Congenital heart defects Flashcards
Foetal lung maturation
Before birth lungs full of fluid containing chloride, protein, mucus, surfactant.
Breathing movements begin before birth – cause aspiration of some amniotic fluid.
As preparation before delivery, fetus removes 60% of fluid out from lungs.
What is the importance of surfactant?
Surfactant is produced by type 2 pneumonocytes (alveolar epithelial cells) from 24 weeks maturing max at 35 weeks
Absolutely critical to ensure that the airways stay open and do not completely collapse after that first breath is taken
Why is breathing movement essential in foetal lung maturation?
Breathing movements essential for normal lung development (incl. resp muscles).
Removal / expulsion of respiratory fluid
- Prelabour hormonal changes 40ml/kg to 10ml/kg
- Compression of vaginal birth removes 1/3
- Adrenaline secreted in labour promotes reabsorption 2/3
Motivators for baby to take first breath
- Temperature change
- Light stimulation
- Physical stimulation
- Lack of oxygen supply
- Negative pressure in chest cavity from recoil after birth.
Adaptations at birth
Inflation of pulmonary sacs
Dilation of pulmonary vascular bed due to fall in capillary pressure.
Passage of blood through feotus
Oxygenated blood enters the fetus through the wide single umbilical vein
Approx 20% of this blood bypasses the liver via the ductus venosus
Blood enters the right atrium, the foramen ovale between the two atria acts as a valve to allow passage of blood from the right to the left side of the heart on the contraction of the atria , thus acting to shunt blood away from entering the right ventricle and instead allow preferential flow to the left ventricle
There is another shunt which then further reduces blood entering the pulmonary circulation by way of the ductus arteriosus
This way approximately 10% of the right ventricular output enters the fetal lung which as seen in this diagram remains small in volume and dense
What happens following birth?
The umbilical vessels degenerate, as does the ductus venosus allowing more blood to enter the liver.
The foramen ovale is forced shut by increased left atrial filling pressure and the ductus arteriosus begins to close therefore reducing the bypass effect
The circulatory system now runs in parallel with each beat of the heart ensuring both pulmonarty and sytemic circulations occurring simultaneously
What initially causes dramatic changes at birth?
Loss of placental blood flow
Start of respiration
Main circulatory changes include closure of:
- Foramen ovale
- Ductus arteriosus
- Ductus venosus
- Umbilical vessels
Foramen ovale
Opening between atria
allows oxygenated blood to bypass lungs
Foramen ovale changes at birth
Closure initially reversible Functional closure - ↑left atrial pressure / ↓ right atrial pressure. Flexible septum primum pressed against rigid septum secundum.
Foramen ovale to fossa ovalis
Apposition of septa leads to final fusion - fossa ovalis (6 to 12 months)
Ductus arteriosis
Vessel connecting pulmonary trunk to arch of aorta
Protects lung against circulatory overload - shunts 90% of blood away from lungs
Blood required for lung development
Ductus arteriosis
Constriction by muscular contraction.
Closure mediated by bradykinin - from lung during initial inflation (requires high O2).
Initial closure at 6-48 hours, full anatomical closure may take 1-3 months.
Ligamentum arteriosum
Lumen obliterated to form ligamentum arteriosum
