Unit 7 - Fetal and Neonatal Circulation

Question 1

what are the 4 unique shunts of fetal circulation?

Answer 1

1. placenta - blood from descending aorta passes mostly through hypogastric arteries into 2 umbilical arteries and into placenta (small portion to hindquarters and mesentery)
2. ductus venosus - O2ated blood from placenta to umbilical vein passes thru DV to bypass liver, then pass into IVC
3. foramen ovale - 40% of IVC –> RA flow passes thru FO into LA; blood that remains in RA is directed thru tricuspid valve to RV
4. ductus arteriosus - most of the de-O2-ated blood in RV passes thru DA to aorta (very small percentage to lungs)

Question 2

where is O2 saturation highest in fetal circulation?

Answer 2

umbilical vein (85%; straight from placenta)
-it drops as deoxygenated blood mixes

Question 3

what happens when placental circulation is lost?

Answer 3

newborn needs to breathe on its own (switch from placenta to lung for gas exchange)

Question 4

what happens to pulmonary blood flow in a newborn? pulmonary vascular resistance? mean pulmonary arterial pressure and why?

Answer 4

there is a dramatic increase in pulmonary blood flow (since fetus has very low usage of lungs) due to a massive fall in vascular resistance
-mean pulmonary arterial pressure also decreases, b/c fall in pulmonary vascular resistance is greater than the rise in blood flow

Question 5

what is the first breath of a baby triggered by?

Answer 5

mild hypoxia, hypercapnia, tactile stimuli, and cold skin

• first inspiratory effort is most difficult; needs transpulmonary pressure of 60 cm H2O to increase lung volume by 40 mL (in adult, need only 2.5 cm H2O for 500 mL air)
• breathing is easier once alveoli are open, and type II alveolar pneumocytes deliver surfactant to air-water interface

Question 6

why does pulmonary vascular resistance fall after birth?

Answer 6

1. pulmonary blood vessels are no longer being crushed
2. breathing causes increased PO2 which, in turn, causes vasodilation
3. local prostaglandins cause vasodilation

Question 7

what 4 essential functions of other organ systems do the placenta do?

Answer 7

gas exchange (lungs)
nutrition (GIT)
waste removal (liver)
fluid/electrolyte balance (kidneys; dialyzer)

Question 8

what does insufficiency of placenta cause?

Answer 8

type II intrauterine growth restriction (IUGR)

• normal cell number, but smaller cells
• anything that interferes with gas exchange may lead to fetal asphyxia

Question 9

when does the fetal heart start beating?

Answer 9

1 month after fertilization; HR increases from 65 to 140 bpm near birth

Question 10

how long does it take for fetal organs to be established?

Answer 10

at one month, all organs of fetus are formed

• 2-3 months for minute details and become similar to newborn
• nervous system, kidneys, and liver need >9 mo for full development

Question 11

what is the intervillous space?

Answer 11

contains pool of maternal blood for gas exchange, and drained by maternal veins

• is between endometrium on maternal side and villi on fetal side
• outer surface of mature chorionic villus is covered with thin layer of syncytiotrophoblast
• under this are cytotrophoblasts, mesenchyme, and fetal blood vessels

Question 12

what are “spiral” arteries?

Answer 12

in mature placenta; from mother and empty directly into intervillous space, which is drained by maternal veins

Question 13

how does the placenta close after birth?

Answer 13

once lung of baby starts breathing and PE2 circulation decreases, umbilical cord reacts to increased PO2 to contract

• placenta receives 50% of CCO in fetal circulation
• closure causes peripheral resistance to double, causing increase in aortic and left atrium/ventricle pressure

Question 14

how does the foramen ovale close? what happens if it fails?

Answer 14

increased pulmonary circulation causes increased venous return to left atrium, causing decrease in right arterial pressure

• this reversal of pressure closes the FO’s valve, and a permanent seal forms in a few months/years (fossa ovalis)
• failure to close = patent FO

Question 15

how does the ductus venosus close? what happens if it fails?

Answer 15

unknown mechanism, probably increased PO2 and reduced prostaglandins

• within 3 hours of birth, constriction of vascular smooth muscle within ductus venosus completely occludes shunt to become ligamentum venosum
• failure to close = portosystemic shunt

Question 16

how does the ductus arteriosus close? what happens if it fails?

Answer 16

increased PO2 and decreased circulating prostaglandin causes constriction; bradykinin from lung is somehow involved

• within a few hours, the DA closes and becomes constriction of wall
• thrombosis obliterates the lumen in a month to become ligamentum arteriosum
• failure to close = patent ductus arteriosus; leads to pulmonary hypertension and maybe congestive heart failure and cardiac arrythmias

Question 17

what happens to the fluid filling the alveoli after birth?

Answer 17

majority is absorbed by pulmonary circulation

• some absorbed by pulmonary lymphatics
• small portion expelled out trachea during birth

Question 18

patent ductus arteriosus

Answer 18

1: 2000; possible to repair with surgery
- if small opening causes murmur, but no symptoms
- if large opening, breathing is hard, feeding is poor, HR and sweating are elevated and weight is low
- endocarditis risk is life-threatening as germs adhere to valves, heart, and/or ductus where turbulance destroys smooth lining of vessels

Question 19

coarctation of aorta

Answer 19

smooth muscle of ductus arteriosus that invaded descending aorta constricts the area following birth
-associated with high BP in regions perfused by aorta above constriction

Question 20

ventricular septal defects

Answer 20

most common birth defects (0.1 - 0.4% of all births, and 20-30% of congenital heart lesions)

• hole in wall between ventricles
• if small, will close by itself w/o problem
• if large, in first few months of life will cause problems b/c blood flows from LV to RV, cause congestive heart failure, and require surgical closure

Question 21

atrial septal defects

Answer 21

1: 1500; patent foramen ovale
- if hole is large enough, there is significant blood movement from LA to RA, possibly leading to volume overload of right heart
- if untreated, can cause pulmonary HTN, right heart enlargement, and heart failure

Question 22

tetralogy of fallot (4 things)

Answer 22

1. pulmonary stenosis (improper development of pulmonary valve, which is 8-12% of all congenital heart defects)
2. dextroposition of aorta, so it overrides the ventricular septum
3. right ventricular hypertrophy
4. ventricular septal defect

most common cause of “blue baby” syndrome