Lecture 18 - Neonatal Physiology Flashcards
What percentage of maternal O2 transfers to the fetus?
Not all of the maternal blood comes into contact with the placenta therefore only 20% of the oxygen in the maternal blood is consumed.
How does the fetus gain O2 and offload CO2 to the mother?
The placental barrier is permeable to both however it is much more permeable to CO2 so this is offloaded to the mother.
What is the difference between fetal and adult haemoglobin? What effect does this have on the dissociation curve?
Until about 30 weeks the fetus has two gamma chains instead of beta chains. It also has lack of 2,3-DPG resulting in an increased affinity for oxygen as there is nothing causing the offloading of oxygen. This keeps the fetal haemoglobin curve to the left of the mothers so oxygen diffuses from the mother to fetus.
What is the difference between fetal and maternal haemoglobin concentrations?
The fetus has increased haemoglobin concentration which means that at the same PO2 the fetal blood can carry greater amounts of O2.
What are the three arterio-venus shunts in the fetal circulation?
Foramen ovale - between atria
Ductus arteriosus - from pulmonary artery to aorta
Ductus venosus - around liver
What determines blood flow through the arteriovenous shunts?
Pressure gradients determine the blood flow. Results in blood with the highest O2 content going to the brain.
Why are the fetal right and left stroke volumes not equal?
Due to the arteriovenous shunts then different amounts of blood are pumped out of the ventricles. Therefore fetal cardiac output is the total output of the ventricles.
How much of the fetal output goes to the pulmonary circulation?
Only 8%
What affects the fetal circulation during development?
Receptors for catecholamines mature early in the fetus, this is independent of the autonomic nervous system. This means that the fetal vessel are under tonic adrenergic vasoconstriction mainly from circulating noradrenaline.
How does fetal HR and ABP change with development?
The fetal heart rate is initially high (160bpm) but will lower to 140bpm by term due to vagal tone developing.
ABP is low at the 11th week but gradually increases as sympathetic activity and the baroreceptor reflex develop towards term.
When do fetal breathing movements begin to occur and what is their function?
They begin from the 11th week and are due to motor pathways for breathing beginning to develop. They result in amniotic fluid aspiration and will decrease towards birth (sign of a healthy fetus). FBMs decrease with fetal hypoxia due to the effect of hypoxia on the respiratory neurones.
What happens during birth and what stimulates first breath?
Amniotic fluid is squeezed out of the lungs during birth. Sensation of cool air on the skin, sensory stimulation and hypoxia from the trapped umbilical cord cause first breath to be triggered. Air moving into lungs causes surfactant to spread across the lungs.
How does the first breath effect the pulmonary vasculature?
It decreases the pulmonary vascular resistance causing blood to flow through the lungs.
What causes closure of the AV shunts?
The increase in return to the left atrium resulting from blood flowing through the lungs causes an increase in pressure in the left atrium causing the foramen ovale to close.
Clamping of the umbilical cause causes a backflow of pressure to the aorta that closes the ductus arteriosus. Reduction in local prostaglandins also cause closure of ductus arteriosus. NSAIDs can be used to promote closure as they block prostaglandins.
Decreased flow to the liver through the umbilical vein due to umbilical clamping causes the ductus venosus to close.
How do wall thicknesses of the heart and associated vessels change in the neonatal period?
The wall of the pulmonary artery and right ventricle decrease in thickness due to decreased pulmonary vascular resistance.
The thickness of the left ventricle increases due to increased total peripheral resistance.
