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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3612_Библиотеки_им_академика_М_И_Перельмана
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Key Questions in CONGENITAL CARDIAC SURGERY
whether or not to disclose an operative risk solely upon whether or
not it crosses a numerical threshold of 1, 0.1 or 0.001 percent is
arbitrary and unreasonable.
Recommended reading
450
1. Bainham A.
2. Wheeler RA. Gillick or Fraser? A plea for consistency over competence in children.
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3. Family Law Reform Act 1969 s8.
4. General Medical Council. Consent: patients and doctors making decisions together.
GMC: London, 2008.
5. Wheeler RA. Numeric threshold for risk.
6. Wheeler RA.
2020.
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2006; 332: 807.
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Bristol: Jordan Publishing, 2005.
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London: CRC Press, Taylor & Francis,
2012; 94(2): 81-2.
Chapter 13
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Atrial septal defects
Narain Moorjani, Nicola Viola
1 What is an atrial septal defect (ASD)?
An atrial septal defect is a congenital lesion that results in
•
communication and therefore shunting of blood between the left and
right atria.
2 How is the atrial septum formed embryologically?
The cavity of the primitive atrium is initially subdivided into right and
•
left chambers by the septum primum, which is a crescentic fold of
endocardial tissue that begins to develop at day 28 in utero (Figure
1).
451
Septum primum
Membrane erosions
Ostium primum
Endocardial cushions
Figure 1. Development of the septum primum.
Key Questions in CONGENITAL CARDIAC SURGERY
The septum primum grows downwards to close the primary
•
interatrial foramen by fusing with the endocardial cushions by
approximately day 35, leaving an opening below its free margin
(ostium primum).
By programmed cell death (apoptosis), a second opening (ostium
•
secundum or foramen ovale) develops in the upper part of the
septum primum at day 33. This opening persists until birth.
At a similar time (day 33), a second endocardial fold (septum
•
secundum), semilunar in shape, develops and descends towards but
never reaches the endocardial cushions, thereby forming the fossa
ovalis (Figure 2).
Ostium secundum
452
Endocardial cushions
Figure 2. Development of the septum secundum.
•
•
Septum
primum
Septum secundum
Foramen interventricularis
Shortly after birth, the foramen ovale closes following fusion of the
septum secundum with the septum primum (Figure 3).
Hence, the component parts of the final true atrial septum (Figure 4)
include the:
Septum secundum
Fused endocardial cushions
a) floor of the fossa ovalis — which is derived from the septum
primum;
b) anteroinferior muscular rim of the fossa ovalis.
13 Atrial septal defects
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Septum secundum
Sealed ostium secundum
Septum secundum
Valve of foramen ovale
Crux of the heart
Sealed ostium
secundum
Figure 3. Closure of the foramen ovale resulting in separation of the left
and right atrial chambers.
453
Superior rim of the fossa
(Waterston’s groove)
True interatrial septum:
Floor of the fossa ovalis
Anteroinferior muscular
rim of the fossa ovalis
Sinus septum
Aortic rim of the fossa
Membranous septum
Tricuspid valve
Atrial component
of the atrioventricular
septum
Figure 4. Components of the true interatrial septum (in white) and
surrounding structures.
454
Key Questions in CONGENITAL CARDIAC SURGERY
3 How are atrial septal defects classified (Figure 5)?
Defects of the interatrial septum:
•
a) fossa ovalis defects (75%):
i) patent foramen ovale (Vieussens valve) — superiorly placed;
ii) ostium secundum defect — centrally placed, can be
single or multi-fenestrated (Figure 5B);
b) vestibular defect (<1%) — muscular defect in the anteroinferior
rim of the fossa ovalis, outside the triangle of Koch (Figure 5C).
Defects outside the interatrial septum:
•
a) sinus venosus defect (5-10%) — which usually occurs at the
superior vena cava-right atrial junction (superior defect), but
may also occur at the inferior vena cava-right atrial junction
(inferior defect) (Figure 5D);
b) coronary sinus defect (1%) — which represents an unroofed
coronary sinus (Figure 5E);
c) other types: cor triloculare biatriatum (<1%).
Defects of the atrioventricular septum:
•
a) ostium primum defect (15-20%) — which is also known as a
partial atrioventricular septal defect (Figure 5F).
Embryologically, only lesions in the fossa ovalis (ostium secundum
•
defects, patent foramen ovale and vestibular defects) are considered
true atrial septal defects.
Surgically, however, all interatrial communications, including ostium
•
primum defects, sinus venosus defects and coronary sinus defects, as
well as the fossa ovalis lesions, are referred to as atrial septal defects.
4 What is the aetiology of atrial septal defects?
An ostium secundum defect is caused by abnormal reabsorption of
•
the septum primum or incomplete overlap of the septum secundum
with the septum primum.
An ostium primum defect is caused by incomplete fusion of the septum
•
primum with the endocardial cushions, resulting in a gap at the level of
the atrioventricular valves. The left atrioventricular valve is usually
displaced, whereas the right atrioventricular valve is rarely involved.
A sinus venosus defect is caused by incomplete fusion of the
•
embryological sinus venosus with the atrial septum, most commonly
near the junction with the superior vena cava.
13 Atrial septal defects
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AB
Ostium secundum defect
C
Vestibular defect
EF
D
Superior sinus
venosus defect
Patent
foramen ovale
Inferior sinus
venosus defect
455
Coronary sinus
defect
Ostium primum defect
Figure 5. Atrial septal defects: A) normal anatomy of the right atrial
cavity; B) multi-fenestrated ostium secundum defect; C) vestibular
defect; D) sinus venosus defect (superior and inferior type) and patent
foramen ovale; E) multi-fenestrated coronary sinus defect; F) ostium
primum defect and patent foramen ovale.
456
Key Questions in CONGENITAL CARDIAC SURGERY
A patent foramen ovale is caused by incomplete fusion of the septum
•
primum with the septum secundum.
Atrial septal defects can also have an iatrogenic aetiology, either
•
during catheter-based procedures (such as percutaneous mitral
balloon valvotomy) or cardiac surgery (such as atrial myxoma
excision or left atriotomy for mitral valve surgery).
5 What abnormalities are often associated with atrial
septal defects?
Trifoliate left atrioventricular valve with or without regurgitation
•
(associated with ostium primum defects).
Anomalous pulmonary venous connections (partial and total,
•
associated with sinus venosus defects or fossa ovalis defects).
Left-sided superior vena cava (associated with coronary sinus
•
defects).
Asplenia.
•
Atrial septal defects are ten times more likely to occur in association
•
with other anomalies than as isolated defects.
6 What is the epidemiology of atrial septal defects?
Account for 7% of all congenital cardiac abnormalities (second only
•
to bicuspid aortic valve).
Female to male ratio is 2:1.
•
Increased incidence in patients with Down’s syndrome.
•
7 What is the pathophysiology of an atrial septal defect?
Due to the difference in compliance between the left and right
•
ventricles, blood usually flows across the defect as a left-to-right
shunt, predominantly in late systole and during diastole.
The magnitude of the shunt depends on the:
•
a) size of the defect;
b) compliance of the left and right ventricles;
c) systemic and pulmonary vascular resistance.
This results in volume overload of the right-sided chambers, with
•
subsequent enlargement of the right atrium, right ventricle and
pulmonary artery.
Initially, the shunt is well tolerated whilst the pulmonary vascular
•
resistance remains low, but if left uncorrected, atrial arrhythmias,
right ventricular failure and possibly pulmonary hypertension
eventually ensue.
13 Atrial septal defects
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8 What is the natural history of atrial septal defects?
Although most patients with an untreated defect usually survive into
•
adulthood, their life expectancy is not normal in comparison to an
age-matched population.
Survival from a cardiac standpoint is influenced by the clinical
•
sequelae of:
a) atrial arrhythmias;
b) right ventricular dysfunction;
c) pulmonary hypertension;
d) stroke;
e) shunt reversal and Eisenmenger syndrome (in rare cases).
9 What are the symptoms of an atrial septal defect?
The timing of the clinical presentation depends on the degree of the
•
left-to-right shunt and subsequent haemodynamic consequences.
Most patients are hence asymptomatic.
•
Patients may eventually present with:
•
a) reduced exercise tolerance and fatigue;
b) recurrent respiratory infections;
c) palpitations (secondary to atrial arrhythmias);
d) syncope;
e) stroke (secondary to paradoxical embolism);
f) congestive cardiac failure.
457
10 What are the signs of an atrial septal defect?
The physical signs elicited depend on the haemodynamic
•
consequences and relate to the:
a) size of the defect;
b) degree of the left-to-right shunt;
c) relative compliance of the left and right ventricles;
d) systemic and pulmonary vascular resistances.
Parasternal heave of the right ventricle (due to increased diastolic
•
filling and subsequent increased right ventricular stroke volume).
Split S1 (caused by increased intensity of T1 due to delayed closure
•
of the tricuspid valve).
Fixed splitting of the 2nd heart sound (which does not vary during
•
respiration).
Pansystolic flow murmur at the left sternal edge in the 2nd intercostal
•
space (due to increased blood flow across the pulmonary valve and
not due to flow across the atrial septal defect).
458
Key Questions in CONGENITAL CARDIAC SURGERY
Soft mid-diastolic murmur at the left sternal edge in the 4th intercostal
•
space (due to the increased flow through the tricuspid valve).
An apical pansystolic murmur of mitral regurgitation may be heard in
•
patients with an ostium primum defect.
If the patient develops pulmonary hypertension, further signs include:
•
a) early diastolic murmur (Graham Still murmur, representing
pulmonary regurgitation);
b) disappearance of the split 2nd heart sound, pansystolic flow
murmur and the mid-diastolic rumble.
Cyanosis and clubbing are only present in patients with shunt
•
reversal (Eisenmenger syndrome).
11 What are the electrocardiographic features of atrial
septal defects (Figure 6)?
Axis deviation:
•
a) ostium secundum defect — right axis deviation;
b) ostium primum defect — left axis deviation;
c) sinus venosus defect — left axis deviation.
Incomplete right bundle branch block (secondary to the right
•
ventricular volume overload), resulting in an rSR’ pattern in V1 and a
wide S wave in leads I, II and V6.
Right ventricular hypertrophy, demonstrated by a tall R wave
•
(>10mm) in V1.
Usually normal sinus rhythm, but atrial fibrillation is often present in
•
patients aged >40 years old.
Figure 6. Electrocardiogram of a patient with an ostium secundum
defect, demonstrating an rSR’ pattern in V1.
13 Atrial septal defects
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12 What are the radiological features of atrial septal
defects (Figure 7)?
Chest radiography may demonstrate:
•
a) cardiomegaly (due to right atrial and right ventricular enlargement);
b) prominent pulmonary arteries (dilated hilar vessels) with
increased pulmonary vascular markings (Figure 7);
AB
459
Figure 7. Chest radiographs demonstrating increased pulmonary
vascular markings, hilar congestion and a distended right atrium in: A) an
18-month-old child with a large ostium secundum defect; and B) a 67year-old patient with a superior sinus venosus defect.
c) in patients with ostium primum defects, left atrial enlargement
may be observed if significant mitral regurgitation is present;
d) in patients with sinus venosus defects, proximal dilation of the
superior vena cava may be seen.
Cardiac magnetic resonance imaging (MRI) may be useful to further
•
delineate associated anomalies and the haemodynamic consequences of
volume overload of the right heart and pulmonary circulation (Figure 8).
os=
is=
os=
is=
Figure 8. Cardiac MRI in a patient with a sinus venosus defect
demonstrating an enlarged right ventricle (RV). LV = left ventricle.
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