Predominantly Left-to-Right (Simple) Shunts
Simple shunts are isolated abnormal
communica-tions between the right and left sides of the heart. Because
pressures are normally higher on the left side of the heart, blood usually
flows across from left to right, and blood flow through the right heart and the
lungs increases. Depending on the size and location of the communication, the
right ventricle may also be subjected to the higher left-sided pres-sures,
resulting in both pressure and volume over-load. Right ventricular afterload is
normally 5% that of the left ventricle, so even small left-to-right pressure
gradients can produce large increases in pulmonary blood flow. The ratio of
pulmonary (Qp) to systemic (Qs) blood flow is useful to determine the
directionality of the shunt.
A ratio greater than 1 usually indicates
a left-to-right shunt, whereas a ratio less than 1 indicates a right-to-left shunt.
A ratio of 1 indicates either no shunting or a bidirectional shunt of opposing
Large increases in pulmonary blood flow
pro-duce pulmonary vascular congestion and increase extravascular lung water.
The latter interferes with gas exchange, decreases lung compliance, and
increases the work of breathing. Left atrial distention also compresses the
left bronchus, whereas distention of pulmonary vessels compresses smaller
Over the course of several years,
chronic increases in pulmonary blood flow produce vascu-lar changes that
irreversibly increase PVR. Elevation of right ventricular afterload produces
hypertro-phy and progressively raises right-sided cardiac pressures. With
advanced disease, the pressures within the right heart can exceed those within
the left heart. Under these conditions, the intracardiac shunt reverses and
becomes a right-to-left shunt (Eisenmenger syndrome).
When a communication is small, shunt
flow depends primarily on the size of the communication (restrictive shunt).
When the communication is large (nonrestrictive shunt), shunt flow depends on
the relative balance between PVR and SVR.
An increase in SVR relative to PVR
favors left-to-right shunting, whereas an increase in PVRrelative to SVR favors
right-to-left shunting. Common chamber lesions (eg, single atrium, single
ventricle, truncus arteriosus) represent the extreme form of nonrestrictive
shunts; shunt flow with these lesions is bidirectional and totally dependent on
relative changes in the ventricular afterload.
The presence of shunt flow between the
right and left hearts, regardless of the direction ofblood flow, mandates the
meticulous exclusion of air bubbles and particulate material from intravenous
fluids to prevent paradoxical embolism into the cerebral or coronary
Ostium secundum atrial septal defects
(ASDs) are the most common type and usually occur as isolated lesions in the
area of the fossa ovalis. The defect is sometimes associated with partial
anomalous pul-monary venous return, most commonly of the right upper pulmonary
vein. A secundum ASD may result in single or multiple (fenestrated) openings
between the atria. The less common sinus venosus and ostium primum ASDs are
typically associated with other car-diac abnormalities. Sinus venosus defects
are located in the upper interatrial septum close to the superior vena cava;
one or more of the right pulmonary veins often abnormally drains into the
superior vena cava. In contrast, ostium primum ASDs are located in the lower
interatrial septum and overlie the mitral and tricuspid valves; most patients
also have a cleft in the anterior leaflet of the mitral, valve and some have an
abnormal septal leaflet in the tricuspid valve.
Most children with ASDs are minimally
symp-tomatic; some have recurrent pulmonary infections. Congestive heart
failure and pulmonary hyperten-sion are more commonly encountered in adults
with ASDs. Patients with ostium primum defects often have large shunts and may
also develop significant mitral regurgitation. In the absence of heart failure,
anesthetic responses to inhalation and intravenous agents are generally not
significantly altered in patients with ASDs. Large increases in SVR shouldbe avoided because they may worsen the
Ventricular septal defect (VSD) is a
common con-genital heart defect, accounting for up to 25% to 35% of congenital
heart disease. The defect is most frequently found in the membranous part of
the interventricular septum (membranous or infracris-tal VSD) in a posterior
position and anterior to the septal leaflet of the tricuspid valve. Muscular
VSDs are the next most frequent type and are located in the mid or apical
portion of the interventricular sep-tum, where there may be a single defect or
multiple openings (resembling Swiss cheese). Defects in the subpulmonary
(supracristal) septum are often asso-ciated with aortic regurgitation because
the right coronary cusp can prolapse into the VSD. Septal defects at the
ventricular inlet are usually similar in development and location to AV septal
defects (see the following section).
The resulting functional abnormality of
a VSD is dependent on the size of the defect, PVR, and the presence or absence
of other abnormalities. Small VSDs, particularly of the muscular type, often
close during childhood. Restrictive defects are associated with only small
left-to-right shunts (pulmonary– systemic blood flow ratios less than 1.75:1).
Large defects produce large left-to-right shunts (shunts larger than 2:1) that
vary directly with SVR and indirectly with PVR. Recurrent pulmonary infec-tions
and congestive heart failure are common with pulmonary–systemic flow ratios of
3–5:1. Patients with small VSDs are treated medically and fol-lowed by electrocardiography
(for signs of right ventricular hypertrophy) and echocardiography. Surgical
closure is usually undertaken in patients with large VSDs before pulmonary
vascular disease and Eisenmenger physiology develop. As with atrial defects, in
the absence of heart failure, anesthetic responses to inhalation and
intravenous agents are generally not significantly altered. Similarly,
increases in SVR worsen the left-to-right shunting.When right-to-left shunting is present, abrupt increases in PVR or
decreases in SVR are poorly tolerated.
Endocardial cushion (AV canal) defects
produce contiguous atrial and ventricular septal defects, often with very
abnormal AV valves. This is a com-mon lesion in patients with Down syndrome.
The defect can produce large shunts both at the atrial and ventricular levels.
Mitral and tricuspid regurgi-tation exacerbate the volume overload on the
ven-tricles. Initially, shunting is predominately left to right; however, with
increasing pulmonary hyper-tension, Eisenmenger syndrome with obvious cya-nosis
Persistence of the communication between
the main pulmonary artery and the aorta can produce restrictive or
nonrestrictive left-to-right shunts. This abnormality is commonly responsible
for the car-diopulmonary deterioration of premature infants and occasionally
presents later in life when it can be corrected thoracoscopically. Anesthetic
goals should be similar to atrial and ventricular septal defects.
This defect is present when one or more
pulmo-nary veins drains into the right side of the heart; the anomalous veins
are usually from the right lung. Possible anomalous entry sites include the
right atrium, the superior or inferior vena cava, and the coronary sinus. The
resulting abnormality produces a variable amount of left-to-right shunting. The
clinical course and prognosis are usually excellent and similar to that of a
secundum ASD. A very large coronary sinus on TEE suggests anomalous drainage
into the coronary sinus, which may complicate the management of cardioplegia
during cardiac surgery. Total anomalous venous return is corrected imme-diately