Chapter 2
Fetal Gas Exchange and
Circulation
http://www.youtube.com/watch?v=OV8wtPYGE-I
Introduction
• The fetus in utero shares the mother’s
circulation for gas exchange
• However the maternal and fetal vascular
networks are separate systems and no blood is
shared between the two
• When a zygote (fertilized egg) first travels to the
uterus, it has no nutrient source. The developing
cells here are called Blastocyst, which must
implant into the uterine lining for nourishment
Introduction
• The outer surrounding layer of the blastocyst is
the trophoblast which combines with tissue from
the endometrium to form the chorionic
membrane around the blastocyst
Introduction
• Inside the blastocyst a group of cells arrange on one side in
the shape of a figure eight
• The central portion is the embryonic disk which forms three
embryonic germ layers; which contain origins for the below
structures
• The ECTODERM: CNS (brain, spinal cord) PNS (craniel
nerves/spinal nerves, eyes, inner ears, nose, glandular tissues,
skin, teeth
• The MESODERM: Cardiovascular system, heart/blood vessels,
lymphatics, connective tissues/blood cells, bone, skeletal muscle,
skin, kidneys/ureters, reproductive tissues, spleen…
• The ENDODERM: Digestive system, respiratory system, urinary
system, liver/pancreas…
• http://www.youtube.com/watch?v=lXN_sDnd1ng
• http://www.youtube.com/watch?v=pp2mWgWAnc8
Introduction
• The outer or top of the figure eight envelops the embryonic
structure and forms the amniotic sac, the inner layer forms
the yolk sac which then turns into the embryo, the amniotic
sac then surrounds the embryo.
• The embryo attaches to the outer layer through the umbilical
stalk later the umbilical cord
• The umbilical cord connects to the finger like projections in
the outer lining of the chorion/chorionic villi
• A capillary network connects the umbilical cord to the
chorionic villi.
• http://www.youtube.com/watch?v=jLTkCQkbkKg
• Abnormal implantation: Ectopic Pregnancy
• http://www.youtube.com/watch?v=45HYJpOF6-0
Introduction
• The villi intertwine into the blood filled lacunar cavities of the
endometrium of the maternal uterus
• O2, CO2, and nutrients diffuse though the vast capillary
surface area of this indirect connection between the mother
and fetus
Maternal-Fetal Gas Exchange
•As fetal development
continues, the region of
this interface becomes
limited to the discusshaped placenta
•The umbilical cord
connects the placenta to
the fetus with one large
vein and two smaller
arteries
•As the cord grows the
vessels tend to spiral
•Wharton’s jelly helps
protect the vessels and
prevents kinking of the
cord
Maternal-Fetal Gas Exchange
• Embryo
• Umbilical stalk
• Umbilical cord
• 2 small arteries
• 1 large vein
• Wharton’s jelly (for protection)
• Chorion (chorionic villi)
• Endometrium (uterus)
• Becomes placental unit
• http://www.youtube.com/watch?v=zv
NPw7m74HE
Cardiovascular Development
• Heart
• First organ to form
• Begins during third week of gestation
• Completed by week 8
Cardiovascular Development (cont.)
•http://www.youtube.c
om/watch?v=aZUDeP
gRQqI
•Cardiovascular
system develops from
the mesoderm layer
•By day 22, cardiac
contractions are
detectable and
bidirectional blood
flow begins
Cardiovascular Development (cont.)
Fourth week of gestation
heart tubes continue to
merge into three
structures: bulbus cordis,
ventricular bulge and the
arterial bulge which empty
into the sinus venosus,
which receives
oxygenated, nutrient rich
blood from the placenta
Continuation of folding,
bending and dilation
continue giving the heart a
S shape
Cardiovascular Development (cont.)
• Simultaneous external changes occur; the
septum primum begins to separate the primitive
atrium. At the same time endocardial cushions
develop which will separate the atriums from the
ventricles.
• The left atrium incorporates the pulmonary
veins, the superior vena cava develops . By end
of the 4th week the dilating ventricular spaces
fold onto each other creating the ventricular
septum and the base of the bulboventricular
loop
Cardiovascular Development (cont.)
• Blood flow matures into a unidirectional path as
the myocardium contiues to strengthen by
recruiting myocytes from surrounding
mesenchymal tissue.
• Weeks 5-6 internal and external structures
mature quickly
• By week 6 the foramen ovale is present (source
of fetal shunting)
• Fetal heart rate is about 95 bpm
• http://www.youtube.com/watch?v=u1x24IdN7V
A
Cardiovascular Development (cont.)
Week 7-8 the
ventricular septum is
finished forming
A small intraventricular
foramen remains and
blood flows between
the two ventricles until
the endocardial
cushions fuse with the
ventricular septum
Tricuspid and Mitral
valves develop
Fetal Circulation
Introduction
• The fetal circulation is markedly different from
the adult circulation
• In the fetus, gas exchange does not occur in the
lungs but in the placenta
• The placenta must therefore receive
deoxygenated blood from the fetal systemic
organs and return its oxygen rich venous
drainage to the fetal systemic arterial circulation
• the fetal cardiovascular system is designed in
such a way that the most highly oxygenated
blood is delivered to the myocardium and brain
Introduction
• These circulatory adaptations are achieved in the
fetus by both the preferential streaming of
oxygenated blood and the presence of
intracardiac and extracardiac shunts
• fetal circulation can be defined as a ‘shuntdependent’ circulation
• In the fetus, deoxygenated blood arrives at the
placenta via the umbilical arteries and is
returned to the fetus in the umbilical vein.
Introduction
• Oxygenated blood travels from the placenta to the fetus
through the umbilical vein
• The ductus venosus, the first fetal shunt, appears
continuous with the umbilical vein, shunting 30-50% of
the oxygenated blood around the fetal liver
• The amount of shunting through the ductus venous
appears to decrease with gestational age
• The shunted oxygen rich blood empties into the inferior
vena cava and mixes with venous blood as it flows to the
right atrium
Fetal Cardiac Shunts
• Foramen ovale
• Between right and left atria bypass the right ventricle
• Ductus arteriosus
• Pulmonary artery to aorta to bypass the right ventricle
• Ductus venosus
• Shunts blood past liver
• the ductus venosus shunts approximately half of the blood
flow of the umbilical vein directly to the inferior vena cava.
Thus, it allows oxygenated blood from the placenta to bypass
the liver. In conjunction with the other fetal shunts, the
foramen ovale and ductus arteriosus, it plays a critical role in
preferentially shunting oxygenated blood to the fetal brain. It
is a part of fetal circulation
• http://www.youtube.com/watch?v=cgccQVcFLi4
Ductus venosus
• The ductus venosus is open at the time of the birth and is the
reason why umbilical vein catheterization works. Ductus
venosus naturally closes during the first week of life in most
full-term neonates; however, it may take much longer to close
in pre-term neonates. Functional closure occurs within
minutes of birth. Structural closure in term babies occurs
within 3 to 7 days.
• After it closes, the remnant is known as ligamentum venosum.
• If the ductus venosus fails to occlude after birth, the individual
is said to have an intrahepatic portosystemic shunt (PSS). The
ductus venosus shows a delayed closure in preterm infants,
Possibly, increased levels of dilating prostaglandins leads to a
delayed occlusion of the vessel
Patent Foramen Ovale (PFO)
http://www.youtube.com/watch?v=yDSTONfL4h8
Fetal Cardiac Shunts
• Shunted oxygen-rich blood empties into the inferior vena cava
and mixes with venous blood as it flows to the right atrium
• In the right atrium most of the blood received from the
inferior vena cava passes through the foramen ovale to the
left atrium
• The remainder of the blood in the right atrium mixes with
desaturated blood from the superior vena cava and empties
into the right ventricle; blood here has slightly higher oxygen
partial pressures. This blood is pumped through the
pulmonary arteries into the developing lungs
• The PVR is high in utero due to compression of the vessels
from low lung volumes, and low lung oxygen concentrations
Fetal Cardiac Shunts
• Since the lungs in utero are void of air, chemical mediators
keep vessels constricted in the pulmonary vascular bed
• 13-25% of the fetal blood flow reaches the lungs
• Blood from the pulmonary veins empties into the left atrium
and the flows into the left ventricle and then out through the
atrium to the head, right arm and coronary circulation
• The high PVR keeps most of the pulmonary artery blood flow
from the right ventricle to bypass the lungs, flowing through
the Ductus arteriosus into the aorta
• Deoxygenated blood from the upper torso returns to the right
atrium via the superior vena cava
• Blood in the descending and abdominal aorta flows through
the two umbilical arteries and back to the placenta for
oxygenation
Transition to Extrauterine Life
• Increase pulmonary blood flow
• Vasodilation
• Initiation of gas exchange
• Increasing PaO2
• Stretching pulmonary units
• Inhabitation of
vasoconstrictors
Transition to Extrauterine Life
• Clamping of the umbilical cord vessels removes the low
pressure system of the placenta from the fetus
• During the first breath several factors improve
pulmonary blood flow and reduce PVR
• Inflating the lungs initiates gas exchange and dilates the
pulmonary arterioles
• Rising PaO2 stimulates release of endogenous pulmonary
vasodilating factors
• Stretching of the pulmonary units stretches open the
vascular units and stimulates the release of anti
vasocontricting agents
Transition to Extrauterine Life
• Once PVR decreases, pressures in the right side of the
heart decrease and pressures in the left side increase
• The foramen ovale closes once the pressure in the left
exceeds the right; this facilitates the increase of blood
flow to the lungs
• Pressure in the aorta increases and becomes greater
than the pressure in the pulmonary artery
• The shunting in the ductus arteriosus decreases
• The PDA typically closes quickly from increases in PaO2
and prostaglandin levels
• Prostaglandins are mediators and have a variety of strong
physiological effects, such as regulating the contraction
and relaxation of smooth muscle tissue
Transition to Extrauterine Life
• Ductus arteriosus closes typically completely within 24 hours
after birth. If they do not close it is termed a PDA
• PDA affects girls more often than boys. The condition is more
common in premature infants and those with neonatal
respiratory distress syndrome
• Infants with genetic disorders, such as Down syndrome, and
whose mothers had rubella during pregnancy are at higher
risk for PDA.
• PDA is common in babies with congenital heart problems,
such as hypoplastic left heart syndrome, transposition of the
great vessels, and pulmonary stenosis.
PDA
A small PDA may not cause any symptoms. However, some
infants may have symptoms such as:
• Fast breathing
• Poor feeding habits
• Rapid pulse
• Shortness of breath
• Sweating while feeding
• Tiring very easily
• Poor growth
• Babies with PDA often have a heart murmur that can be heard
with a stethoscope. However, in premature infants, a heart
murmur may not be heard. The health care provider may
suspect the condition if the infant has breathing or feeding
problems soon after birth.
PDA
• Changes may be seen on chest x-rays. The diagnosis is
confirmed with an echocardiogram.
• Sometimes, a small PDA may not be diagnosed until later in
childhood.
• To assess a babies oxygenation after birth the probe is placed
preductal on the right hand or wrist
• We can then compare SpO2 readings pre and post ductally to
assess the severity of a PDA
PDA
• If the rest of the baby's heart and blood flow is normal or
close to normal, the goal is to close the PDA. If the baby has
certain other heart problems or defects, keeping the ductus
arteriosus open may be lifesaving. Medicine may be used to
stop it from closing
• Sometimes, a PDA may close on its own. In premature babies
it often closes within the first 2 years of life. In full-term
infants, a PDA rarely closes on its own after the first few
weeks.
• When treatment is needed, medications such as indomethacin
or a special form of ibuprofen are often the first choice.
Medicines can work very well for some newborns, with few
side effects. The earlier treatment is given, the more likely it is
to succeed.
PDA
• A transcatheter device closure is a procedure that uses a thin,
hollow tube placed into a blood vessel. The doctor passes a
small metal coil or other blocking device through the catheter
to the site of the PDA. This blocks blood flow through the
vessel. These coils can help the baby avoid surgery.
• Surgery may be needed if the catheter procedure does not
work or it cannot be used. Surgery involves making a small cut
between the ribs to repair the PDA. Surgery has risks,
however. Weigh the possible benefits and risks with your
health care provider before choosing surgery.
PFO
• Normally the foramen ovale closes at birth when increased
blood pressure on the left side of the heart forces the opening
to close.
• If the atrial septum does not close properly, it is called a
patent foramen ovale. This type of defect generally works like
a flap valve, only opening during certain conditions when
there is more pressure inside the chest. This increased
pressure occurs when people strain while having a bowel
movement, cough, or sneeze.
PFO
• If the pressure is great enough, blood may travel from the
right atrium to the left atrium. If there is a clot or particles in
the blood traveling in the right side of the heart, it can cross
the PFO, enter the left atrium, and travel out of the heart and
to the brain (causing a stroke) or into a coronary artery
(causing a heart attack).
• People with PFO do not need any treatment if there are no
associated problems, such as a stroke. Patients who have had
a stroke or transient ischemic attack (TIA) may be placed on
some type of blood thinner medication, such as aspirin, plavix
(clopidogrel), or coumadin (warfarin) to prevent recurrent
stroke.
• Surgical repair may be indicated
Development of Baroreceptors
and Chemoreceptors
Baroreceptors
• Baroreceptors are stretch receptors in the wall of some blood
vessels. They are involved in the control of arterial pressure
through the discharge of impulses to the cardiovascular centre
when there is distension due to a change in the blood
pressure.
• Baroreceptors are found in the carotid sinus (dilation in the
left and right internal carotid arteries), the aortic arch, and the
elastic arteries of the neck and chest and some veins.
• http://www.youtube.com/watch?v=5-bruUXxGKA
Baroreceptors
• Any decline in the blood pressure stretches the vascular wall
which stimulates the baroreceptors.
• These receptors send impulses to the cardiovascular center
which in turn decreases parasympathetic stimulation of the
heart via the vagus nerves and increases the sympathetic
stimulation of the heart.
• The cardiovascular center stimulates the secretion of
adrenaline and noradrenaline from the medulla of the adrenal
gland. The effect on the heart and blood vessels is to
accelerate heart rate and contractility and promote
vasoconstriction, resulting in an increase in blood pressure
Baroreceptors
• If there is an increase in blood pressure, the
baroreceptors send impulses to the
cardiovascular center
• In response the cardiovascular center increases
the parasympathetic stimulation of the heart,
and decreases its sympathetic stimulation.
• The heart rate and contractility will decrease
leading to low cardiac output and the peripheral
resistance will decline due to vasodilation. Low
cardiac output and low peripheral resistance
cause a decrease in blood pressure
Baroreceptors
• The baroreceptors in the carotid sinus are responsible for the
regulation of the blood pressure in the brain, while those in
the aortic arch are responsible for regulation of systemic
blood pressure.
Chemoreceptors
• Chemoreceptors are found close to the carotid and aortic
baroreceptors in small structures called carotid bodies and
aortic bodies.
• They are sensitive to any change in the chemical composition
of the blood, such as a decrease in oxygen level and pH of the
blood or an increase in the carbon dioxide level. These
receptors send impulses to the cardiovascular center which in
turn increases the sympathetic stimulation to the blood
vessels causing an increase in blood pressure.
• Chemoreceptors also stimulate the respiratory centers in the
brain to increase the rate of respiration.
• http://www.youtube.com/watch?v=DvYWFKAQNS8
Adult vs Fetal Circulation
Adult circulation sequence
• Non-oxygenated blood enters the right atrium
via the inferior and superior vena cava.
• Increase level of blood in the right atrium causes
the tricuspid valve to open and drain the blood
to the right ventricle.
• Pressure of blood in the right ventricle causes
the pulmonic valve to open and non-oxygenated
blood is directed to the pulmonary artery then to
the lungs.
Adult vs Fetal Circulation
Adult circulation sequence
• Exchange of gases occurs in the lungs. Highly oxygenated
blood is returned to the heart via the pulmonary vein to
the left atrium.
• From the left atrium the pressure of the oxygenated
blood causes the mitral valve to open and drain the
oxygenated blood to the left ventricle.
• Left ventricle then pumps the oxygenated blood that
opens the aortic valve. Blood is then directed to the
ascending and descending aorta to be distributed in the
systemic circulation
Adult vs Fetal Circulation
• Fetal Circulation Sequence
• Exchange of gases occurs in the placenta. Oxygenated
blood is carried by the umbilical vein towards the fetal
heart.
• The ductus venosus directs part of the blood flow from
the umbilical vein away from the fetal liver (filtration of
the blood by the liver is unnecessary during the fetal life)
and directly to the inferior vena cava.
• Blood from the ductus venosus enters to the inferior
vena cava. Increase levels of oxygenated blood flows into
the right atrium.
Adult vs Fetal Circulation
• Fetal Circulation Sequence
• In adults, the increase pressure of the right atrium causes
the tricuspid valve to open thus, draining the blood into
the right ventricle. However, in fetal circulation most of
the blood in the right atrium is directed by the foramen
ovale (opening between the two atria) to the left atrium.
• The blood then flows to the left atrium to the left
ventricle going to the aorta. Majority of the blood in the
ascending aorta goes to the brain, heart, head and upper
body.The portion of the blood that drained into the right
ventricle passes to the pulmonary artery.
Adult vs Fetal Circulation
• Fetal Circulation Sequence
• As blood enters the pulmonary artery (carries blood to
the lungs), an opening called ductus arteriosus connects
the pulmonary artery and the descending aorta. Hence,
most of the blood will bypass the non-functioning fetal
lungs and will be distributed to the different parts of the
body. A small portion of the oxygenated blood that
enters the lungs remains there for fetal lung maturity.
• The umbilical arteries then carry the non-oxygenated
blood away from the heart to the placenta for
oxygenation.
Anatomic and physiologic differences
between the infant and adult
• The respiratory mechanism of the pediatric
patient varies from the adult in both anatomy
and physiology. As children grow, the airway
enlarges and moves more
• caudally as the c-spine elongates. The pediatric
airway overall has poorly developed
cartilaginous integrity allowing for more laxity
throughout the airway.
• Another important distinction is the narrowest
point in the airway in adults is at the cords
versus below the cords for children.
Anatomic and physiologic differences
between the infant and adult
Anatomy
Pediatric
Adult
Tongue
Large
Normal
Epiglottis shape
Floppy, omega shaped
Firm, flatter
Epiglottis Level
Level of C3-4
Level of C5-6
Trachea
Smaller, shorter
Wider, longer
Larynx Shape
Funnel Shaped
Column
Larynx Position
Angles posteriorly away
Straight up and down
Narrowest Point
TLC
Ventilator set VT
At level of Vocal cords
250 ml
4-6 ml/kg
6 Liters
5-10 ml/kg
Anatomic and physiologic differences
between the infant and adult
• There are also many physiologic differences in
respiratory mechanisms between children and adults.
• Children have a more complaint trachea, larynx, and
bronchi due to poor cartilaginous integrity.
• This in turn allows for dynamic airway compression, i.e. a
greater negative inspiratory force “sucks in” the floppy
airway and decreases airway diameter.
• This in turn increases the work of breathing by increasing
the negative inspiratory pressure generated.
Anatomic and physiologic differences
between the infant and adult
• A vicious cycle is created which may eventually
lead to respiratory failure:
• Subglottic stenosis ⇒ ⇑ negative inspiratory
force ⇒ airway collapse ⇒ ⇑ subglottic stenosis
⇒ ⇑ negative inspiratory force ⇒ ⇑ work of
breathing ⇒⇒ respiratory
• failure. Pediatric patients also have more
compliant chest walls also increasing the work of
breathing – i.e. the outward pull of the chest is
greater..
Anatomic and physiologic differences
between the infant and adult
• Infants are dependent on functional diaphragms for adequate
ventilation. The accessory muscles contribute less to the
overall work of breathing in infants as compared to older
children and adults.
• Therefore, a non-functional diaphragm often leads to
respiratory failure. Diaphragmatic fatigue is one amongst
several potential causes of respiratory failure and apnea in
young patients with RSV bronchilitis.
• Finally, the respiratory muscles themselves have a significant
oxygen and metabolite requirement in children. In pediatric
patients the work of breathing can account for up to 40% of
the cardiac output, particularly in stressed conditions
Thermoregulation
• Hyperthermia is usually secondary to
• overheating due to an external source; however it can be
secondary to other factors including sepsis,
hypermetabolism, neonatal abstinence syndrome, and
maternal hyperthermia at delivery.
• Clinically hyperthermia may present with
• irritability, poor feeding, flushing, hypotension,
tachypnea or apnea, lethargy and abnormal posturing, in
addition to an elevated peripheral or core temperature.
If untreated then seizures, coma, neurological damage
and ultimately death may occur
Thermoregulation
• Hypothermia: All neonates are at risk of hypothermia
within the first twelve hours of life, particularly the
extremely premature and growth retarded infants.
• Other risk factors include abnormal skin integrity
including gastroschisis, exmphalos and neural tube
defects and neonates with neurological impairment –
global or to the hypothalamus in particular.
• Hypoglycemic infants or those already significantly
metabolically stressed are also at risk
Thermoregulation
• The mainstay of care is to maintain the newborn
in a neutral thermal environment which ensures
minimal metabolic activity and oxygen
consumption are required to conserve body
temperature
• Incubators are now specifically designed to
minimize losses by radiation, convection,
conduction and evaporation whilst allowing clear
visibility and access to the patient
Thermoregulation
• Ambient temperature and humidity are
easily controlled. A skin temperature
probe is placed away from regions where
brown fat metabolism occurs and should
be reflective if under a radiant warmer.
• All newborns should have a hat to prevent
excessive heat loss from the head. Plastic
wrapping and increased vigilance
regarding maintaining temperature control
should be instigated for any transfers.