It is easiest to begin with disorders where chambers and valves
are in the normal, or relatively normal position. Such disorders
comprise those of atrial and/or ventricular septal defects and
those disorders where chambers and valves are absent or small,
causing obstruction. As a consequence, these types of defects
will be divided into those when shunting (abnormal blood flow
between the left and right circulations), obstruction, or regurgitation
are predominant.
When Shunting is Predominant
In the structurally normal heart, right and left sides are divided
and are without communication. Shunting refers to those flow anomalies
where there is an abnormal communication, such as an atrial or
ventricular septal defect, that allows abnormal flow between the
right and left sides. Since right-sided pressures are normally
lower than those on the left side, when such defects are encountered
abnormal flow is usually left-to-right and increased flow into
the lungs results. Normally, the lungs can accommodate the increased
flow without significant symptoms if the degree of shunting is
small or moderate.
When significant shunting is present and exceeds the ability
of the lungs to accept the increase, the lungs are literally flooded
and symptoms of cardiac failure ensue. Where no failure is clinically
evident, such shunting, over time, may result in reactive changes
in the pulmonary vasculature where normally low pressures then
rise within the right heart and exceed those on the left side.
Depending on the clinical situation, such changes may be permanent
and cause right-to-left shunting (Eisenmenger's Syndrome). Without
early recognition and correction, these simple problems may result
in permanent, inoperative damage to the lungs. In addition, any
communication between right and left sides of the heart may allow
for the possibility of venous emboli entering the arterial circuit.
2004