1. development of the heart and great vessels

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Development of the Heart and Great

Vessels




Development of the Heart

When does the heart begin to develop?

Towards the end of the 3rd week of gestation.

How does the development of the heart begin?

A pair of endothelial tubes fuse to become the primitive heart tube.

This develops within the pericardial cavity, suspended from its dorsal wall by a dorsal mesocardium.

Development of the heart

Grooves develop in the primitive heart tube to divide it into 5 regions:

POSTERIORANTERIOR


The arterial and venous ends of the tube are surrounded by a layer of visceral pericardium.

Development of the heart The primitive heart tube then elongates within

the pericardial cavity. As the bulbus cordis and ventricle grow more

rapidly than the attachments at either end, the heart first takes a U-shape and later an S-shape.


At the same time the heart rotates slightly anticlockwise and twists so that the right ventricle lies anteriorly and the left atrium and ventricle posteriorly.

Despite this, and an increase in the number of vessels entering and leaving, they still continue to be enclosed together in this single tube of pericardium.


As the tube develops,› the sinus venosus becomes

incorporated into the atrium › the bulbus cordis becomes

incorporated into the ventricle.


Endocardial cushions develop between the primitive atrium and ventricle.

An interventricular septum develops from the apex up towards the endocardial cushions.

Development of the heart The division of the atrium is

slightly more complicated.› The septum primum grows

down to fuse with the endocardial cushions, but leaves a hole in the upper part which is termed the foramen ovale.

› The septum secundum develops just to the right of the septum primum and foramen ovale.

› Thus a valve-like structure develops which allows blood to go from the right to the left side of the heart in the fetus.


At birth, when there is an increased blood flow through the lungs and a rise in the left atrial pressure, the septum primum is pushed across to close the foramen ovale.

Usually the septa fuse, obliterating the foramen ovale and leaving a small residual dimple (the fossa ovalis).


The sinus venosus joins the atria, becoming the 2 venae cavae on the right and the 4 pulmonary veins on the left.

The Aortic Arches

Development of the aortic arches

A common arterial trunk (the truncus arteriosus), continues from the bulbus cordis and gives off 6 pairs of aortic arches.


The aortic arches

The aortic arches curve around the pharynx to join to dorsal aortae which join together lower down as the descending aorta.

These arches are equivalent to those supplying the gill clefts of a fish.


1st and 2nd arches: disappear early.

3rd arch: remains as the carotid artery.


4th arch: › On the right:

becomes the subclavian artery

› On the left: becomes the arch of the aorta, giving off the left subclavian.

5th arch disappears early.


Ventral part of 6th arch:› becomes the

right and left pulmonary artery

› The connection to the dorsal aortae disappears on the right but continuing as the ductus arteriosus on the left connecting with the aortic arch.

The aortic arches and their derivatives


In the early fetus the larynx is at the level of 6th aortic arch, and the vagus gives off its nerve to it below the 6th arch.

As the neck elongates and the heart migrates caudally, the recurrent nerves become dragged down by the aortic arches. › On the right: the 5th and 6th

absorb leaving the nerve to hook round the 4th (subclavian) in the adult.

› On the left: the nerve remains hooked around the 6th arch (the ligamentum arteriosum) of the adult.

Congenital Anomalies of the Heart and Great

Vessels

Malposition

This includes: › Dextrocardia: a mirror image of the normal

anatomy › Situs inversus: inversion of all the viscera.

Appendicitis may present as left iliac fossa pain in this condition.

In pure dextrocardia there is no intracardiac shunting and cardiac function is normal.

Malposition

Very rare in normal life, but slightly more common in exams!

Left to right shunts (late cyanosis)

Atrial septal defect (ASD)Ventricular septal defect (VSD)Patent ductus arteriosus (PDA)

Atrial septal defect (ASD)

This may be from the ostium primum, secundum or sinus venosus and represents failure in the primary or secondary septa.

Treatment:› Closure with a pericardial patch › Catheter-introduced atrial baffles made

of Dacron (more recently)

Atrial septal defect (ASD)

Clinical differences between ASD with intracardiac shunting and a persistent patent foramen ovale:› A probe may be passed obliquely

through the septum.› In PFO, flow of blood does not occur

after birth, because of the higher pressure in the left atrium.

› This condition is said to occur in 10% of subjects, but it is not normally of any significance.

Ventricular septal defect (VSD)

VSD is the most common abnormality. › Small defects in the

muscular part of the septum may close.

› Larger ones in the membranous part just below the aortic valves do not close spontaneously and may require repair.

Patent ductus arteriosus (PDA)

This normal channel in the fetus may occasionally fail to close after birth.

It should be corrected surgically because it causes increased load to the left ventricle and pulmonary hypertension.

Along with septal defects, PDA may later cause reverse flow and, therefore, late cyanosis.

Eisenmenger’s syndrome

Pulmonary hypertension may cause reversed flow (right to left shunting).

This is due to an increased pulmonary flow resulting from either an ASD, or VSD or PDA.

When cyanosis occurs from this mechanism it is known as Eisenmenger’s syndrome.

Right to left shunts (cyanotic)

Fallot’s tetralogy


The 4 features of this abnormality are › VSD, › Stenosed pulmonary

outflow tract› Wide aorta which

overrides both the right and left ventricles

› Right ventricular hypertrophy


Because there is a right to left shunt across the VSD there is usually cyanosis at an early stage, depending mainly on the severity of the pulmonary outflow obstruction.

Obstructive non-cyanotic abnormalities

Coarctation of the aortaAbnormalities of the valves

Coarctation of the aorta

This is a narrowing of the aorta, normally just distal to the ductus arteriosus.

Thought to be an abnormality related to the obliterative process of the ductus.


Clinical sequelae:› Hypertension in

the upper part of the body

› Weak delayed femoral pulses


Extensive collaterals develop to try and bring the blood down to the lower part of the body, resulting in large vessels around the scapula, anastomosing with the intercostal arteries and the internal mammary and inferior epigastric arteries.› These enlarged intercostals

usually cause notching of the inferior border of the ribs (a diagnostic feature seen on chest x-ray).


Treatment:› This condition used to require a

major thoracic operation but now can frequently be treated by balloon angioplasty.

Abnormalities of the valves

Any of these may be imperfectly formed and tend to cause either stenosis or complete occlusion (atresia).

The pulmonary and the aortic valves are more frequently affected than the other two.

Type of Defect Mechanism

Ventricular Septal Defect (VSD) There is a defect within the membranous or muscular portions of the intraventricular septum that produces a left-to-right shunt, more severe with larger defects

Atrial Septal Defect (ASD) A hole from a septum secundum or septum primum defect in the interatrial septum produces a modest left-to-right shunt

Patent Ductus Arteriosus (PDA) The ductus arteriosus, which normally closes soon after birth, remains open, and a left-to-right shunt develops

Tetralogy of Fallot Pulmonic stenosis results in right ventricular hypertrophy and a right-to-left shunt across a VSD, which also has an overriding aorta

Transposition of Great Vessels The aorta arises from the right ventricle and the pulmonic trunk from the left ventricle. A VSD, or ASD with PDA, is needed for extrauterine survival. There is right-to-left shunting.

Truncus Arteriosus There is incomplete separation of the aortic and pulmonary outflows, along with VSD, which allows mixing of oxygenated and deoxygenated blood and right-to-left shunting

Coarctation of Aorta Either just proximal (infantile form) or just distal (adult form) to the ductus is a narrowing of the aortic lumen, leading to outflow obstruction

References

Raftery AT (2008): Applied Basic Science for Basic Surgical Training, 2nd ed. Elsevier, Edinburgh.

Ellis H (2006): Clinical Anatomy, 11th ed. Blackwell Publishing

1. development of the heart and great vessels

Health & Medicine

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