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Detailed information about postnatal circulatory changes in various beds, including skin, hepatic, and cardiac output. It also discusses the characteristics of fetal circulatory dynamics and the distribution of cardiac output in fetal lambs and human fetuses. The document also covers venous return to the heart and shunts in fetal circulation, as well as pulmonary circulation and oxygen exchange function.
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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.
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 Mechanisms
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.
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 distribution Fetal 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 distribution Fetal 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 distribution human 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 distribution human 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
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.
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.
Ductus venosus
Foramen ovale
Ductus arteriosus or aortic isthmus
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.
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.
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.
