the IVC blood to cross the foramen ovale.The lower margin of septum secundum [crista dividens] helps to direct the left posterior stream to preferentially across the foramen ovale.SVC blood is directed aross the TV.

THE PRENATAL PULMONARY CIRCULATIONPhysiologic regulation of pulmonary vascular resistance Pulmonary vascular resistance in the fetal lung is initially highThe most prominent factor associated with high fetal pulmonary vascular resistance is the normally low blood O2 tension (pulmonary arterial blood pO2 = 17 to 20 torr).PVR in FETUS is highIn the fetus and newborn, all small pulmonary arteries have a thicker medial smooth muscle layer in relation to diameter than similar arteries in adults.This increased muscularity is partly responsible for the increased vasoreactivity and pulmonary vascular resistance in the fetus, particularly near term.PULMONARY CIRCULATIONExperiments show fetal PBF increases dramatically in response to increase in maternal PO2.This response is evident only in latter part of gestation.Doppler studies indicate similar changes in humans as well.Modulation of pulmonary vascular toneOxygen modulates the production of both prostacyclin and endothelium-derived nitric oxide (EDNO); two potent vasoactive substances that may in part underlie the responses of the developing pulmonary circulation to changes in oxygenationVasoconstrictors in the pulmonary circulation of the fetus, such as alpha agonists, thromboxane, and the leukotrienes are other mediators of increased pulmonary vascular toneCombined ventricular outputThe combined ventricular outputThe term 'combined ventricular output(CVO)' has been applied to the output of the two ventricles, and it also represents total venous return to the fetal heart.

The right ventricle ejects about 55-65%, and the left only 35-45% of CVO.

Adapted from Hursts THE HEARTHursts THE HEART43Cardiac output and its distribution

CVO is 450 ml/kg/wt

UV flow is 200 ml/mt/kg [45% of CVO]

Of this,110 ml/mt [24%] passes through DV and 90 ml/mt[21%] passes through hepatic circulationCardiac output and its distribution

Portal venous flow forms 7% and of CVO and abdominal IVC blood forms 30% of CVO.

Total venous return to heart from IVC is 315 ml/mt and represents 70% of CVO.

Of this 115 ml/mt [25% of CVO] passes through FO and and 200 ml/mt [44%] passes through TV.Cardiac output and its distribution

Venous return to heart from SVC is 90 ml/mt/ and represents 21% of CVO most of this passes through tricuspid valve.

RV ejects about 300 ml/mt or about 55% of CVO.

About 35 ml/mt [8% of CVO] enters the pulmonary circulation Cardiac output and its distribution

About 265 ml/mt [60%]passes through ductus arteriosus.

LV ejects 150 ml/kg [ 45% ].

Of this,90 ml/mt [20%] distributed to head and upper half and 45 ml/mt [10%]passes through isthmus.

3% of CVO enters coronary circulation.

Cardiac output in human fetus Fetus increases the cardiac output by increasing the heart rate as it is incapable of increasing the stroke volume Myocardium is underdeveloped Fluid content is more Myocardium surrounded by fluid filled lungBLOOD OXYGEN SATURATIONS

Adapted from Hursts THE HEART

Blood oxygen saturationsThe highest partial pressure of oxygen (Po2) is found in the umbilical vein (32 mm Hg) The brain and coronary circulation receive blood with higher oxygen saturation (Po2 of 28 mm Hg) when compared to the saturation in the blood supply to the lower body (Po2 of 24%)The atrial and ventricular pressures The wide communication at the atrial level (foramen ovale) allows for near equalization of atrial and ventricular end-diastolic pressures. Similarly, at the great vessel level, the nonrestrictive ductus arteriosus allows equalization of systolic pressures in the aorta and the pulmonary artery and, in the absence of aortic or pulmonic stenosis, at the ventricular level

Adapted from Hursts THE HEART

Rudolph congenital heart diseases55 What happens at birth?The change from fetal to postnatal circulation happens very quickly.Changes are initiated by babys first breath.

Post natal circulationThe changes in the central circulation at birth are primarily caused by external events rather than by primary changes in the circulation itself

What are the changes?rapid and large decrease in pulmonary vascular resistancedisruption of the umbilical-placental circulationAbruptly at birth, the ductus arteriosus changes from a right-to-left conduit of blood to the descending aorta, to a left-to-right conduit of blood to the lungsPOST NATAL PULMONARY CIRCULATION PULMONARY CIRCULATIONBreathing at birth is associated with a marked fall in PVR and rise in PBF.PA pressure does not fall as rapidly and remain elevated till the ductus is widely patent.Once the ductus is closed, PA pressure can vary independent of systemic pressure. The transitional circulationIn humans by 24 hours of age, mean pulmonary arterial blood pressure may be only half systemic. After the initial rapid decrease in pulmonary vascular resistance and pulmonary arterial blood pressure, there is a slow, progressive decrease, with adult levels reached after 2 to 6 weeks. This is due to vascular remodeling, muscular involution, and rheologic changes.

In the first 4 to 6 weeks after birth, there is progressive involution of the circumferential medial smooth muscle with overall reduction in medial muscular thickness of the walls of the small pulmonary arteries

CLOSURE OF THE SHUNTSDuctus arteriosus closurePostnatal closure of the ductus arteriosus is effected in two phases. Immediately after birth, contraction and cellular migration of the medial smooth muscle in the wall of the ductus arteriosus produce shortening, increased wall thickness, and protrusion into the lumen of the thickened intima (intimal cushions or mounds), resulting in functional closure . This commonly occurs within 12 hours after birth in full-term human infants The second stage usually is completed by 2 to 3 weeks in human infants, produced by infolding of the endothelium, disruption and fragmentation of the internal elastic lamina, proliferation of the subintimal layers, and hemorrhage and necrosis in the subintimal region. The mounds enlarge progressively, and there is connective tissue formation and replacement of muscle fibers with fibrosis and permanent sealing of the lumen to produce the ligamentum arteriosumClosure of the ductus arteriosusThe increased level of oxygen probably causes vasoconstriction of the ductal musculature, but there are strong suggestions that a reduction in circulating prostaglandins of the E series plays a roleOxygen, prostaglandin E2 (PGE2) levels, and maturity of the newborn are important factors in closure of the ductus. Acetylcholine and bradykinin also constrict the ductus.

Closure of the umbilical vein and ductus venosusAs the placental circulation stops at birth the flow to the umbilical vein and hence the ductus venosus stopsIt closes by a process of fibrous obliteration of the lumen almost similar to the process involved in the closure of the ductus arteriosusIt completely obliterates by the seventh postnatal dayClosure of the foramen ovaleClosure of the foramen ovale at birth is entirely passive, secondary to alterations in the relative return of blood to the right and left atriaAt birth combined ventricular output returning from the lung changes from 8% prenatally to >50% Left atrial pressure thus exceeds right, and the redundant flap of tissue of the foramen ovale that previously bowed into the left atrium is now pressed against the septumFate of the shuntsForamen ovaleCloses shortly after birth, fuses completely in first year.Ductus arteriosusCloses soon after birth, becomes ligamentum arteriosum in about 3 months.Ductus venosusLigamentum venosumUmbilical arteriesMedial umbilical ligamentsUmbilical vein Ligamentum teresBlood circulation in various regional vascular bedsPost natal changes in various circulatory bedsSkin blood flow is high in utero as the vessels are dilated because the skin is exposed to warm amniotic fluid.Cutaneous vasoconstriction occurs post natally as evaporation from skin starts.Cutaneous flow falls and the vascular resistance increases. Post natal changes in various circulatory bedsCoronary Blood flow decreases dramatically as the oxygen content increases.

Cerebral circulation also behaves in the same fashion as coronary circulation. Post natal changes in various circulatory bedsHepatic blood flow falls rapidly post natally with reduction in umbilical venous return and then increases as the GI flow is re established.Hepatic blood flow progressively increases after birth and by 7 days after birth reaches a level of 250 ml/minute /100 g by which time there is no flow through ductus venosus.Changes in Cardiac outputOxygen consumption increases from 6-8 ml/mt/kg body weight pre natally to 15 20 ml/mt/kg post natallyChanges in Cardiac outputMechanismsNeonate has to increase the metabolism to increase the body temperature as it is exposed to external temperature.Improved diastolic function due to removal of compression by maternal organs and uterus causes increased cardiac filling and hence the cardiac output.Changes in Cardiac outputMechanismsPerinatal increase in thyroid hormones is the principal mechanism for increase in cardiac output.

Improvement in myocardial growth and maturation brought about by cortisol may also play important role.Changes in hemoglobin and tissue oxygen deliveryHuman new born has a high hemoglobin level (about 16g/dl) so that the oxygen carrying capacity is quiet high and the total amount of oxygen transported to tissues is quiet high.Since the Hb F levels are still high facilitation at tissue site is not as great as in adults.Over the first 8-10 weeks after the birth,Hb concentration falls to 10-11 g/dl. This is accompanied by loss of Hb F and almost 100% is adult type. Fetal circulation in pathological conditionsFetal circulation in pathological conditionsDevelopment of a structural abnormality will modify the fetal circulation.This will affect the development of other components and can lead to other defects.The impact of a defect will depend on its severity and time of gestation at which it occurs. Fetal circulation in pathological conditionsCardiovascular malformations may: cause hydrops fetalis by increasing venous pressure change the volume or direction of blood flow cause obstruction to blood flow alter the oxygen saturation of blood delivered to various organs.Fetal circulation in pathological conditionsMany of the defects, though it modifies the circulation, will not significantly affect fetal perfusion and hence the growth and development.This is because of the presence of shunts and mixing of blood.Fetus tolerates the obstructive lesions very much.Fetal circulation is jeopardized by regurgitant lesions and myocardial disease.

Pulmonary edema in a fetusIncreased fluid accumulation in the lung of the fetus is manifested as pulmonary lymphangiectasia.This occurs only in those conditions in which pulmonary venous pressure can be raised to high levels, such as total pulmonary venous drainage with obstruction and aortic or mitral atresia with a small or closed foramen ovale.

PDA O2PGE2,PGE1 and PGI2 concentrationPulmonary vascular resistance Surfactant replacement therapyAbility of fetal myocardium to handle cardiac outputSeptal defectsThey in general do not modify the fetal circulation significantly.VSD may have a transient left to right shunt in systole.In OP ASD, due to close proximity of defect with TV, more than normal amount of SVC blood may enter the LA. Septal defectsIn atrioventricular septal defects, the obligatory flow from LV to RA will result in decrease in LV output and an increase in RV output.This will reduce the flow across the isthmus and can predispose to coarctation.It is the degree of severity of AV valve lesion and regurgitation which will determine the outcome.PATENT FORAMEN OVALEASSOCIATIONSCryptogenic strokedecompression sickness (arterial gas embolism from the venous side) migraine headaches.Platypnea-orthodeoxia syndrome (dyspnea and arterial desaturation in the upright position, which improves on lying down)LVOT ObstructionSevere obstruction developing early result in a small LV with an increased mass.

RV is able to compensate fully if LVOTO develops slowly.

LVOT ObstructionSVC flow courses normally.

Majority of IVC blood flow crosses TV to RV.

Flow across the ductus increases.

PBF has higher than normal saturation.

A retrograde flow in arch and ascending aorta indicates severe obstruction

706065626075/490/575/5070/45Aortic arch abnormalitiesMost of the alteration in the circulation are due to co existing intra cardiac defects.Common features are, reduced flow in to ascending aorta, increased flow in to the pulmonary trunk and greater proportion of CVO carried across ductus to descending aorta.The decreased volume loading of LV may possibly interfere with its development. Mitral and aortic atresiaAll blood must pass through RV and ductus has to provide for both AA and DA blood flow.Complete mixing of blood occurs in RA and saturation in PA,AA and DA are all same.If the foramen ovale is sufficiently large and ductus accomodates whole of systemic blood flow, there will be no significant interference with intrauterine development and survival. Left to right flow through foramen ovale seen.

704060606070/270/40Mitral and aortic atresiaIf the foramen ovale is restrictive, severe pulmonary venous hypertension develops.If the ductus does not enlarge to accommodate the whole of systemic blood flow increased blood flow to lungs and pulmonary hypertension develops.Both these can lead to increased development of smooth muscle in the pulmonary vasculature.

70406363 6363100/470/3RVOTO with intact IVSIn pulmonary atresia, all systemic venous blood is carried to left side through foramen ovale and all blood supply to DA is provided through isthmus.Larger than normal foramen ovale, left sided chambers,AA and aortic isthmus.RVOTO with intact IVSIf severe RVOTO develops early in gestation,the flow through the ductus is reversed and carries only 8 to 10 % of cardiac output.The ductus will be narrower and will make an acute inferior angle with aorta.The ductus will remain patent for longer than normal durationRVOTO with intact IVSIf the fetus develops significant TR,RV pressure remains low and myocardial sinusoids and coronary fistulae do not develop.If TR does not develop,significant RV systolic pressure develops and if occurs early in gestation, intramyocardial sinusoids and coronary fistulae develop.TAPVCUsually does not affect the development of fetus.If whole of PV return drains to RA,LV will be totally free of PV blood and hence will be of higher saturation.Left atrium and left ventricle will be relatively small in TAPVC.TOF and related disordersDoes not appear to affect fetal circulation adversely.The volume and direction of flow across the PA and ductus are dependent on the severity of obstruction.Total flow through the ductus will be reduced considerably if there is severe RVOTOTOF and related disordersThis can markedly reduce the diameter of fetal ductus and also reduce the development of smooth muscle in its wall.If blood flows from aorta to PA in fetal life,the orientation of ductus changes and it forms an acute inferior angle with aorta.AA and the isthmus carries large than normal amount of blood and they tend to be larger.

705565636070/470/470/45TOF with absent PVDuctus is frequently either atretic or not developed.RV would be subjected to both volume and pressure overload and this can develop in utero.Significant pulmonary regurgitation can seriously affect perfusion of pulmonary vessels and cause abnormal development of intrapulmonary vessels.

TGACompatible with fetal survival and normal intrauterine development.Does not affect the pattern of venous return.Blood with higher oxygen saturation will go to lungs. This will reduce the PVR and hence increase in PBF. TGAThis will reduce the blood flow across the ductus and increases the flow across the isthmus.Blood with lower oxygen saturation perfuses coronary and cerebral circulation.Hence cerebral and coronary blood flow are increased considerably.

70405570556570/470/370/4570/45TGA with VSDBidirectional shunting occur depending on after load of each ventricle.

The difference in saturation between AA and DA will be lesser.TGA with VSD and PSAlmost complete admixture of SVC and IVC streams in RV.AA and DA will have similar oxygen saturations.Blood flow in the ductus will be from aorta to PA.

70405565 656570/470/470/45Truncus arteriosusVarious degree of mixing occurs just above the semilunar valve.Degree of mixing depends on morphology.In type 1,there is differential streaming of blood and in others there will be complete mixing. Ductus is usually small or absent and increased flow traverses through isthmus and it is large.Ebsteins anomalySevere TR can manifest as in utero cardiac failure especially if foramen ovale is restrictive.Marked enlargement of RA and atrialised RV can cause septal displacement and compromise LV output.Functional pulmonary atresia can result and ductal flow may be reversed.

Ebsteins anomalyMarked enlargement of right atrium can cause pulmonary hypoplasia Severe TR alters the preferential drainage of venacaval blood and causes complete mixing of blood in right atrium. Tricuspid atresiaAll of the venous return traverses foramen ovale and it is considerably larger than normal .Complete admixture of blood in the left atriumIt is compatible with normal intrauterine development and survival. Tricuspid atresiaIf IVS is intact, whole PBF is from aorta through ductus and ductal flow is lesser than normal.75% of combined VO traverses the isthmus and it tends to be larger.In the presence of VSD, the flow pattern is decided by the size of the defect and presence of pulmonary stenosis.

Some clinical pearlsUmbilical vein catherisation Umbilical vein catheterization may be a life-saving procedure in neonates who require vascular access and resuscitation. Indications To gain vascular access during emergency resuscitation,central venous access in neonate,exchange transfusionsContraindications Omphalitis,peritonitis,NECSome clinical pearlsThe fact that the right lobe of the liver receives blood of considerably lower oxygen saturation probably explains the frequent presence of a larger number of hemopoietic cells in the right as compared with the left lobe of the liverREFERENCES1.BRAUNWALDS HEART DISEASES2.HURSTS THE HEART3.RUDOLPHS CHD4.MYUNG PARKS PEDIATRIC CARDIOLOGY5. Distribution and regulation of blood flow in the fetal and neonatal lamb;A M RUDOLPH; Circ Res. 1985;57:811-8216.Preferential streaming of ductus venosus blood to the brain and heart in fetal lambs;Edelstone DI, Rudolph AM; 1979 Dec;237(6):H724-9; Am J Physiol.7. The ductus venosus;Kiserud T; Semin Perinatol. 2001 Feb;25(1):11-20.8. Assessment of Flow Events at the Ductus VenosusInferior Vena Cava Junction and at the Foramen Ovale in Fetal Sheep by Use of Multimodal Ultrasound Klaus G. Schmidt,MD; Norman H. Silverman,MD; Abraham M. Rudolph,MD; Circulation. 1996; 93: 826-833

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Flow Chart of Fetal Circulation