FETAL CIRCULATION

FETAL CIRCULATION IN HEALTH AND DISEASE

Characteristics of fetal circulatory dynamics • Parallel arrangement of two main arterial systems and their respective ventricles. • Mixing of venous return and preferential streaming. • High impedance and low flow of pulmonary circulation. • Low impedance and high flow of placental circulation. • Presence of shunts.

Cardiac output and its distributionFetal lamb • CVO is 450 ml/kg/wt • RV ejects 2/3 and LV ejects 1/3 of CVO • 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 circulation

Cardiac output and its distributionFetal lamb • 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 distributionFetal lamb • 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 66% of CVO. • About 35 ml/mt [8% of CVO] enters the pulmonary circulation

Cardiac output and its distributionFetal lamb • About 265 ml/mt [60%]passes through ductus arteriosus. • LV ejects 150 ml/kg [ 33% ]. • 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.

60% 20% AA 33% 21% 8% 28 19 18 66% 70% 24% 24 70% 21% 32 45% 55%

65% 65% 50% 55% 35%

Cardiac output and its distributionhuman fetus • Limited data only is available based on doppler studies. • Umbilical blood flow is 180 ml/mt /kg of estimated fetal weight. • Pulmonary blood flow is estimated to be 75 ml/kg of fetal weight

Cardiac output and its distributionhuman fetus • CVO appears to be similar to that in lamb, 450 ml/mt/kg fetal weight • Ratio of RV output to LV output is only 1.2 to 1.3 as compared to 2:1 in fetal lamb

140 45 175 15 75 250 200 220 95 125 140 75 180 15 220 40

31 10 39 3 17 58 44 49 31 28 21 17 3 49 39 10

Venous return to heart • Umbilical vein gives branches to left lobe of liver and then divides into DV and arcuate vien. • Arcuate vien joins the portal vein and then gives of branches to right lobe of liver. • Left hepatic vein joins the DV at it’s entry to IVC and Right hepatic vein joins the IVC directly.

Venous return to heart • Right lobe of liver 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 heart • Posterior 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 heart • Eustechian valve helps to direct the IVC blood to cross the foramen ovale. • The lower margin of septum secundum [christa dividens] helps to direct the left posterior stream to preferentially across the foramen ovale. • SVC blood is directed aross the TV.

Shunts in fetal circulation • Ductus venosus • Foramen ovale • Ductus arteriosus or aortic isthmus

Shunts in fetal circulation • The blood returning to heart through venacavae and then redistributed to tissues without being delivered to placenta represents effective R to L shunt. • The blood which passes through DV and then reaches DA and goes to placenta without getting distributed to tissues represent effective L to R shunt. • Combined R to L and L to R shunts forms 33% of CVO.

PULMONARY CIRCULATION • Fetal lung does not serve gas exchange function. • PVR is high and PBF is low. • This helps to reduce workload of fetal heart.

PULMONARY CIRCULATION • MPA continues as Ductus and RPA and LPA arise as branches. • Medial layer is composed of smooth muscle predominantly in small pre acinar and large acinar level arteries. • Further branches have no muscular component.

PULMONARY CIRCULATION • PA pressure rises gradually paralleling the rise in aortic pressure. • TPR falls gradually but this fall when correlated with rise in lung weight, there is actually an increase in PVR towards term . • PBF increases gradually.

PULMONARY CIRCULATION • MPA has forward flow throughout systole with a short period of backflow at end of systole. • DA also has forward flow throughout systole. • BPA has forward flow only through initial one third of systole followed by back flow through rest of systole and diastole. • In humans forward flow is more prolonged.

PULMONARY CIRCULATION • Experiments 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.

PULMONARY CIRCULATION • Fetal pulmonary endothelium behaves in a similar fashion as adult endothelium to vasodilators. • Adrenomedullin has a potent and prolonged vasodilatory effect. • Leukotriens may be responsible for maintaining high fetal PVR.

PULMONARY CIRCULATION • Breathing 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.

Oxygen exchange function • Higher 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 function • These 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. • This is not significant in fetal hemoglobin.

Oxygen exchange function • As 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.

Oxygen exchange function • As O2 is released,maternal Hb acts as a buffer that removes H+ from local environment. • This encourages production of bicarbonate from H2O and CO2 thereby reducing local PC02 and facilitating diffusion of CO2 from fetus.

Post natal changes • Gas exchange function is transferred from placenta to the lungs. • Separation of systemic and pulmonary circulations • Increased metabolism to maintain body temperature and hence increased cardiac output.

Post natal changes in various circulatory beds • Coronary 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 beds • Skin 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 increaes.

Post natal changes in various circulatory beds • Hepatic 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 output • Oxygen consumption increases from 6-8 ml/mt/kg body weight pre natally to 15 –20 ml/mt/kg post natally. • CVO of fetal lamb is 450 ml/mt/kg. • C.O of neonatal lamb is 300 -425 ml/mt/kg.So the CVO will be 600 -850 ml/mt/kg. • So the increase is 1.5 to 2 times.

Changes in Cardiac outputMechanisms • Neonate 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.