Fetal Gas Exchange and Circulation

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Chapter 2
Fetal Gas Exchange and Circulation
http//www.youtube.com/watch?vOV8wtPYGE-I
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Introduction

• The fetus in utero shares the mothers
  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

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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

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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?vlXN_sDnd1ng
• http//www.youtube.com/watch?vpp2mWgWAnc8

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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?vjLTkCQkbkKg
• Abnormal implantation Ectopic Pregnancy
• http//www.youtube.com/watch?v45HYJpOF6-0

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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

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Maternal-Fetal Gas Exchange

• As fetal development continues, the region of
  this interface becomes limited to the
  discus-shaped 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
• Whartons jelly helps protect the vessels and
  prevents kinking of the cord

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Maternal-Fetal Gas Exchange

• Embryo
• Umbilical stalk
• Umbilical cord
• 2 small arteries
• 1 large vein
• Whartons jelly (for protection)
• Chorion (chorionic villi)
• Endometrium (uterus)
• Becomes placental unit
• http//www.youtube.com/watch?vzvNPw7m74HE

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Cardiovascular Development

• Heart
• First organ to form
• Begins during third week of gestation
• Completed by week 8

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Cardiovascular Development (cont.)

• http//www.youtube.com/watch?vaZUDePgRQqI
• Cardiovascular system develops from the mesoderm
  layer
• By day 22, cardiac contractions are detectable
  and bidirectional blood flow begins

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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
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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

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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?vu1x24IdN7VA

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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
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Fetal Circulation
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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

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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
  shunt-dependent circulation
• In the fetus, deoxygenated blood arrives at the
  placenta via the umbilical arteries and is
  returned to the fetus in the umbilical vein.

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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

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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?vcgccQVcFLi4

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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

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Patent Foramen Ovale (PFO)
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http//www.youtube.com/watch?vyDSTONfL4h8
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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

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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

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Transition to Extrauterine Life

• Increase pulmonary blood flow
• Vasodilation
• Initiation of gas exchange
• Increasing PaO2
• Stretching pulmonary units
• Inhabitation of vasoconstrictors

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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

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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

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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.

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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.

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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

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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.

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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.

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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.

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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

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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?v5-bruUXxGKA

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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

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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

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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.

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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.

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• http//www.youtube.com/watch?vDvYWFKAQNS8

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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.

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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

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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.

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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.

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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.

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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.

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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 At level of Vocal cords
TLC Ventilator set VT 250 ml 4-6 ml/kg 6 Liters 5-10 ml/kg
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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.

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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..

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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

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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

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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

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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

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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.

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