Lect 4: Fetal & Neonatal Circulation

Question 1

Fetal Circulation has four unique shunts:

Answer 1

Since the lungs are non-functional and the liver only partially functional, special vessels help blood bypass these organs. Gas exchange occurs through the placenta.

Question 2

The four shunts are

Answer 2

1. placenta
2. ductus venosus (Umbilical vein to IVC)
3. foramen ovale (RV to LV)
4. ductus arteriosus (PulmArtery to Aorta

Question 3

Shunts 1 & 2

Answer 3

1. Shunt 1: 50% oxygenated blood returning from the placenta for gas exchange
2. Shunt 2: instead of going to liver blood goes along the umbilical vein to IVC though the ductus venosus. (oxygenated blood)

Question 4

Shunts 3 & 4

Answer 4

3. Shunt 3: from IVC 40% flows thru foramen ovale into left atrium while the rest flows into the RV. Blood also enters the RA from the SVC and is directed through the tricuspid valve into the RV.
4. Shunt 4: About ¾ of the blood from the RV is pumped through the ductus arteriosus into the aorta, bypassing the lung. Thus the right and left ventricle both pump to the descending aorta and their output is largely in parallel rather than in series (as in adults). Therefore the sum of both ventricles is considered to be the combined cardiac output. A small amount of blood from the right ventricle passes through the lungs and then enters the LA.

Question 5

Resistance in the Lungs:

Answer 5

The lungs react to hypoxia and hypoxic vasoconstriction

Blood vessels are crushed because the lungs are not yet inflated
Ductus arteriosus is open b/c fetus has higher levels of prostaglandin E2

Question 6

Oxygen partial pressure and saturation

Answer 6

o Highest in the umbilical vein (until it mixes with bl from IVC so PO2 and O2 sat is reduced in RA. The further it goes (RV) the more reduced it gets
o In the L.A. PO2 is similar to the bl in the RA that is mixed with bl from lung. It gets reduced as it goes further
o Mixing is good bc all organs will get moderate amt of blood

Question 7

Adjustments at or after birth

Answer 7

o Loss of placental circulation requires the newborn to breathe on its own.
o Dramatic increase in pulmonary flow (bc resistant is decreased)
o Closure of the ductus venosus, foramen ovale and ductus arteriosus

Question 8

First breath

Answer 8

o Is triggered by mild hypoxia, hypercapnia, tactile stimuli and cold skin (Not oxygen or reduced umbilical bl flood)
o Requires a transpulmonary pressure of 60 cm H20 to increase the lung volume and the type 2 pneumocytes deliver….RDS due to deficient surfactant

Question 9

Fetal to Neonatal Circulation Switch

Answer 9

o (In the fetus, pulmonary vascular resistance is high, pulmonary blood flow is low, and mean pulmonary arterial pressure is high. At birth all of these reverse. )The primary event is the fall in resistance which occurs because of three things pulmonary blood vessels are no longer, being crushed, breathing causes increased PO2, which in turn causes vasodilation and local prostaglandins cause vasodilation
o →results in Increased blood flow to lung b/c PVR is decreased and MeanPulmAP is decreased

Question 10

The Placenta

Answer 10

o has four essential functions. In the lungs it is involved in gas exchange; in the GI tract it is involved in nutrition; in the liver it is involved in nutrition and waste removal and in the kidneys it is involved in fluid and electrolyte balance
o if you have too little placenta it cause type II intrauterine growth restriction (IUGR)..small baby

Question 11

The Intervillous Space

Answer 11

o Contains a pool of maternal blood for gas exchange. Maternal blood gets trapped in the intervillous space for exchange
o In the mature placenta, spiral arteries from the mother empty directly into the intervillous space, which is drained by maternal veins (CO2). Mom gives O2.

Question 12

Maternal and Fetal oxygen Levels

Answer 12

o IV space PO2 is low bc most is delivered
o Umbilical vein: PO2 is 30 but Hb sat is 85%. O2 is identical to uterine vein but Hb sat is higher because of fetal Hb has: a higher affinity to O2 than mom’s. Also, baby’s heart rate is faster, so greater CO. Also fetal Hb has more content.

Question 13

Closure of placental circulation after birth

Answer 13

o Increased oxygen partial pressure
o Decreased prostaglandin circulation
o Its closure causes peripheral resistance to double causing an increase in aortic BP and left ventricle pressure

Question 14

Closure of foramen ovale

Answer 14

o Is caused by reversal of the right/left atrial pressure. Pressure drop in RA; becomes fossa ovalis
o Failure of closure leads to patent foramen ovale

Question 15

Closure of the ductus venosus

Answer 15

o It becomes the ligamentum venosum. The closure may be due to increased PO2 and reduced prostaglandins: not established
o Failure of closure leads to portosystemic shunt.

Question 16

Closure of the ductus arteriosus

Answer 16

o becomes ligamentum arteriosum. Increased PO2 and decreased prostaglandin E2 cause vasoconstriction. Also bradykinin from lung.
o Failure of closure: patent ductus arteriosus
o Leads to pulmonary hypertension and possibly congestive heart failure and cardiac arrhythmias

Question 17

Changes in Circulation at Birth (1)

Answer 17

o Closure of these two shunts establishes separate right and left circulatory systems. As the pressure in the LA rises higher than the pressure in the R.A. owing to the large decrease in pulmonary vascular resistance, the flap of the foramen ovale pushes against the septum, this prevents blood flow from the left to the right atrium. Eventually this flap seals shut.

Question 18

Changes in Circulation at Birth (2)

Answer 18

o As aortic pressure exceeds the pressure of the pulmonary artery, blood flow through the ductus arteriosus reverses. Well oxygenated aortic blood now flows though the ductus arteriosus within a few hours. Falling prostaglandin levels also contribute to the rapid closure. The elimination of the fetal shunts and the oxygenation of blood in the lungs leads to major increases in the O2 sat and PO2 in the circulation.

Question 19

Patent Ductus Arteriosus

Answer 19

no symptoms but a murmur…with larger openings breathing is difficult, feeding is poor, HR and sweating are elevated and weight is low; endocarditis risk

Question 20

Coarctation of the Aorta

Answer 20

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

Question 21

Ventricular Septal Defects

Answer 21

due to a hole in the wall between the ventricles. A small VSD rarely causes problems and often closes by itself. A large VSD during the first few months of life due to blood flow from the from the left to the right ventricle. Can cause CHF

Question 22

Atrial Septal Defect

Answer 22

if hole is large enough there is significant movement from the left atria to the right atria, possibly leading to volume overload of the right lung. If untreated can lead to pulmonary hypertension, right heart enlargement and heart failure. Foramen ovale not permanently sealed

Question 23

Teratology of Fallot

Answer 23

a combination of 1. Pulmonary stenosis (improper development of the pulmonary valve →RV hypertrophy 2. Dextroposition of the aorta so it overrides the ventricular septum 3. Right ventricular hypertrophy and 4. Ventricular septal defect. “Blue baby syndrome”

Question 24

The Neonatal circulation is characterized by

Answer 24

a reversed flow though the ductus arteriosus to the lung. This enhances O2 uptake by the blood as it passes though the lung
1. loss of blood flow though the placenta (umbilical cord constricts due to stretch and increased po2) produces a doubling of the systemic vascular resistance. This results in an increase in the aortic pressure as well as increased pressures in the LV and LA.

Question 25

Neonatal circulation continued

Answer 25

2. As a result of respiratory activity the pulmonary vascular resistance is greatly decreased
   o Upon lung expansion the pulmonary vessels are no longer compressed
   o Highter blood oxygen levels produce pulmonary vadodilation as does the release of prostaglandins

Question 26

Neonatal circulation continued again

Answer 26

3. These lung effects reduce the pulmonary arterial pressure and the RV and RA pressure
4. Following lung inflation a large amount of blood is now directed to the LA from the lungs. This increases left atrial pressure.

Question 27

Neonatal circulation continued again again

Answer 27

5. The increase in LA and decrease in RA pressures close the valve of the foramen ovale and flow from the inferior vena cave is not limited to the RA.
6. Within 1 to 3 hours the muscular wall of ductus venosus contracts and closes this avenue to flow.

Question 28

Neonatal circulation continued again again again

Answer 28

7. With increased aortic pressure and decreased pulmonary arterial pressure the blood flow though the ductus arteriosus is reversed. This increases blood flow through the lung, increasing O2 uptake. Only in neonates (~4 weeks old)

Question 29

Neonatal circulation continued one last time

Answer 29

8. After a few hours of increased plasma PO2 and decreased blood prostaglandin levels the muscular wall of the ductus arteriosus constricts markedly and within 2-3 days the constriction is stong enough to stop most blood flow.
9. With closure of the ductus arteriosus the adult circulation is attained