It is also
possible that all the cardiac chambers and most valves are in normal
sequence but some valves are not well formed, leading to absent
valvular connections. When such valves are absent, the term "atresia"
is also used, implying that no antegrade (or forward) flow is possible
across the valve. Thus, atresia of any valve may occur. Valvular
atresia on the right side prevents blood from reaching the lungs;
valvular atresia on the left side prevents blood from reaching the
systemic circuit. For any infant with valvular atresia to survive
past the first few hours of life, a shunt lesion must be present
to allow blood to progress antegradely through the heart.
Not all obstructive lesions prevent the total forward flow of blood.
Some, such as subvalvular aortic stenosis, offer partial obstruction
to flow. The degree of obstruction relates to the anatomic severity
of the lesion.
Absent atrioventricular connections
Absent atrioventricular valve connections (tricuspid or mitral atresia)
are less common than those lesions previously discussed. Atresia
of either of these valves may have one of two underlying causes:
an absence of the atrioventricular connection, or an imperforate
membrance blocking the valve orifice as noted in Fig. 3. Gradations
between the two extremes also exist.
Where an imperforate membrane has caused atrioventricular valve
atresia, there is a formed (but usually hypoplastic) atrioventricular
valve ring blocked by an imperforate membrane. Atrioventricular
communication is potentially possible by excision of the membrane.
 |
| Fig.22 |
More commonly, atretic valves have completely absent tissue. Hypoplasia
of proxima or distal chambers to the atretic valve is also possible.
Fig. 22 demonstrates an absent right atrioventricular connection
(tricuspid atresia) form the subcostal approach. The right ventricle
is hypoplastic and difficult to visualize. The left-sided atrioventricular
valve is present.
In this patient, survival would be impossible because no route for
antegrade blood flow to the lungs is present. Note, however, that
there is a large secundum atrial septal defect that allows blood
to immediately mix with oxygenated blood in the left atrium and
then transit the left ventricle into the system circuit. A central
shunt (a synthetic tube between the aorta and the pulmonary artery)
was surgically placed to allow some mixed blood to then indirectly
travel back to the lungs for oxygenation.
 |
| Fig.23 |
Fig. 23 shows an apical four-chamber view from another patient with
tricuspid atresia. A thick ridge of tissue replaces the tricuspid
valve. The right ventricle is poorly formed. The left-sided atrioventricular
valve is present and the left ventricle is normal. Note that the
atrial septum is bowed from the right atrium into the left atrium.
 |
| Fig.24 |
This patient was treated in infancy, similar to the previous patient.
Later, the atrial septum was closed and the central shunt removed.
Blood flow to the pulmonary artery was established by placing a
conduit from the right atrium directly to the proximal pulmonary
artery (the Fontan procedure). With no intervening right ventricle
to pump blood into the lungs, hydrostatic pressure rises on the
venous side. This is frequently sufficient to provide adequate blood
for oxygenation. An absent left atrioventricular connection (mitral
atresia) is somewhat less common. Fig. 24 shows an infant with a
normally formed tricuspid valve from the apical four-chamber view.
The mitral valve has been replaced by a thick band of tissue. Survival
in this case is impossible unless some route of blood flow returning
to the lungs from the left atrium is provided. In this case, the
atrial septum was emergently removed to allow for mixing of oxygenated
blood with venous blood in the right atrium. Note that there is
only one ventricle. The ventricle gave rise to both the pulmonary
artery and aorta, assuring some blood flow to both the lungs and
periphery.
2004