Neonatal Cardio-Respiratory Physiology

Question 1

When is the placenta fully formed?

Answer 1

The placenta is fully formed by the 11th to 14th week of pregnancy

Question 2

What is the role of the placenta?

Answer 2

The placenta allows the fetus to receive O2 and nutrients while removing CO2.

Question 3

Describe the maternal side of the placenta.

Answer 3

The maternal side consists of maternal venules and arterioles. Maternal arterioles send branches towards the placenta, delivering well-oxygenated blood into the intervillous space, which is drained by maternal venules.

Question 4

Describe the fetal side of the placenta.

Answer 4

The fetal side contains umbilical arteries and one umbilical vein. The umbilical arteries arise from the fetal aorta and carry de-oxygenated blood back to the placenta for oxygenation, while the umbilical vein returns oxygenated blood to the fetus.

Question 5

What are the PO2 levels in mother and fetus?

Answer 5

Mother: Uterine artery PO2 = 12.7 kPa, Uterine vein PO2 = 5.6 kPa. Fetus: Umbilical artery PO2 = 3.2 kPa, Umbilical vein PO2 = 4.2 kPa. Equilibrium is not achieved for O2.

Question 6

What are the PCO2 levels in mother and fetus?

Answer 6

Mother: Uterine artery PCO2 = 5.3 kPa, Uterine vein PCO2 = 6.1 kPa. Fetus: Umbilical artery PCO2 = 6.6 kPa, Umbilical vein PCO2 = 5.8 kPa. Fetal umbilical vein blood almost achieves equilibrium with maternal blood for CO2.

Question 7

Why is there a discrepancy between O2 and CO2 equilibrium?

Answer 7

The discrepancy is due to the placental barrier being more permeable to CO2 than O2, not all maternal blood contacting the villi, and the placental tissue consuming O2 (~20%).

Question 8

What type of hemoglobin is present in the fetus?

Answer 8

The fetus has fetal hemoglobin (HbF), which comprises 2α and 2γ polypeptide chains, unlike adult hemoglobin (2α and 2β).

Question 9

How do levels of HbF change over gestation?

Answer 9

HbF reaches peak levels at 10 weeks, is maintained until 30 weeks, declines to 80% of total at term, and gradually disappears after birth.

Question 10

Compare HbF with adult Hb.

Answer 10

HbF has a higher affinity for O2 than adult Hb due to its γ polypeptide chain not interacting with 2,3-DPG.

Question 11

Describe fetal HbO2 saturation compared to adult and explain the difference.

Answer 11

P50 of HbF is much lower than adult Hb. O2 diffuses from maternal to fetal Hb at PO2 ~4.2 kPa, with HbF being ~75% saturated, much more than adult Hb.

Question 12

What is the maternal vs fetal blood Hb concentration?

Answer 12

Fetal blood has a higher Hb concentration than maternal blood: 18 g/dl vs 15 g/dl, allowing fetal blood to carry more O2 at any PO2.

Question 13

How does fetal circulation optimize O2 delivery?

Answer 13

Fetal circulation optimizes O2 delivery through the development of three shunts: ductus venosus, foramen ovale, and ductus arteriosus.

Question 14

What does flow through each shunt depend on?

Answer 14

Flow through each shunt depends on an intravascular pressure gradient across that shunt.

Question 15

Describe the passage of blood from the placenta to the RA.

Answer 15

Blood comes from the placenta via the umbilical vein, is about 75% saturated with O2, goes through the ductus venosus into the inferior vena cava, which carries less oxygenated blood from the legs.

Question 16

Why is the brain supplied with the most well-oxygenated blood?

Answer 16

Oxygenated blood from the ductus venosus preferentially goes through the foramen ovale into the left atrium, minimizing mixing and ensuring the brain receives well-oxygenated blood.

Question 17

How does blood enter the ductus arteriosus and where does it go?

Answer 17

Less oxygenated blood leaves the right ventricle via the pulmonary artery, goes through the ductus arteriosus into the descending aorta due to high vascular resistance in the collapsed lung vessels.

Question 18

Describe differences in fetal and adult cardiac outputs.

Answer 18

In adults, R stroke volume equals L stroke volume. In fetuses, shunting means RV receives ~65% and LV ~35% of venous return, defining fetal cardiac output as combined ventricular output (CVO).

Question 19

What factors control fetal circulation?

Answer 19

Circulating catecholamines, hormones, and locally released vasoactive substances control fetal circulation, acting on α and β-adrenoreceptors.

Question 20

Describe the neural control in fetal circulation.

Answer 20

By 11 weeks, HR is ~160 b/min, slowing to ~140 b/min by 28 weeks due to vagus nerve tone. ABP rises gradually due to sympathetic tone.

Question 21

When do fetal breathing movements (FBMs) develop?

Answer 21

FBMs develop from ~11 weeks, playing a role in respiratory muscle development and allowing aspiration of amniotic fluid.

Question 22

What two types of breathing movements can be recognized?

Answer 22

Two types of breathing movements are regular and irregular breathing, with irregular movements occurring during REM sleep.

Question 23

What happens around 11+ weeks between HR and FBM?

Answer 23

From 11+ weeks, a coupling between FBM and HR develops, with rapid breathing associated with increased HR and primary apnoea linked to lower HR.

Question 24

When does the incidence of FBM decrease?

Answer 24

The incidence of FBMs decreases during fetal hypoxia and prior to delivery, predicting delivery in a healthy fetus.

Question 25

What are the main events at birth?

Answer 25

Main events at birth include the initiation of breathing, clearance of fluid from the lungs, and establishment of independent circulation.

Question 26

What does a decrease in fetal breathing movements (FBMs) prior to delivery indicate?

Answer 26

It is a predictor of delivery in a healthy fetus.

Question 27

What is the peripheral chemoreceptor reflex effect on respiration prior to delivery?

Answer 27

It is not yet present.

Question 28

What local effect does hypoxia have on the brain regarding FBMs?

Answer 28

It acts in a depressive way on the essential respiratory neurons, causing a decrease in FBMs.

Question 29

What are the main respiratory events at birth?

Answer 29

1. Baby takes its first breath. 2. Umbilical cord is clamped.

Question 30

What is required for the baby's first breath?

Answer 30

Enormous ventilatory effort.

Question 31

What happens to the fluid in the lungs during the birthing process?

Answer 31

Fluid is squeezed out during the birthing process, but this is less effective during a Caesarian birth.

Question 32

What triggers the first breath?

Answer 32

1. Cooling. 2. Sensory stimulation. 3. Chemoreceptor stimulation (central and peripheral).

Question 33

How is the first breath made possible?

Answer 33

By surfactant secreted by Type II cells around 28-30 weeks, under the influence of fetal cortisol.

Question 34

What happens as air moves into the lungs during the first breath?

Answer 34

It forces the lung fluid across the alveoli, and surfactant is adsorbed onto the alveoli surface.

Question 35

How does fluid displacement from airways occur during the first breath?

Answer 35

Negative pressure in the alveoli pulls fluid across the alveoli surfaces into the interstitium and then into the lymphatics.

Question 36

What changes occur in compliance and functional residual capacity (FRC) two weeks after birth?

Answer 36

Lung compliance and FRC gradually increase, making each subsequent breath easier.

Question 37

What general cardiovascular changes occur at birth?

Answer 37

Closure of the foramen ovale, ductus arteriosus, and ductus venosus.

Question 38

What happens to pulmonary vascular resistance at the first breath?

Answer 38

It decreases, leading to increased pulmonary perfusion and closure of the foramen ovale.

Question 39

What effect does clamping the umbilical cord have on the cardiovascular system?

Answer 39

It increases total peripheral resistance (TPR) and causes the ductus arteriosus to close.

Question 40

What neonatal changes occur in the heart after birth?

Answer 40

The septal leaflets of the foramen ovale fuse, and the ductus arteriosus closes within 2 days.

Question 41

What initiates the closure of the ductus arteriosus?

Answer 41

High PO2 in the lungs, which reduces local prostaglandin synthesis.

Question 42

What happens to the wall thickness of the pulmonary arteries and ventricles after birth?

Answer 42

Wall thickness of pulmonary arteries and right ventricle decreases, while left ventricle wall thickness increases.

Question 43

What happens to peripheral chemoreceptors after birth?

Answer 43

They 're-set' over ~2 weeks, shifting the threshold for activation to higher PO2 levels.

Question 44

What occurs to carotid chemoreceptors after birth?

Answer 44

They re-set and become more sensitive to PO2 levels likely experienced during air-breathing.

Question 45

What issues can arise if the foramen ovale does not close?

Answer 45

It can lead to pulmonary remodeling, pulmonary hypertension, and potentially Eisenmenger’s syndrome.

Question 46

What happens if the ductus arteriosus remains open after birth?

Answer 46

It can divert left ventricular output into pulmonary circulation, leading to pulmonary hypertension and heart failure.

Question 47

What is pre-eclampsia?

Answer 47

High ABP and proteinuria in the mother, which can lead to convulsions and requires urgent delivery.

Question 48

What is intra-uterine growth retardation (IUGR)?

Answer 48

A condition where the fetus develops retardedly, often due to maternal factors like high blood pressure.

Question 49

What is fetal programming?

Answer 49

Adverse conditions in utero lead to increased risk of cardiovascular disease, diabetes, and obesity in adult life.

Question 50

What mechanisms lead to hypertension in fetal programming?

Answer 50

Chronic stress, gestational diabetes, and other maternal insults can alter fetal growth and increase CV risk.