Fetal-Neonatal Transition Flashcards
At birth the placenta is removed. What else needs to occur to ensure a smooth transition for the fetus?
- aeration of the lung
- transition of circulation
both of these need to occur so the baby function
What are the three shunts in the fetal circulation that allow for oxygenated blood from the placenta to circulate accurately for fetal growth?
- foremane ovale
- ductus venosus
- ductus arteriosus
Where are the 3 fetal shunts located?
- foreman ovale
- located on the atrial septum shunting blood from the right atrium to the left - ductus arteriosus
- located in the main pulmonary artery shunting blood into the descending aorta - Ductus venousus
- located in the liver connecting the umbilical vein to the inferior vena cava to get oxygenated blood straight to the fetal heart
Explain the route of oxygenated blood from the placenta through the fetal circulation including the shunts
- oxygen blood travels from the placenta through the umbilical vein
- the blood passes through the ductus venosus and into the inferior vena cava approaching the right atrium
- blood enters the right atrium and majority gets shunted through the foreman ovale into the left atrium and the left ventricle
- blood that does not get shunted into the foreman ovale goes through the right ventricle and through the pulmonary artery and gets shunted through the ductus arteriosus into the descending aorta
- the blood then goes into the lower body and then the umbilical vein back into the placenta to be re-oxygenated
In placental circulation explain the roles of the umbilical vein and umbilical arteries
2 umbilical arteries
- deoxygenated blood from fetus to placenta
1 umbilical vein
- oxygenated blood carried away from placenta to fetus
How can a fetus and their mother have a different blood type?
Immunoglobulin M (blood type immunoglobulin)
does not cross the placenta
** IgG can cross the placenta as an immune defence
How does Rhesus incompatibility occur?
occurs when mother is rhesus negative but fetus is rhesus positive
- when fetal blood enters maternal circulation (can occur through labour) IgG form anti-D antibodies
- This does not affect her first pregnancy at her second pregnancy if left untreated and the fetus is also rheusus positive the IgG anti-D antibodies can be transported through the placenta and haemolyse fetal RBC’s
This is treated by an injection of IgG anti-D immunoglobulin
How does the foreman ovale remain patent during fetal growth?
its remains patent as RV pressure is greater then LV pressure and blood will flow where there is less pressure/ resistance
How much of fetal ventricular output will go into lung circulation
about 10% ad 90% of ventricular output goes through the ductus arteriosus into descending aorta
How is the ductus arteriosus kept patent during fetal development?
- the fetus live in levels of high hypoxia in the womb which is good as oxygen is a potent vasoconstrictor which is not helpful when keeping the ductus arteriosus patent
- high levels of prostaglandins vasodilator the DA
Why does sensitivity to prostaglandins in the fetus decrease during development?
this is to help prime the fetus for transition to an adult circulatory system and close the DA after birth
Prostaglandin inhibitors like ibuprofen are advised not to be taken during pregnancy why is this?
if prostaglandins are inhibited during pregnancy the fetal DA will not remain patent and cause circulatory issues in the fetus
How does pulmonary circulation occur during fetal development?
With each fetal heart contraction blood is pushed into the lungs via the force however during diastole the blood movement is dependant on pressure.
As pressure in the pulmonary vasculature is high as they are filled with fluid the blood therefore moves through the ductus arterious into the descending aorta as there is a lower pressure
*note pressure in main pulmonary artery is 45-50 mmHg whereas the descending aorta pressure is 40-45 mmHg
How does the fluid filled lungs create a high pressure in the pulmonary vasculature?
liquid in the lungs cause tissues to be hyperextended to help lung development and growth but in turn squish the pulmonary vasculature bed creating high pulmonary vasculature resistance and low pulmonary blood flow.
Blood flow still occurs due to force of systole but during diastole the high pressure in the lungs cause the blood to be pushed back into the main pulmonary artery and into DA
During adult circulation the LV wall is thicker than the RV. Why is this not in the case in a fetus and how does when fetus transitions to lung aeration?
the RV wall is thicker in fetus as it work harder to pump blood against the pulmonary vasculature resistance.
Postnatally the lung aerate decreasing pulmonary vascular resistance. Over time the LV wall becomes thicker as there is a lot less pressure in the pulmonary circulation compared to systemic
If the LV wall is thicker in the fetus what does this indicate?
this indicates that blood is pushing against high resistance in the umbilical veins. Indicating there is something wrong with placental circulation
What is combined ventricular output and how is it measured
combined output of left and right ventricles as a fetus has two unequal parallel circulations
measured by heart rate x stroke volume
In a fetus 33% of combined ventricular output will go to the LV in comparison to adult circulation where 100% of cardiac output goes through the LV. In a fetus what does the left ventricular output supply?
mostly the upper body
10% goes to heart
65% goes to upper body
25% into descending aorta
Fetal heart rate is used as an indicator for well being of the fetus. What does fetal heart rate tell us?
Heart rate increase:
- increases with fetal activity and acute fetal hypoxia
- due to an increase in sympathetic drive
Heart rate decrease:
- decreases due to severe asphyxia
- caused by chronic hypoxia and vagal stimulation
What is stroke volume and what is the relationship between stroke volume and fetal development?
stroke volume in CVO is based on venous return, down stream resistance and contractility
during gestation stroke volume increases due to an increase in ventricular size and maturity of the heart
What are the critical components to ensure a smooth transition for the fetus at brith?
removal of the placenta
initiation of breathing
transition in circulation so fetus remains oxygenated without the placenta
How does pressure and resistance determine blood flow?
- blood flows from high pressure to low pressure
- blood flows along the path of least resistance
In fetal circulation what are the differences between pulmonary circulation and systemic circulation that leads to right to left flow in the ductus arteriosus?
pulmonary circulation:
- high pressure
- high resistance
therefore low blood flow
systemic circulation:
- low pressure
- low resistance
In newborn circulation what are the changes that occur to the pulmonary and systemic circulation that change the flow of blood through the ductus arteriosus to left to right?
Pulmonary circulation:
- low pressure
- low resistance
therefore high blood flow now occurs
systemic circulation:
- high pressure
- higher resistance
As the DA does not close right after birth blood changes direction. What can occur during the this change is DA flow?
sometimes blood will move from the systemic circulation in the descending aorta back through the DA into the pulmonary vein and into the lungs. If this occurs long term then less oxygenated blood will be travelling into the systemic circulation
During newborn circulation:
- 100% of right ventricular output enters the lung
- pulmonary blood flow is high and pulmonary vascular resistance is low
- systemic arterial pressure is greater than pulmonary pressure
- DA flow is left to right
What needs to occur to achieve these changes in newborn circulation from fetal circulation?
- decrease in pulmonary vascular resistance
- occurs by establishing lungs as gas exchange immediately - reversal of pulmonary-systemic pressure gradient
How is pulmonary vascular resistance decreased at birth?
overall mechanism is unknown but the theory is the increase in blood O2 leads to an increase in arterial oxygenation
- this may release vasodilators or an effect of ventilation
A reduction in lung fluid volume is known to be caused by lung aeration however
What are the changes of pulmonary blood flow following birth including delivery, clamping of the cord and the start of ventilation?
at delivery:
- there is not much difference in blood flow
clamping of cord:
- slight increase to pulmonary blood flow
start of ventilation:
- dramatic increase in pulmonary blood flow which indicates there is a decrease in pulmonary vascular resistance
- aeration causes removal of lung fluid which helps vessels to open = lowering PVR
How does reversal of pulmonary-systemic pressure gradient occur
by decrease PVR you in turn reverse the pressure gradient however there are other factors like clamping the umbilical cord
How does clamping the umbilical cord reverse the pulmonary-systemic pressure gradient?
by clamping the cord 1/3 of fetal blood volume is lost
- there is a 50% loss of right ventricular output
If cord clamping occurs before ventilation occurs there is a rise in systemic blood pressure. When ventilation occurs lungs decrease in pressure reversing the pressure gradient
this is driven by the removal of the placenta
When and why does the ductus arteriosus close?
closes up to 4 days after birth due to:
- increase in oxygen content and removal of prostaglandins E2
Why would the dustus arteriosus remain patent?
may be due to being delivered preterm or having a growth restriction
this can occur because of hypoxia when lungs do not take over for lung exchange
requires surgical closure
When does the ductus venosus closes and why?
functional closure (minutes after birth)
structural closure (a few weeks after birth)
it closes due to the removal of the placenta as the umbilical veins are no longer present to move blood into this shunt
What is the causes the closure of the foreman ovale?
- no placenta means that there is no high velocity flow of highly oxygenated blood through the IVC. Lowering the pressure in the RA
- An increase in pulmonary flow increases pulmonary venous to the heart and increasing the left atrial pressure
These two things cause the blood to preferentially move into the RV due to the lower pressure than that LA
A patent foremen ovale is present in up to 25% of the adult population. What does this indicate about the shunt?
If it remains open there is no hemodynamic significance
Early cord clamping can lead to hypoxia. Why does this occur?
if clamping occurs before aeration of the lungs:
- high pulmonary resistance and low pulmonary blood flow
- increase in systemic pressure and resistance
- loss of umbilical venous return
- left ventricle therefore has increased after load and decreased preload leading to decreased cerebral blood flow
Overall no oxygen being pumped
What is physiological cord clamping and how to help neonate transition?
physiological cord clamping: clinican aerates lungs while the placenta is still attached
- lung aeration decreases PVR and moves lung fluid out of alveoli to help fetus breath on own
- placenta still attached allow for umbilical venous return and pulmonary venous return from lung aeration allows for stable LV output and cerebral blood flow stops hypoxia from occurring
What occurs when breathing onset happens before cord clamping in regard to blood pressure and cardiac output?
there is no dramatic increase in blood pressure:
- avoiding the decreased LV output
Cardiac output is also sustained instead of a dramatic decrease when cord is clamped early
How does physiological based cord clamping affect heart rate?
Heart rate doesnt show a dramatic increase then plateau
Instead heart rate gradually increases not putting to much stress on the heart
Breathing onset before cord clamping confer what benefits?
- prevents sudden rise in blood pressure
- sustains cardiac output
- potential for placental to infant blood transfusion
(allowing less blood to be lost)
There are many risk factors that cause problems during transition for the fetus. Broadly most of the risk factors lead to what two large complications?
respiratory and cardiovascular complications
What are the respiratory complications that can occur in newborns?
transient tachypnea of newborn
- failure of the lung to drain lung fluid
- cause: unknown
respiratory distress syndrome
- surfactant deficiency
- cause: lung immaturity, premature birth
meconium aspiration
- meconium reduces the efficacy of surfactant
- cause: exposure to stress in utero
persistent pulmonary hypotension of newborn
- failure of the pulmonary vascular resistance to reduce at birth
- cause: not entirely known though to be prenatal hypoxia and FGR
What are the cardiovascular complications that can occur in newborns?
congenital heart disease
failure of shunts to close
fetal growth restriction
