Flashcards in Lect 4: Fetal & Neonatal Circulation Deck (29)
Fetal Circulation has four unique shunts:
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.
The four shunts are
2. ductus venosus (Umbilical vein to IVC)
3. foramen ovale (RV to LV)
4. ductus arteriosus (PulmArtery to Aorta
Shunts 1 & 2
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)
Shunts 3 & 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.
Resistance in the Lungs:
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
Oxygen partial pressure and saturation
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
Adjustments at or after birth
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
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
Fetal to Neonatal Circulation Switch
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
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
The Intervillous Space
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.
Maternal and Fetal oxygen Levels
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.
Closure of placental circulation after birth
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
Closure of foramen ovale
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
Closure of the ductus venosus
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.
Closure of the ductus arteriosus
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
Changes in Circulation at Birth (1)
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.
Changes in Circulation at Birth (2)
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.
Patent Ductus Arteriosus
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
Coarctation of the Aorta
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
Ventricular Septal Defects
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
Atrial Septal Defect
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
Teratology of Fallot
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”
The Neonatal circulation is characterized by
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.
Neonatal circulation continued
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
Neonatal circulation continued again
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.
Neonatal circulation continued again again
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.
Neonatal circulation continued again again again
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)