Fetal circulation
Dr. S. Parthasarathy
MD., DA., DNB, MD (Acu),
Dip. Diab.DCA, Dip. Software statistics
PhD (physio)
Mahatma gandhi medical college and research
institute, puducherry, India
What are the needs of the CVS ??
• Give oxygen to tissues
• Give metabolic nutrients to tissues
Fetus
to
neonate
Placenta to lungs
Fetus
Essence
• The entire cardiac output cannot go to the lungs
• Hence we have shunts in the circulation
• But adequate oxygen supply has also be there
• After delivery, lungs take over, shunts disappear
• The foramen ovale, ductus arteriosus, and ductus
venosus
Some changes take place
• That is transitional circulation
• Word is important – transitional !!!
• If it is permanent – think of preterm, critically
ill neonate or congenital cardiac illness
Special characters
• Parallel arrangement of two main arterial
systems and their respective ventricles.
• But series in adults
• Mixing of venous return and preferential
streaming.
• High resistance, low flow of pulmonary
circulation.
• Low resistance and high flow of placental
circulation.
• Presence of shunts
The pathway
• Deoxygenated blood of fetus
• Descending aorta
• Umbilical arteries
• Intervillous spaces
oxygenated blood
Placenta
- gas exchange
Then what happens
• Umbilical vein
• Preferably through ductus venosus to IVC
• Streaming
• Liver bypassed
From the IVC, it bypasses the right atrium through
foramen ovale to Left atrium
Streaming
• Eustechian valve helps to direct the IVC blood to cross the
foramen ovale to left atrium
• The lower margin of septum secundum [crista dividens] helps
to direct the left posterior stream to preferentially across the
foramen ovale.
• Posterior and left stream of IVC blood carries oxygenated
blood while anterior and right stream carries poorly
oxygenated blood
• SVC blood is directed across the TV to right ventricle
• Left atrium to left ventricle through mitral valve
• LV to ascending aorta
• Supplies oxygenated blood to three main arteries
• Mixes with ductus arteriosus blood
• most highly oxygenated blood is delivered to
the myocardium and brain.
The pathway
• Deoxygenated blood from the SVC and part of
IVC enter Right atrium,
• Tricuspid valve
• Right ventricle
• Pulmonary artery
• Then lungs ?? No
• But through ductus arteriosus to descending
aorta
• Left to right (blue)
• And
• Right to left shunt (Pink)
• follows
SVC and IVC
60
50
65
80 %
25%
RV vs LV
• The RV receives about 65% of the venous
return and the LV about 35%.
• Thus, in the shunt dependent circulation of
the fetus, the situation is much more complex
and cardiac output must be defined in
different terms.
• Hence CVO = combined ventricular output
• 45 % to placenta 8 % to lungs
The big three
• high hemoglobin (16gm%)
• fetal haemoglobin
• high CVO
• help maintain oxygen delivery in the fetus
despite low oxygen partial pressures
HbF
Normal Hb
• The transition from fetus to neonate
Transition
• Gas exchange function is transferred from
placenta to the lungs.
• Shunts closure
• Separation of systemic and pulmonary
circulations.
• LV output must increase
• Increased metabolism to maintain body
temperature
What is done
• Placenta removed
• Cord clamped
• Baby cries and lung starts to inspire
Cord clamped and placenta removed
• The umbilical vessels are reactive and
constrict in response to longitudinal stretch
and the increase in blood PO2.
• Obviously external clamping of the cord will
augment this process.
• Placenta removed
• No flow through ductus venosus
The ductus venosus closes passively
3–10 days after birth.
Placenta (low resistance) cut off,
SVR increases
Lungs expand
• At birth, after expansion of the lungs, there is
a dramatic fall in PVR and an 8–10-fold
increase in pulmonary blood flow.
• Expansion
stimulation of stretch receptors
vasodilation
• Not oxygen , even any gas
LA flow increase – RA flow decrease FO closes
• Initial closure of the foramen ovale occurs
within minutes to hours of birth.
• Anatomical closure occurs later via tissue
proliferation.
• Ductus venosus is closed FO closed , what next
• Ductus arteriosus
• Concomitant with the drop in PVR, the shunt at
the level of the DA becomes bi-directional.
• The exact mechanism of ductal closure is not
known
• increased PO2 in neonatal blood- direct
constriction of smooth muscle within the duct.
• concentrations of PGE2, produced in the
placenta, fall rapidly after birth, causes ductal
constriction.
PVR change
Ductus arteriosus
• Functional closure by 96 hours
• anatomical closure via endothelial and fibrous
tissue proliferation later
• Hemoglobin
• HbF becomes adult Hb – better oxygen delivery
Cardiac output
• Term fetus -- CVO = 400 ml / kg/min.
• Neonate CVO = upto 400 ml / kg/min.
• Upto 4.0 litres / sq. metre
• Necessary for increased metabolic demands
• LV and RV outputs equalize
Persistent fetal circulation
By then-- why we should know about
fetal circulation
• Take for example – A case of truncus or
transposition of vessels come for other surgery –
It should be known to us that Ductus arteriosus
should be kept patent –
• Yes they need shunts
• We can start PGE1 infusions
• Congenital diaphragmatic hernia – PVR has not
come down – What should be done to make fit !!
• Thank you all