Atrial septal defect

Question 1

Definition & types according to septum development?

Answer 1

Arial septal defect = opening between the right and left atria.

90% are due to septum secondum defect (hole in upper part, ostium secondum remains); 10% are due to septum primum not growing all the way down & fusing with the fused endocardial cushions to close off the ostium primum.

Question 2

Risk Factors

Answer 2

• Down syndrome: associated with ostium primum defects
• Foetal alcohol syndrome

Question 3

Normal atrial septum development during embryogenesis & changes in fetal circulation at birth- focus on ASD?

Answer 3

IN THE FETUS:

• During development, the septum primum grows downwards from the top of the heart towards the fused endocardial cushions to separate the left & right atria. It slowly creates 2 separate chambers from 1 atrium by closing the gap (ostium primum).
• The septum primum then fuses with the endocardial cushion and closes the gap completely. A hole appears in the upper area of the septum primum, called the ostium secundum.
• The septum secundum grows downward, to the right of the septum primum, and covers the ostium secundum, leaving a small opening called the foramen ovale- allows blood to be shunted from RA→ LA to bypass nonfunctional lungs.

(The foetus receives oxygenated blood via the umbilical vein from the placenta→ this goes to the right atrium, through the foramen ovale into the left atrium and then out of the left ventricle to supply the body. )

AT BIRTH:

O2 is a pulmonary vasodilator so when the 1st breath is taken, pulmonary vascular resistance drops hence blood flow from the RV increases→ increased venous return to LA increases left-sided pressures so foramen ovale shuts.

Question 4

ASD pathophysiology (types, blood flow, complications)

Answer 4

An atrial septal defect describes when the septum between the right and left atrium doesn’t close up all the way and remains open even after birth.

Types of ASD from most to least common are:

• 90% = Ostium secondum, where theseptum secondumfails to fully close, leaving a hole in the wall.
• Patent foramen ovale, where theforamen ovalefails to close (although this not strictly classified as anASD).
• Ostium primum, where theseptum primumfails to fully close, leaving a hole in the wall. This tends to lead toatrioventricular valve defectsmaking it anatrioventricular septal defect (BIG HOLE IN MIDDLE OF HEART).

BLOOD FLOW WITH AN ASD:

• Blood is shunted from left atrium → right atrium as pressures are higher in left atrium. This is an acyanotic shunt (not blue) as there is only O2 blood getting to the RHS & being pumped to the pulmonary circulation, not deoxygenated blood bypassing lungs & being pumped to the LHS to the systemic circulation.
• The extra blood volume passing through the pulmonary valve causes a delay in the closure of the pulmonic valve relative to the aortic valve. This slight delay in closure of the pulmonary valve can be heard as splitting of the S2 sound, & systolic murmur in some cases.
• Increased pulmonary flow leads to right ventricular hypertrophy (compensation for increased afterload) & eventually right heart failure & pulmonary hypertension.

Complication = EISENMENGER’S SYNDROME

Pulmonary pressures on RHS > systemic pressures on LHS → shunt reverses & blood flows right to left across the atria septal defect. This means deoxygenated blood bypasses the lungs & is pumped to the systemic circulation, resulting in cyanosis.

Question 5

Clinical manifestations (signs & symptoms)

Answer 5

Often asymptomatic in childhood. LESS SEVERE THAN VSD, SIMILIAR SYMPTOMS.
SIGNS:
- Ejection systolic murmur loudest at the upper left sternal border, 2nd intercostal space.
- Fixed split S2 sound (due to pulmonary valve closing after the aortic valve due to increased blood flow through the pulmonary valve. Fixed split means the split does not changes with inspiration or expiration).
- Cyanosis (if reversal of shunt, this indicates Eisenmenger’s syndrome)

Typical symptoms in childhood
- Dyspnoea (increased pulmonary blood flow, less blood flow to systemic circulation)
- Exercise intolerance
- Difficulty feeding
- Poor weight gain
- Lower respiratory tract infections

Question 6

Investigations

Answer 6

• ECHO: hypertrophy and dilation of right side of heart and pulmonary arteries. = diagnostic
  CXR & ECG are not diagnostic.
• CXR: (large pulmonary arteries; may show large heart)
• ECG: (may show right bundle branch block due to right ventricle dilatation)

Question 7

Management

Answer 7

For right to left shunt

1st line = Watch and wait: for small ASD as it may close spontaneously

2nd line = Surgical closure of ASD

• Transvenous-catheter closure (via the femoral vein)
• Open heart surgery

& Anticoagulants: used to reduce the risk of clots and stroke in adults (in patients with ASD & DVT, the DVT can embolise to the internal carotids & cause an ischaemic stroke).

Question 8

What is a paradoxical embolsim?

Answer 8

In Px with ASD & DVT, it can embolise & travel through the RA → LA→ LV → internal carotids & cause ischemic stroke. This is known as a paradoxical embolism as normally a DVT should result in a pulmonary embolism. Hence these patients should be given anticoagulants.

Question 9

Complications (3)

Answer 9

• Eisenmenger’s syndrome (pulmonary pressures > systemic pressures so the shunt reverses & blood is shunted from right to left across the atrial septal defect. Results in cyanosis as deoxygenated blood bypasses the lungs & is pumped to the pulmonary circulation)
• Cor pulmonale (right heart failure due to pulmonary hypertension)
• May develop atrial arrythmias due to right atrial dilatation (volume overload on RHS of heart)