Outline the evidence for the primitive heart tube in the adult heart.
- The primitive atrium contributes a (small) component to each atrium
- The bulbus cordis give rise to (part of) the right ventricle
- The left ventricle is derived from the primitive ventricle.
- The truncus arteriosus ultimately gives rise to the roots and proximal portions of the pulmonary trunk and aorta.
Identify the features of the primitive heart tube.
- Aortic roots
- Truncus arteriosus
- Bulbus cordis
- Sinus venosus
Relate the embryological structures to the corresponding mature derivatives.
- Sinus venosus
- Primitive atrium
- Primitive ventricle
- Bulboventricular sulcus
- Bulbus cordis (proximal 1/3, Conus cordis, truncus arteriosus)
Describe the process of septation.
- Therefore, in the process of septation the primitive heart tube becomes divided into chambers and the outflow tract is subdivided into pulmonary trunk and aorta.
- Firstly, the junction between the atrium and ventricle becomes constricted, creating a narrow channel called the atrioventricular canal.
-This narrowing provides a framework by which the inter-atrial and interventricular septa are formed.
- Central to the process of septation are the endocardial cushions that form both in the region of the atrioventricular canal and truncus arteriosus.
- In the atrioventricular canal, endocardial cushions provide a platform toward which the septa grow inferiorly (inter – atrial) or superiorly (inter – ventricular), dividing the heart into left and right sides
- Endocardial cushions forming in the truncus arteriosus contribute to the formation of a spiral septum dividing the outflow into pulmonary trunk and aorta.
Describe the principles of foetal circulation in terms of the fate of foetal shunts.
- Atrial septation is complicated by the fact that the circulatory needs of the embryo/foetus are different to those of the adult.
- Thus, a right – to – left shunt (the foramen ovale) must be maintained during life in utero, but this must be instantly sealable at birth.
Outline foetal circulation.
Outline the fate of foetal shunts.
Describe the formation and looping of the primitive heart tube.
- The cardiogenic field from which the heart, blood vessels and blood cells will develop is created during gastrulation and at first lies at the cranial end of the embryo before folding occurs.
- In the 3rd week of development, the endocardial tubes are formed.
- These pair of tubes are brought together during embryonic folding and fuse in the mid-line to create the primitive heart tube.
- The primitive heart tube is linear at first, receiving blood (inflow) at its caudal pole and pumping blood (outflow) from its cranial pole. The primitive heart tube is described as having four segments.
Briefly describe the process of looping.
- The symmetrical arrangement of embryonic/foetal blood vessels is systematically remodelled over the course of development.
- The process by which the adult disposition of the heart is achieved begins with the process of looping of the primitive heart tube. Put simply, looping places both the inflow and outflow cranially with the inflow dorsal to (behind) the outflow.
Name the regions of the developing heart.
Describe the partitioning of the heart into four chambers.
Describe, briefly, the development of the great vessels.
- Early arterial system begins as a bilaterally symmetrical system of arched vessels.
- They undergo extensive remodelling to create the major arteries leaving the heart.
Describe the aortic arch derivatives.
- The aortic arches are remodelled to create the mature disposition.
- As the heart “descends” the nerve hooks around the 6th aortic arch and “turns back on itself”.
- The left recurrent laryngeal nerve becomes hooked around the shunt between the PT & aorta.
- Fourth arch:
I. R = proximal part of R Subclavian A
II. L = arch of aorta
- Sixth arch:
I. R = R pulmonary artery
II. L = L pulmonary artery and Ductus Arteriosis
Outline recurrent laryngeal nerves.
- Each aortic arch has a corresponding nerve.
- The nerve corresponding to the 6th is the recurrent laryngeal nerve (br vagus, CN X) – right descends to T1-T2, & left descends to T4-T5
- Two factors influence the course of the nerve on left & right sides:
I. Caudal shift of the developing heart & expansion of the developing neck region.
II. The need for a foetal shunt between PT & aorta.
Outline Hypoplastic left heart syndrome.
- Exact cause not known
- Some embryological speculation
I. Defect in development of mitral and aortic valves, resulting in atresia and limited flow.
II. Or ostium secundum too small
III. Therefore, right to left flow inadequate in utero
Describe types of atrial septal defects.
B - Excessive resoprtion of septum primum
C - Short septum primum, septum secundum, large oval foramen
D - Absence of septum secundum
F - Absence of septum primum and septum secundum
Outline ventricular septation.
- Ventricular septum has 2 components
- Muscular portion forms most of the septum and grows upwards towards the fused endocardial cushions, leaving a small gap - the primary interventricular foramen:
Outline the closure of the Primary Interventricular foramen.
The membranous portion of the interventricular septum formed by connective tissue derived from endocardial cushions to "fill the gap"
Outline Tetralogy of Fallot
- Large ventricular septal defect
- Overriding aorta
- Right ventricular outflow tract obstruction
- Right ventricular hypertrophy
- Conotruncal septum formation defective – importance of neural crest cells
- Neural crest cells are very sensitive to alcohol. Hence, if the mother drinks during pregnancy, the cells die – breaking down the intraventricular septum.
- An atrial septal defect (ASD) is an opening in the septum/wall between the two atria which persists following birth.
- Because left atrial pressure is normally higher than right atrial pressure, flow will mainly be from left to right and there is no mixing of deoxygenated blood with the oxygenated blood being pumped around the systemic circulation.
Describe ostium secundum atrial septal defect.
- Ostium secundum atrial septal defect is the most common variety of ASD affects about 70% of children with the condition.
-It occurs when a part of the atrial septum fails to close completely while the heart is developing, causing an opening to develop in the centre of the wall separating the two atria.
Describe patent foramen ovale.
- A patent foramen ovale (PFO) is not a true ASD. A foramen ovale is a hole in the heart. It normally exists in babies who are still in the womb. It should close soon after birth. If it doesn’t close, the condition is called patent foramen ovale (PFO).
- PFOs may be present in around 20% of the population and are generally clinically silent since the higher left atrial pressure causes functional closure of the flap valve.
- In most cases, people with PFO have no symptoms or complications. PFO is usually not a concern unless you have other heart conditions.
- However, there is some evidence that adults with PFO may have a higher risk of stroke. In addition, there may be a connection between PFO and migraines.
- A ventricular septal defect (VSD) is an abnormal opening in the interventricular septum. This most commonly occurs in the membranous portion of the septum, but can occur at any point.
- Since left ventricular pressure is much higher than right, blood will flow from left to right, the amount of flow depending on the size of the lesion.
Explain the effects of a left to right shunt.
Although left to right shunting of blood does not cause cyanosis it can be problematic later on if untreated, with the extent of the problems depending on the degree of shunting.
Explain the causes of congenital cyanotic heart defect.
- Tetralogy of Fallot is a group of 4 lesions occurring together as the result of a single developmental defect which places the outflow portion of the interventricular septum too far in the anterior and cephalad directions.
- The four anomalies are VSD, overriding aorta, and a variable degree of pulmonary stenosis and right ventricular hypertrophy.
- Pulmonary stenosis causes persistence of the foetal right ventricular hypertrophy as the right ventricle must operate at a higher pressure to pump blood through the pulmonary artery.
- The increased pressure on the right side of the heart along with the VSD and overriding aorta allow right to left shunting and mixing of deoxygenated blood with the oxygenated blood going to the systemic circulation, resulting in cyanosis.
- The magnitude of the shunt and level of severity depend on the severity of the pulmonary stenosis.
- Affected individuals may present with cyanosis or spells of cyanosis in infancy, but mild cases can present in adulthood.
Describe the functional importance of transposition of the great vessels.
- Transposition of the great arteries results in two unconnected parallel circulations instead of two circulations in series. In this defect the right ventricle is connected to the aorta and the left ventricle to the pulmonary trunk.
- In some cases, the left ventricle and ascending aorta fail to develop properly resulting in a condition called hypoplastic left heart. A PFO or ASD are also present and blood supply to the systemic circulation is via a PDA. Without surgical correction, this condition would be lethal.
- A baby with transposed arteries is cyanotic. For a baby with transposed arteries to survive, blood flow between the right and left sides of the heart must be increased.
- Sometimes, other heart abnormalities may temporarily minimize the problem caused by this condition such as: VSD, ASD, PDA.
- Permanent treatment requires a surgical procedure to switch the arteries to their proper places. Another procedure, called an atrial septostomy, may be used to enlarge an atrial septal defect.
Describe the functional importance of stenosis and atresia of the aorta and pulmonary valve.
Tricuspid atresia (lack of development of the tricuspid valve) leaves no inlet to the right ventricle. There must be a complete right to left shunt of all the blood returning to the right atrium (ASD or PFO) and a VSD or PDA to allow blood flow to the lungs.
Explain the significance of a patent ductus arteriosus.
- The ductus arteriosus is a vessel that exists in the foetus to shunt blood from the pulmonary artery to the aorta before the lungs are functioning.
- This vessel should close shortly after birth as the pressure in the pulmonary artery drops following perfusion of the lungs.
- Failure to close leads to a patent ductus arteriosus(PDA). Blood flow through a PDA will be from aorta to pulmonary artery after birth (high to low pressure)
Describe the effects of coarctation of the aorta.
- Coarctation of the aorta: a narrowing of the aortic lumen in the region of the ligamentum arteriosum (former ductus arteriosus).
- The narrowing of the aorta increases the afterload on the left ventricle and can lead to left ventricular hypertrophy. Because the vessels to the head and upper limbs usually emerge proximal to the coarctation, the blood supply to these regions is not compromised. However, blood flow to the rest of the body is reduced. The extent of the symptoms depends on the severity of the coarctation.
- Arterial hypertension in the arms with low blood pressure in the lower extremities is classic. In the lower extremities, weak pulses in the femoral arteries and arteries of the feet are found. The coarctation typically occurs after the left subclavian artery.