474
U N I T 5
Circulatory Function
adulthood and consider having children of their own.
Recent evidence suggests that the genetic contribution
to congenital heart disease has been underestimated in
the past.
65,66
Some heart defects, such as aortic stenosis,
atrial septal defect of the secundum type, pulmonary
valve stenosis, tetralogy of Fallot, and certain ventricu-
lar septal defects, have a stronger familial predisposition
than others. Chromosomal abnormalities are also asso-
ciated with congenital heart defects, as evidenced by the
observation that as many as 30% of children with con-
genital heart disease have an associated chromosomal
abnormality. Heart disease is found in nearly 100% of
children with trisomy 18, 50% of those with trisomy
21, and 35% of those with Turner syndrome.
66
Congenital heart diseases are commonly classified
according to their anatomic site (atrial septal or ventric-
ular septal defects), the hemodynamic alterations caused
by the anatomic defects (left-to-right or right-to-left
shunts), and their effect on pulmonary blood flow and
tissue oxygenation (cyanotic or noncyanotic defects).
Shunting
Shunting of blood refers to the diversion of blood flow
from one system to the other—from the arterial to the
venous system (i.e., left-to-right shunt) or from the
venous to the arterial system (i.e., right-to-left shunt).
64
The shunting of blood in congenital heart defects is
determined by the presence of an abnormal opening
between the right and left circulations and the degree of
resistance to flow through the opening. The shunting of
blood can affect both the oxygen content of the blood
and the volume of blood being delivered to the vessels in
the pulmonary circulation.
The direction of shunting (right-to left or left-to-
right) is largely determined by the vascular resistance
of the systemic and pulmonary circulations. Due to the
high pulmonary vascular resistance in the neonate, atrial
and ventricular septal defects usually do not produce a
significant shunt during the 1st weeks of life. As the pul-
monary vascular smooth muscle regresses in the neo-
nate, the resistance in the pulmonary circulation falls
below that of the systemic circulation, causing a left-to-
right shunt in uncomplicated atrial or ventricular septal
defects. In more complicated ventricular septal defects,
increased resistance to outflow may affect the pattern of
shunting. For example, defects that increase resistance
to aortic outflow (e.g., aortic valve stenosis, coarctation
of the aorta, hypoplastic left heart syndrome) increase
left-to-right shunting, whereas defects that obstruct pul-
monary outflow (e.g., pulmonary valve stenosis, tetral-
ogy of Fallot) increase right-to-left shunting. Crying,
defecating, or even the stress of feeding may increase
pulmonary vascular resistance and cause an increase in
right-to-left shunting in infants with septal defects.
Cyanosis, a bluish color of the skin most notable
in the nail beds and mucous membranes, develops
when sufficient deoxygenated blood from the right
side of the heart mixes with oxygenated blood in the
left side of the heart.
67
Abnormal color becomes obvi-
ous when the oxygen saturation falls below 80% in
the capillaries (equal to 5 g of deoxygenated hemo-
globin). Defects that result in a right-to-left shunting
or obstruction of pulmonary blood flow are catego-
rized as cyanotic disorders and those involving left-
to-right shunting are usually categorized as acyanotic
disorders. Of the congenital defects discussed in this
chapter, patent ductus arteriosus, atrial and ventricu-
lar septal defects, endocardial cushion defects, pulmo-
nary valve stenosis, and coarctation of the aorta are
considered acyanotic; tetralogy of Fallot, transposition
of the great vessels, and single-ventricle anatomy are
considered cyanotic defects.
A right-to-left shunt results in deoxygenated blood
moving from the right side of the heart to the left side
and then being ejected into the systemic circulation.
With a left-to-right shunt, oxygenated blood intended
for ejection into the systemic circulation is recirculated
through the right side of the heart and back through the
lungs. This increased volume distends the right side of
the heart and pulmonary circulation and increases the
workload placed on the right ventricle.
68
Alterations in Pulmonary Blood Flow
Many of the complications of congenital heart disor-
ders result from a decrease or an increase in pulmonary
blood flow. Defects that reduce pulmonary blood flow
(e.g., pulmonary stenosis) typically cause symptoms of
fatigue, dyspnea, and failure to thrive. In contrast to the
arterioles in the systemic circulation, the arterioles in the
pulmonary circulation are normally thin-walled vessels
that can accommodate the various levels of stroke vol-
ume that are ejected from the right heart. The thinning
of the pulmonary vessels occurs during the 1st weeks
after birth, during which the vessel media thin and pul-
monary vascular resistance decreases. In a term infant
who has a congenital heart defect that produces mark-
edly increased pulmonary blood flow (e.g., ventricular
septal defect), the increased flow stimulates pulmonary
vasoconstriction and delays or reduces the normal invo-
lutional thinning of the small pulmonary arterioles. In
most cases pulmonary vascular resistance is only slightly
elevated during early infancy, and the major contribution
to pulmonary hypertension is the increased blood flow.
However, in some infants with a large right-to-left shunt,
the pulmonary vascular resistance never decreases.
Congenital heart defects that persistently increase
pulmonary blood flow or pulmonary vascular resistance
have the potential of causing pulmonary hypertension
and producing irreversible pathologic changes in the
pulmonary vasculature. When shunting of systemic
blood flow into the pulmonary circulation threatens per-
manent injury to the pulmonary vessels, a surgical pro-
cedure should be done to reduce the flow temporarily
or permanently. Pulmonary artery banding consists of
placing a constrictive band around the main pulmonary
artery, thereby increasing resistance to outflow from the
right ventricle. The banding technique is a temporary
measure to alleviate symptoms and protect the pulmo-
nary vasculature in anticipation of later surgical repair
of the defect.
