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Chapter 2 • Thorax

Fetal Circulation

Venoussystems associated with thefetal heart

There are 3 major venous systems that flow into the sinus venosus end of the heart tube.

• Viteline (omphalomesenteric) veins drain deoxygenated blood from the yolk stalk; they will coalesce and form the veins of the liver (sinusoids, hepatic portal vein, hepatic vein) and part of the inferior vena cava.

•Umbilicalvein carries oxygenated blood from the placenta.

•Cardinalveins carry deoxygenated blood from the body of the embryo;

they will coalesce and contribute to some of the major veins of the body (brachiocephalic, superior vena cave, inferior vena cava, azygos, renal).

Arterialsystems associated with thefetal heart

During fetal circulation, oxygenated blood flood from the placenta to the fetus passes through the umbilical vein. Three vascular shunts develop in the fetal circulation to bypass blood flow around the liver and l gs (Figure IIl-2-12).

1 . The ductus venosus allows oxygenated blood in the umbilical vein to bypass the sinusoids of the liver into the inferior vena cava and to the right atrium. From the right atrium, oxygenated blood flows mostly through the foramen ovale into the left atrium then left ventricle and into the systemic circulation.

2. The foramen ovale develops during atrial septation to allow oxygenated blood to bypass the pulmonary circulation. Note that this is a right-to­ left shunting of blood during fetal life.

3. During fetal circulation, the superior vena cava drains deoxygenated blood from the upper limbs and head into the right atrium. Most of this blood flow is directed into the right ventricle and into the pulmonary trunk. The opens into the underside of the aorta just distal to the origin of the left subclavian artery and shunts this deoxy­ genated blood from the pulmonary trunk to the aorta to bypass the pul­ monary circulation.

The shunting of blood through the foramen ovale and through the ductus arteriosus (right to left) during fetal life occurs because of a

sure gradient.

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Chapter 2 • Thorax

Postnatal circulation

Following birth, these 3 shunts will close because of changes in the pressure gra­ dients and in oxygen tensions. The umbilical vein closes and reduces blood flow into the right atrium. The ductus venosus also closes. Lung expansion reduces pulmonary resistence and results in increased flow to the lungs and increased venous return to the left atrium.

•Closure of the foramen ovale occurs as a result of the increase in left atrial pressure and reduction in right atrial pressure.

•Closure of the ductus venosus and ductus arteriosus occurs over the next several hours as a result of the contraction of smooth muscles in its wall and increased oxygen tension.

•The release of bradykinin and the immediate drop of prostaglandin E at birth also facilitate the closure of the ductus arteriosus.

The changes which occur between preand postnatal circulation are summarized in Table ill-2-2.

Table 111-2-2. AdultVestiges Derived from the Fetal Circulatory System

SEPTATION OF THE HEART TUBE

Except for the sinus venosus of the embryonic heart tube that initially develops into right and left horns, the ventricular, atrial, and truncus parts ofthe heart tube, which are originally a common chamber, willundergo septation into a right and left heart structure. The septation of the atria and ventricles occurs simultane­ ously beginning in week 4 and is mostly finished in week 8. Most ofthe common congenital cardiac anomalies result from defects in the formation ofthese septa.

Atrial Septation

During fetal life, blood is shunted from the right to the left atrium via the fora­ men ovale (FO). Note that during fetal circulation, right atrial pressure is higher than left due to the large bolus of blood directed into the right atrium from the placenta and to high pulmonary resistance.

The FO has to remain open and functional during the entire fetal life to shunt oxygenated blood from the right atrium into the left atrium.

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Section Ill • Gross Anatomy

Truncus arteriosus defects

Three classic cyanotic congenital heart abnormalities occur with defects in the development of the aorticopulmonary septum and are related to the failure of neural crest cells to migrate into the truncus arteriosus:

1 . TetralogyofFallot (Figure III-2- 1 9) is the mostcommon cyanotic congenital heart defect. Tetralogy occurs when the AP septum fails to align properly and shifts anteriorly to the right. This causes right-to-left shunting of blood with resultant cyanosis that is usually present sometime after birth. Imaging typi­ cally shows a boot-shaped heart due to the enlarged right ventricule.

•There are 4 major defects in Tetralogy of Fallot:

-Pulmonary stenosis (most important)

Overriding aorta (receives blood from both ventricles) Membranous interventricular septal defect

-Right ventricular hypertrophy (develops secondarily)

Aorta

1 . Pulmonary stenosis

2. Ventricular septa! defect

3. Hypertrophied right ventricle

4. Overriding aorta

Figure 111-2-19. Tetralogy of Fallot

2.Transposition ofthe great vessels (Figure Ill-2-20) occurs when the AP sep­ tum fails to develop in a spiral fashion and results in the aorta arising from the right ventricle and the pulmonary trunk arising from the left ventricle. This causes right-to-left shunting of blood with resultant cyanosis.

•Transposition is the most common cause of severe cyanosis that per­ sists immediately at birth. Transposition results in producing 2 closed circulation loops.

•Infants born alive with this defect usually have other defects (PDA, VSD, ASD) that allow mixing of oxygenated and deoxygenated blood to sustain life.

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Chapter 2 • Thorax

Anterior Mediastinum

The anterior mediastinum is the small interval between the sternum and the anterior surface of the pericardium. It contains fat and areolar tissue and the infe­ rior part ofthe thymus gland. A tumor ofthe thymus (thymoma) can develop in the anterior or superior mediastina

Posterior Mediastinum

Theposteriormediastinumis located between the posterior surface ofthe pericar­ dium and the T5-Tl2 thoracic vertebrae. Inferiorly, it is closed by the diaphragm.

There are 4 vertically oriented structures coursing within the posterior medias­ tinum:

•Thoracic (descending) aorta

-Important branches are the bronchial, esophageal, and posterior inter­ costal arteries

-Passes through the aortic hiatus (with the thoracic duct) at the T12 vertebral level to become the abdominal aorta

•Esophagus

-Lies immediately posterior to the left primary bronchus and the left atrium, forming an important radiological relationship

-Is constricted (1) at its origin from the pharynx, (2) posterior to the arch of the aorta, (3) posterior to the left primary bronchus, and (4) at the esophageal hiatus of the diaphragm.

•Thoracic duct

-Lies posterior to the esophagus and between the thoracic aorta and azygos vein

-Ascends the posterior and superior mediastina and drains into the junction of the left subclavian and internal jugular veins

-Arises from the cisterna chyli in the abdomen (at vertebral level L1) and enters the mediastinum through the aortic hiatus of the diaphragm

•Azygos system ofveins

-Drains the posterior and thoracic lateral wall

-Communicates with the inferior vena cava in the abdomen and ter- minates by arching over the root of the right lung to empty into the superiorvena cava above the pericardium

-Forms a collateralvenous circulation between the inferior and superior vena cava

Middle Mediastinum

The middle mediastinum contains the heart and great vessels and pericardium and willbe discussed later.

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Chapter 2 • Thorax

The relationships ofthese structures in the superior mediastinum are best visual­ ized in a ventral to dorsal orientation between the sternum anteriorly and the vertebrae posteriorly:

• Thymus: Located posterior to the manubrium, usually atrophies in the adult and remains as fatty tissue

• Rightandleftbrachiocephalicveins: Rightvein descends almostvertically and the left vein obliquely crosses the superiormediastinumposterior to the thyrnic remnants.

The 2 veins join to form the superiorvenacava posterior to the right first costalcartilage.

The superiorvena cava descends and drains into the right atrium deep to the right third costal cartilage.

• Aortic arch and its 3 branches: Aortic arch begins and ends at the plane ofthe sternal angle and is located just inferior to the left brachiocephalic vein. As a very important radiological landmark, the origins ofthe

3 branches ofthe aortic arch (brachiocephalic,left common carotid,and left subclavian) are directly posterior to the left brachiocephalic vein.

• Trachea: Lies posterior to the aortic arch and bifurcates at the level of

T4 vertebra to form the right and left primary bronchi. The carina is an internal projection of cartilage at the bifurcation.

• Esophagus: Lies posterior to the trachea and courses posterior to the left primary bronchus to enter the posterior mediastinum

ln addition to these structures, the superior mediastinum also contains the right and leftvagus and phrenic nerves and the superior end ofthe thoracic duct.

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