4- fetal circulation & changes at birth Flashcards
what are differences for foetus in uterus?
- baby contained in bag of fluid & lungs full of fluid, they’re not used in gas exchange
- liver has little role in nutrition & waste management
- gut is not in use
- placenta is used in circulation for gas exchange, nutrition, waste excretion, homeostasis
what is explanation of foetal circulation?
blood with oxygen comes from placenta to heart then pushed on to supply growing brain & body - only small amount of blood goes to lungs to help them develop
oxygenated blood from placenta runs into umbilical cord up to liver through ductus venosus and joins inferior vena cava. inferior & superior (deoxygenated form top end of foetus) vena cava blood. most from vena cava shoots through foramen ovale to LA. right & left pulmonary arteries are high resistance
RA →most goes through foramen ovale to LA (to join the very little blood that comes from lungs), pumped into ascending aorta, arch of aorta and joined by other blood from duct →umbilical arteries blood goes back to umbilical cord
what are the 3 shunts that are specific to foetal life?
- Ductus venosus
- Foramen Ovale
- Ductus Arteriosus
what is ductus venosus and it’s purpose?
extra blood vessel that connects umbilical vein to the inferior vena cava
- nutrients come from placenta, don’t need further processing in liver so just cut straight to IVC. (Ductus venosus carries majority of placental blood straight into IVC bypassing portal circulation)
what is foramen ovale and it’s purpose?
= membranous flap opening in atrial septum connecting RA to LA
- allows blood to flow from RA to LA allowing the best oxygenated blood to enter LA then to LV to go to aorta and body (because lungs are high pressure)
what is ductus arteriosus and it’s purpose?
= connects pulmonary bifurcation to descending aorta
- small fraction of RV output goes to lungs (the lungs in foetus are actually very high vascular bed - in us they’re low resistance)
- the majority goes through path of least resistance via ductus arteriosus to join descending aorta to supply rest of body (left)
- patency maintained by circulating prostaglandin E2 produced by placenta
what is change in systemic vascular resistance once baby is born?
= sudden increase in systemic vascular resistance
- flow in umbilical cord stops
- cord clamped & cut
- removal of large low resistance vascular bed from systemic circulation
what is change in pulmonary vascular resistance once baby is born?
= dramatic drop in pulmonary vascular resistance
- baby breathes & cries
- lungs expand and fill with air
- air filled lungs lead to higher oxygen tension in pulmonary circulation
- oxygen is very effective vasodilator in pulmonary arterioles (so they relax, from small tight high resistance →big, open, low resistance)
what happens to ductus venosus after birth?
just stops functioning and doing anything
what happens to foramen ovale?
= like door shutting due to pressure changing on 2 sides of heart
- As PVR falls and SVR rises the LA pressure exceeds the RA pressure
- The flap is pushed closed
- PFO remains in up to 35% of the population
- Implicated in stroke, migraine
what happens to ductus arteriosus after birth?
- left side pressure changes becoming higher than right (so gradient toward pulmonary arteries)
- flow changes in duct - aorta→pulmonary artery
- as O2 tension in circulation rises, smooth muscle constricts (duct gets smaller as O2 increases)
- decreased circulating prostaglandin E2 due to increased lung metabolism
- combination of decreased flow, oxygen and prostaglandin it closes and after a few days it remodels to fibrous ligament (functional closure within hours to days and anatomical closure within 7-10 days)
how do you keep arteriosus duct open and why would you want to?
when interrupted aortic arch →means no circulation past arch of aorta, pulmonary artery is working to supply descending aorta
- Some congenital heart disease causes a “duct dependent circulation”
- IV prostaglandin E2 can be used to keep the duct open until an alternative shunt established or definitive surgery carried out
how long does it take for pulmonary resistance to reach normal level?
- important to realise this continues to change for some time after birth
- continues to drop
- reaches “normal” adult type levels by 2-3 months
when is failure of adaptation more likely?
in sick babies:
- Sepsis
- Hypoxic ischaemic insult
- Meconium aspiration syndrome
- Cold stress
where do you place oxygen saturation probes?
on right arm (measure stuff from purple branch of aortic arch) and one on left foot
