Week 3 - Development of the CVS, Heart Chambers and Congenital Heart Defects Flashcards
Which part of the adult heart do the primitive ventricles form?
Left ventricles
Which part of the adult heart does the trunks arteriosus form?
- Aortic roots
- Proximal aorta
- Pulmonary trunk
Which part of the adult heart does the bulbus arteriosus form?
Gives rise to the ventricles (along with the primitive ventricle)
Which part of the adult heart does the sinus venosus form?
It is incorporated into the wall of the right atrium to form a smooth part called the sinus venosum
Which part of the adult heart does the primitive atrium form?
Auricles of definitive atria
What is the first step of septation?
- Endocardial cushions develop in the atrioventricular region, on the dorsal and ventral sides of the atrioventricular canal
- These grow towards each other to divide the heart into right and left channels
How does atrial septation occur?
Involves the formation of 2 septa with 3 holes
- Septum primum grows down towards the fused endocardial cushions
- The ostium primum is the hole present before the septum primum fuses with the endocardial cushions (underneath the septum primum)
- Before the ostium primum closes, a 2nd hole, the ostium secundum appears in the septum primum (this occurs by programmed cell death)
- Finally, a second crescent-shaped septum, the septum secundum grows
- The hole in the septum secundum is the foramen ovale
What is the foramen ovale?
The right to left shunt, used in-utero to bypass the lungs
What is the fossa ovalis?
The adult remnant of the foramen ovale
What is the ventricular septum made of?
2 components:
- Muscular (forms most of the septum)
- Membranous (formed by connective tissue from endocardial cushions)
How does ventricular septation occur?
- The muscular portion grows upwards towards the fused endocardial cushions
- It leaves a hole at the top (the primary interventricular foramen)
- This primary interventricular foramen can be filled by the membranous portion of the interventricular septum
How does septation of the outflow tract occur?
- Endocardial cushions appear in the trunks arteriosus
- As they grow towards each other, they twist around each other
- This forms a spiral septum
Describe foetal circulation
- Lungs are non-functional
- Circulation by-passes lungs and liver
- Receives oxygenated blood from mother via placenta and umbilical vein
- Returns to the placenta via a pair of umbilical arteries
- The change in circulation when born must happen immediately
- Blood flow = placenta -> (via ductus venosus) inferior vena cava -> right atrium (some blood passes to the right ventricle then the pulmonary trunk) -> (via foramen ovale) left atrium -> left ventricle -> aorta (also receives blood from the pulmonary trunk via the ductus arteriosus) -> body -> placenta
What happens to foetal circulation when respiration begins?
Pressure increases in the left atrium, above that of the right atrium - The ductus arteriosus contracts - The foramen ovale closes Placental support is removed - Ductus venosus closes
What is the fate of the foetal shunts?
- Foramen ovale –> fossa ovalis
- Ductus arteriosus –> ligamentum arteriosum
- Ductus venosus –> ligamentum venosum
- Umbilical veins –> ligementum teres
Describe the internal structure of the right atrium
- Smooth, thin-walled posterior part (sinus venarum) on which the venue cavae and coronary sinus open, bringing poorly oxygenated blood into the heart
- Rough, muscular anterior wall composed of pectinate muscles
- Right AV orifice through which the right atrium discharges the poorly oxygenated blood it has received into the right ventricle
- The smooth and rough parts of the atrial wall are separated externally by a shallow vertical groove (the sulcus terminalis)
- They are separated internally by a vertical ridge, the crust terminalis
Describe the internal structure of the left atrium
- A larger smooth-walled part and a smaller muscular auricle containing pectinate muscles
- 4 pulmonary veins entering the smooth posterior wall
- A slightly thicker wall than that of the right atrium
- An interatrial septum that slopes posteriorly and to the right
- A left AV orifice through which the left atrium discharges the oxygenated blood it receives from the pulmonary veins into the left ventricle
Why is the left ventricle wall thicker than the wall of the right ventricle?
- Arterial pressure is much higher in the systemic than in th pulmonary circulation
- So the left ventricle performs more work than the right ventricle
- Hence the walls are 2-3 times thicker to withstand the high pressure
Where is the tricuspid valve found?
In the right ventricle, guarding the right atrioventricular orifice
When does the tricuspid valve open/close?
The right atrium contracts when the right ventricle is empty
- The cusps of the valve are pushed aside, opening it
- Blood is forced through the tricuspid valve
- The valve closes when the right ventricle contracts
What is the role of papillary muscles?
They attach to the cusps of the atrioventricular valves, preventing their separation and inversion
- This blocks regurgitation of blood
- Provide support
- Allows the valves to resist pressure developed during contractions
Where is the mitral valve found?
In the left ventricle
- Attached to the papillary muscles
When does the mitral valve open/close?
At the same time as the tricuspid valve
What are the semilunar valves?
Pulmonary and aortic valves
- They have 3 cusps
- Not supported by muscles
When do the semilunar valves open/close?
- Open when the ventricular pressure > arterial pressure
- After relaxation of the ventricle, the elastic recoil of the wall of the pulmonary trunk or aorta forces the blood back towards the heart
- This regurgitation of blood causes the cusps to snap shut
Describe the circulation of blood in the left-side of the heart
- Oxygen-rich blood from the lungs enters the left atrium from the pulmonary vein
- Blood flows from the left atrium to the left ventricle via the mitral valve
- When the ventricle is full, the mitral valve shuts
- The ventricle contracts and the aortic valve opens
- Blood flows into the aorta from the left ventricle and goes to the body
Describe the circulation of blood in the right-side of the heart
- Oxygen-poor blood enters the right atrium via the inferior and superior vena cava
- Blood flows from the right atrium to the right ventricle through the tricuspid valve as the atrium contracts
- Tricuspid valve shuts when the ventricle is full
- The ventricle contracts and blood enters the pulmonary artery (then the lungs) via the pulmonary valve
What can congenital heart defects be caused by?
- Genetics (e.g. Down’s, Turner’s, Marfan’s)
- Environment (e.g. teratogenicity from drugs, alcohol)
- Maternal infections (e.g. rubella)
How common are congenital heart defects?
Common
- Incidence = 6-8 per 1000 births
- Most common = ventricular septal defects, then atrial septal defects
How can you classify congenital heart defects?
They are cyanotic (i.e. cause cyanosis; low O2 saturation) or acyanotic
What are ventricular septal defects?
An abnormal opening in the inter ventricular septum
Are ventricular septal defects cyanotic or acyanotic?
Acyanotic
Where can ventricular septal defects occur?
Most commonly in the membranous portion of the septum
- Can occur at any point
How is blood flow affected in a ventricular septal defect?
- Since left ventricular pressure is much higher than the right, blood will flow from left to right through the abnormal opening
- The amount of flow depends on the size of the lesion
- Causes pulmonary venous congestion, and eventually pulmonary hypertension
How do ventricular septal defects present?
In infancy, with left heart failure
- Unless very small
How do atrial septal defects present?
Usually asymptomatic until adulthood
- Eventually, there is right heart failure and late onset arrhythmia
Are atrial septal defects cyanotic or acyanotic?
Acyanotic
What are atrial septal defects?
An opening in the septum/wall between the 2 atria which persists following birth
- The foramen ovale exists prenatally to permit prompt right to left shunting of oxygenated placental venous return to the left side of the circulation
- It normally closes after birth
- Failure to close, or an abnormal communication, allows blood to continue to flow between the 2 atria postnatally
How is blood flow affected in an atrial septal defect?
Because left atrial pressure is normally higher than the right, flow will mainly be from left to right
- There is no mixing of deoxygenated blood with the oxygenated blood being pumped around the systemic circulation
Where can atrial septal defects occur?
Can occur almost anywhere along the atrial septum
- Most common site = foramen ovale (an ostium secundum ASD)
- An ostium septum ASD occurs in the inferior portion of the septum and is less common
What are the effects of a left to right shunt?
- Requires a hole
- Blood from the left heart is returned to the lungs instead of going to the body
- Increased lung blood flow by itself is not damaging, but increased pulmonary artery/venous pressure is
- The extent of problems depends on the degree of shunting
- Can lead to vascular remodelling of the pulmonary circulation and an increase in pulmonary resistance
- If the resistance of the pulmonary circulation increases beyond that of the systemic circulation, the shunt will reverse direction
What are some cyanotic congenital heart defects?
- Tetralogy of Fallot
- Transposition of the Great Arteries
- Total anomalous pulmonary venous drainage
- Univentricular heart
What are some acyanotic congenital heart defects?
- Left to right shunts (ASD, VSD, patent ductus arteriosus)
- Obstructive lesions: aortic stenosis, pulmonary stenosis, coarctation of the aorta, mitral stenosis
What is tetralogy of Fallot?
A group of 4 lesions occurring together as the result of a single development defect
- It places the outflow portion of the interventricular septum too far in the anterior and cephalad directions
What are the 4 anomalies seen in tetralogy of Fallot?
- Ventricular septal defect
- Overriding aorta
- A variable degree of pulmonary stenosis
- Right ventricular hypertrophy
What is the effect of pulmonary stenosis?
It causes persistence of the foetal right ventricular hypertrophy, because:
- The right ventricle most operate at a higher pressure to pump blood through the pulmonary artery
What is the effect of tetralogy of Fallot?
The increased pressure on the right side of the heart along with the VSD and overriding aorta allows:
- Left to right shunting
- Mixing of deoxygenated blood with the oxygenated blood going to the systemic circulation, resulting in cyanosis
What determines the severity of tetralogy of Fallot?
The severity of the pulmonary stenosis
How does tetralogy of Fallot present?
With cyanosis or spells of cyanosis in infancy
- Mild cases can present in adulthood
What are the different types of pulmonary stenosis?
- Valvar: the valve leaflets are thickened and/or narrowed
- Supravalvar: the portion of the pulmonary artery just above the pulmonary valve is narrowed
- Subvalvar: the muscle under the valve area is thickened, narrowing the outflow tract from the right ventricle
- Branch peripheral: the right or/and left pulmonary artery is narrowed
What is tricuspid atresia?
Lack of development of the tricuspid valve
- Leaves no inlet to the right ventricle
How can tricuspid atresia be survivable?
There must be a complete right to left shunt of all the blood returning to the right atrium
- ASD or patent foramen ovale
And to allow blood flow to the lungs:
- A VSD or patent ductus arteriosus
What is transposition of the great vessels?
2 unconnected parallel circulations instead of 2 circulations in series
- The right ventricle is connected to the aorta and the left ventricle to the pulmonary trunk (wrong way round!)
What are the effects of transposition of the great vessels?
Not compatible with life after birth:
- Unless a shunt exists to allow the 2 circulations to communicate
- A shunt must be maintained or created immediately following birth to sustain life
- E.g. the ductus arteriosus can be maintained patent, and/or an ASD formed
What is 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
- If it fails to close after birth, it causes a PDA
- An cyanotic defect
How does patent ductus arteriosus affect blood flow?
Blood flow through the PDA, from the aorta to the pulmonary artery (high to low pressure)
What is coarctation of the aorta?
A narrowing of the aortic lumen in the region of the ligament arteriosum (former ductus arteriosus)
- Acyanotic defect
What is the effect of coarctation of the aorta?
Increased afterload on the left ventricle
- 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
- Blood flow to the rest of the body is reduced
What determines the severity of coarctation of the aorta?
The severity of the narrowing
How does coarctation of the aorta present?
- If severe: an infant may present with symptoms of heart failure shortly after birth
- If mild: defect may be detected in adult life
What is patent foramen ovale?
The foramen ovale remains open after birth
- Not a true ASD
- May be present in 20% of the population
What is the effect of patent foramen ovale?
Generally clinically silent:
- The higher left atrial pressure causes functional closure of the flap valve
But it may be the route by which a venous embolism reaches the systemic circulation if pressure on the right side of the heart increases even transiently
What is hypoplastic left heart?
Failure of the left ventricle and ascending aorta to develop properly
- A PFO and ASD are also present
What is the effect of hypoplastic left heart?
It is lethal without surgical correction
How is blood flow affected in hypoplastic left heart?
Blood supply to the systemic circulation is via a PDA
