Lecture 18 Fetal Circulation PDF

Lecture 17:Fetal and Neonatal Circulation MABS Physiology PRESENTED BY: Jim Mahaney, PhD WRITTEN BY: Krista Johansen, MD Recommended Reading: Note there is no chapter dealing with fetal cardiovascular physiology in the required textbook Constanzo, although some of the concepts are well described th...

Lecture 17:Fetal and Neonatal Circulation MABS Physiology PRESENTED BY: Jim Mahaney, PhD WRITTEN BY: Krista Johansen, MD Recommended Reading: Note there is no chapter dealing with fetal cardiovascular physiology in the required textbook Constanzo, although some of the concepts are well described there and in the previous cardiac lectures. Learning Objectives 1. 2. 3. 4. 5. 6. 7. 8. Recall the main physical characteristics of fetal circulation that are different from neonatal-adult circulation. Relate the relative differences in oxygen saturation and pressure for blood in the major blood vessels and cardiac chambers of the fetus. Relate why fetal cardiac output is defined as “combined cardia output.” Relate why fetal systemic resistance is low and pulmonary resistance is high compared to the opposite for newborn babies. Relate the role of shunts in fetal circulation and recall the function of the four shunts in fetal circulation. Relate how a baby’s first breaths fundamentally change the pulmonary and systemic circulations. Relate how the relative differences in oxygen saturation and pressure for blood in the major blood vessels and cardiac chambers change at birth. Relate the mechanisms causing closure of the ductus venosus, foramen ovale and ductus arteriosus at birth, and distinguish between anatomical closure versus physiologic closure. 2 Two Central Questions • What is the role of fetal circulation? • What is different between fetal and postnatal circulation? 3 Two Questions: • What is the role of Fetal Circulation? – Transport nutrients, gases and waste – Maximize exchange of transported material between mother and fetus – Provide higher oxygen supply to developing organs with high metabolic demand • What is different between fetal and postnatal circulation? – – – – – Fetal lungs are not functional Fetal liver is minimally functional Fetus relies on placenta Right and left circulations (pulmonary and systemic) are not separate Resistance: • • • Pulmonary vascular resistance is high in fetus Systemic vascular resistance is relatively low in fetus Right and left ventricular pressures are about equal in the fetus – There are 4 major shunts in the fetal circulation not present in adults 4 Fetal Circulatory Anatomy: In Class Exercise • • • Identify the unlabeled structures: – Heart Chambers – Vessels – Four major shunts of the fetus: • Placenta, Ductus Venosus, Foramen Ovale and Ductus Arteriosus Put arrowheads on the lines to indicate direction of flow and discuss it with your colleagues Put an “H” where you think the areas of highest O2 content are and an “L” on the areas of lowest and an “I” for intermediate 5 Fetal Circulatory Anatomy • Circulatory Flow Basics – Heart – Vessels – Shunts Objective 1 See notes for description of flow 6 Oxygenation • • • Note that areas of higher metabolic demand get relatively higher oxygenated blood (brain, upper body, coronary vessels) Areas of lower metabolic demand get less oxygenated blood (lungs, lower body) The umbilical vein has the highest oxygen saturation and the abdominal IVC has the lowest. Note the umbilical arteries have an intermediate oxygen saturation. Objective 2 7 Boron and Boulpaep Figures Showing Fetal Blood Flow, O2 Saturation B. Oxygen saturation/PO2 A B See notes section for description of diagrams 8 Note in this diagram the heart is drawn with the atria inferior to more easily follow the flow Cardiac Output • • • • • Postnatally there are two circulations: pulmonary and systemic (AKA right and left) Cardiac output in postnatal and adult life is the volume of blood being pumped out of the left ventricle per unit time. Usually in L/min. – Represented by CO = SV x HR “Combined Cardiac Output” or “CCO” is used in the fetus where the pulmonary and systemic circulations are not separate. CCO is the sum of the volume from the right and left ventricles. Heart is beating by week 4 at 60 BPM and at 140 BPM at term Cardiac output is increased mainly by increasing heart rate as stroke volume is near maximal (CO=HR x SV) See notes for more information Objective 3 Boron and Boulpaep 9 Note in this diagram the heart is drawn with the atria inferior to more easily follow the flow Vascular Resistance • Vascular Resistance is the force that needs to be overcome in order for blood to flow through the circulatory system. • Represented by the equation R = 8Lɳ/πr4 – Vasoconstriction increases resistance. – Vasodilation reduces resistance. • In fetus: – Systemic vascular resistance is very low • High cardiac output and flow to tissues and placenta – Pulmonary vascular resistance is high • Not much flow through lungs (7% CCO) See notes for more on resistance Objective 4 What is a shunt? Boron and Boulpaep 10 Shunts • • • What is a shunt? – Site of communication between two separate circulatory systems Why are shunts important? – They allow for highly oxygenated blood to be diverted to areas of highest oxygen demand (brain, upper extremities, coronary arteries) – Blood shunted to the placenta allows for effective maternal-fetal exchange – They allow the post-natal circulation to be in place so it is ready for action right after birth May be classified as right to left or left to right See notes for more information Objective 5 Next: What does the placenta do?11 Shunts: Placenta • • • The placenta – An organ made up of fetal and maternal tissue that develops during pregnancy – Site of exchange between mother and fetus – Connected to the fetus via the umbilical cord Functions: – Respiration – Nutrition – Metabolism – Fluid and ion regulation – Waste removal – Protection – Hormone production Why does it do all these functions? Objective 5 http://lostinnocentsorthodox.blogspot.com/p/the-actual-process.html 12 Shunts: Placenta • • • • • • ~50% of fetal blood is shunted to the placenta by the umbilical arteries Blood is returned to the fetus via a single umbilical vein These vessels travel in the umbilical cord The placenta has a HUGE surface area for transfer and is a relatively low pressure environment “Parallel circulation” of fetal system and placenta leads to LOW FETAL SYSTEMIC VASCULAR RESISTANCE Umbilical cord Which has higher oxygenation umbilical vein or arteries? Next: What does the ductus venosus do? Objective 5 Diagram shows %O2 in vessels 13 Shunts: Ductus Venosus • • • • • • Oxygenated blood from umbilical vein passes through the ductus venosus to the IVC, bypassing the liver The ductus venosus contains a sphincter which can regulate flow. Approximately 50% of the CCO passes through the ductus venosus (DV), amount in umbilical vessels/placenta Blood from DV joins the IVC to enter the right atrium So how does the liver get supplied? – Portal vein, aorta, umbilical vein – The liver is not highly functional in the fetus and does not need as much blood as in postnatal life There is an anastomosis between the distal part of the umbilical vein and the portal vein that allows for some mixing See notes Objective 5 Boron and Boulpaep Next: What does the foramen ovale do? 14 Shunts: Foramen Ovale (FO) • • • • • Foramen ovale is a hole in the interatrial septum It allows passage of blood from right atrium to left atrium, bypassing the pulmonary system 27% of the combined CO, mostly derived from the ductus venosus via the IVC passes through FO (“preferential streaming”) Blood from the SVC passes through the right atrium to the right ventricle This allows relatively highly oxygenated blood to enter the left ventricle and supply the upper body and coronary vessels via the ascending aorta Next: What does the ductus arteriosus do? See notes Objective 5 15 Shunts: Ductus Arteriosus • • • • Ductus arteriosus is a wide muscular vessel that diverts blood from pulmonary artery to descending aorta – A “right to left” shunt Patency is maintained by high levels of prostaglandins which relax the smooth muscle Only 7% of CCO enters pulmonary circulation due to HIGH RESISTANCE – Collapsed vessels – Hypoxia causing pulmonary vasoconstriction 59% of CCO enters the ductus arteriosus to the descending aorta to supply the lower body and be shunted to placenta See tip in notes section Objective 5 Diagram shows % CCO 16 Birth • What are some major differences between in utero life and ex utero life? (cardiovascularly speaking) 17 Birth • What are some major differences between in utero life and ex utero life (cardiovascularly speaking)? Need to start breathing air Placenta no longer available, need to switch to neonatal circulation • Metabolic state of the newborn: – O2 level? – Metabolic fuel? – Temperature? • Hypoxic Hypoglycemic Hypothermic Better fix this quick! 18 Birth: That first breath Objective 6 • “The hardest breath you’ll ever take” • Resistance in the pulmonary system is very high. • First breath must create enormous transpleural pressure to overcome the collapsed lungs. It gets easier after that. • What causes that first breath? – Hypoxia (not enough O2) – Hypercapnia (too much CO2) – Hypothermia (temperature too low) – Tactile stimulation – High sympathetic and other nervous system mechanisms What’s so good about respiration? 19 Birth: Breathing and the Cardiovascular System • What’s so good about respiration? – Reduces pulmonary vascular resistance • Lungs are inflated, vessels are less compressed • Higher O2 levels lead to pulmonary vessel dilation – Result: • Higher blood flow to lungs • Lower pressure in respiratory system • Lower pressure in right ventricle • Key point about high O2: – It causes pulmonary artery vasodilation – It causes systemic artery vasoconstriction Objective 6 First Breath: https://www.youtube.com/watch?v=5u5fEVTi3Xw#action=share 20 Fetal vs Neonatal Circulation What do you notice? What has to happen? Objective 7 21 Fetal VS. Neonatal Circulation: Shunt Closure Objective 8 In fetal circulation shunts are open, postnatally the shunts close. Two types of closure: • Physiologic closure: physiological changes lead to the reduction or absence of flow through the shunt: ‒ Example: Change in pressure preventing flow • Anatomic closure: structural changes lead to the absence of flow through the shunt. ‒ Example: Fibrous obliteration of shunt Physiologic closure usually happens first and is more rapid, anatomic closure may take place over time 22 Birth: Placental Independence • During gestation, the placenta received 40-50% of the fetal cardiac output. • Even before the umbilical cord is cut, umbilical arteries constrict because of higher oxygenation, increasing systemic vascular resistance (physiologic closure). • Loss of the placental circulation doubles the total peripheral resistance • The systemic circulation is now a “high pressure” circulation and aortic pressure and left ventricular pressures increase. Objective 8 http://ghanahealthnest.com/heal/wp-content/uploads/2013/07/cord-clamping1.jpg “Cutting the cord” 23 Shunt Closure: Ductus Venosus • • • • At birth, flow ceases through the umbilical vein however there is still portal flow through the ductus venosus via an anastomosis Within 3 hours, ductus venosus constricts (physiologic closure, mechanism unknown) and shunt is closed Result: – Increase in portal vein pressure and increase in blood flow to the liver – Decrease in venous return to the right atrium (IVC only carries systemic blood, not blood from ductus venosus) leads to decrease in right atrial pressure Ductus venosus fills with connective tissue (anatomic closure) becomes ligamentum venosum in the liver and the umbilical vein becomes the round ligament of the liver (we will see this in the next unit) See notes Objective 8 Fetus Neonate 24 Shunt Closure: Foramen Ovale • Decrease in pulmonary vascular resistance leads to – INCREASE in pulmonary venous return and – INCREASE in left atrial pressure • Venous return to right atrium DECREASES which leads to DECREASE in right atrial pressure • Result: – No more right to left shunt • Why doesn’t the flow reverse? – Valve in left atrium prevents backflow of blood (physiologic closure) – Valve fuses to atrial wall over time (anatomic closure) • Foramen ovale becomes the fossa ovalis in the interatrial septum Yellow pin piercing fossa ovalis in right atrium Objective 8 Valve of foramen ovale seen from 25 left atrium (fused) Shunt Closure: Ductus Arteriosus • • • • Immediately after birth the ductus arteriosus is STILL OPEN Which direction will the blood flow? – Opposite from fetal life: higher pressure in the aorta and lower pressure in the pulmonary trunk reverses the flow (LEFT to RIGHT shunt) Within a few hours, smooth muscle in ductus arteriosus constricts, ceasing flow (physiologic closure) – This is caused by increased oxygenation of blood flow through this vessel (from aorta) – There is also a decrease in circulating prostaglandins Within about a month, connective tissue obliterates the lumen of the ductus arteriosus resulting in the ligamentum arteriosum (anatomic closure) See notes Objective 8 Ligamentum arteriosum 26 Summary of the Newborn Cardiovascular System • • • • • Breathing leads to decrease in pulmonary vascular resistance and decrease right ventricular pressure. Lungs become functional. Loss of placenta leads to increase in systemic vascular resistance and increase pressure in aorta and left ventricle. Cardiac output of the left ventricle increases (increase in stroke volume). Shunts critical for fetal survival close Right and left circulations are separated But they are not adults: – O2 saturation is similar to adults but fetal hemoglobin persists and O2 saturation curve is left shifted (you learned about this curve in respiratory physiology) – The systemic vascular resistance (SVR) is higher than in fetal life but still lower than in adults (average blood pressure 75/40) – Because of low SVR neonates have relatively high blood flow despite lower perfusion pressure – Average heart rate of newborn 100-160 (sleeping vs. crying) 27 Summary: Questions to Ponder • • • • • • • • • • • What is the role of fetal circulation? What is different between fetal and postnatal circulation? Where is resistance high in the fetus? Low? What is the pattern of circulation in the fetus? Where is oxygenation highest in fetus? Where is it lowest? What are the four shunts? What are their functions? What does breathing do for the newborn? What effect does high oxygen levels have on pulmonary vasculature? On systemic vasculature? What happens when the placental connection is lost? What causes the umbilical arteries to constrict even before cutting the cord? How and when do each of the shunts close? What is the stimulus for each? What is the result? What is the difference between physiologic and anatomic closure of a shunt? 28 Sample Problem 1 Which of the following fetal vessels would be expected to contain blood with the greatest oxygen saturation? A. B. C. D. Aorta Inferior vena cava at the right atrium Pulmonary artery Superior vena cava at the right atrium 29 Sample Answer 1 Which of the following fetal vessels would be expected to contain blood with the greatest oxygen saturation? A. B. C. D. Aorta Inferior vena cava at the right atrium Pulmonary artery Superior vena cava at the right atrium 30 Sample Problem 2 Which of the following is NOT a shunt found in the fetal circulation? A. B. C. D. E. Ductus arteriosus Ductus venosus Foramen ovale Inferior vena cave Placenta 31 Sample Answer 2 Which of the following is NOT a shunt found in the fetal circulation? A. B. C. D. E. Ductus arteriosus Ductus venosus Foramen ovale Inferior vena cava Placenta 32