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

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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?v=lXN_sDnd1ng http://www.youtube.com/watch?v=pp2mWgWAnc8

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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?v=jLTkCQkbkKg Abnormal implantation: Ectopic Pregnancy http://www.youtube.com/watch?v=45HYJpOF6-0

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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

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 • Wharton’s jelly helps protect the vessels and prevents kinking of the cord

Maternal-Fetal Gas Exchange

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Embryo

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Umbilical stalk

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Umbilical cord

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2 small arteries

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1 large vein

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Wharton’s jelly (for protection)

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Chorion (chorionic villi)

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Endometrium (uterus)

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Becomes placental unit

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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 4 th 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 ‘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.

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

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Increase pulmonary blood flow

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Vasodilation

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Initiation of gas exchange

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Increasing PaO

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Stretching pulmonary units

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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.

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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

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

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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

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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

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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.

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

Tongue Epiglottis shape Epiglottis Level Trachea Larynx Shape Larynx Position Narrowest Point TLC Ventilator set VT

Pediatric

Large Floppy, omega shaped Level of C3-4 Smaller, shorter Funnel Shaped Angles posteriorly away 250 ml 4-6 ml/kg

Adult

Normal Firm, flatter Level of C5-6 Wider, longer Column Straight up and down At level of Vocal cords 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 ⇒ ⇑ force ⇒ airway collapse negative inspiratory ⇒ ⇑ 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.