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By Richard A. Jonas, MD
The Blalock Shunt There are a number of disadvantages
in using the modified Blalock
shunt with the Norwood procedure.
Blood flows into the lungs during both
systole and diastole. It is during diastole
that blood normally flows into the
heart muscle itself since it cannot do so
during ventricular contraction because
of the high intramyocardial pressure.
Because the shunt is stealing blood
away from the arterial system to the
lungs during diastole, there is a risk
that there will be inadequate pressure to
supply the coronary arteries themselves.
Furthermore, there are often changes in
the resistance in the lungs depending
upon whether a child is beginning to
wake up, perhaps has had the endotracheal
tube suctioned recently, or has
experienced a change in inspired oxygen
concentration. The competition between
lung blood flow and coronary artery
blood flow probably explains the instability
that has frequently been seen in
babies after a Norwood procedure. It
was this instability that contributed to
an ongoing risk of cardiac arrest and
death reported in up to 25% of babies
who had the Norwood procedure with
a modified Blalock shunt.
The Sano Shunt The Sano shunt modification
avoids the problem of competitive flow
between the lungs and coronary arteries.
The shunt is constructed from a slightly
larger Gortex tube graft than that used
for the modified Blalock shunt. Generally
a 5 mm tube graft is selected in contrast
to the 3.5 mm graft used for averagesize
babies for a Blalock shunt. Distally,
the graft is connected to the main
pulmonary artery between the right
and left pulmonary artery takeoffs. The
proximal end of the shunt is connected
to a limited infundibular incision in
the right ventricle.
In the past there was concern over
the need for a ventricular incision,
which led to many surgeons avoiding
this technique. However, Dr. Sano's
experience suggests that there is such a
marked improvement in coronary
blood flow that this compensates for
the localized injury caused to a small
area of the right ventricle by the shunt
incision. Many centers that have started
to use this modification have reported much improved
stability in babies postoperatively. At Children's Hospital
Boston, for example, since the Sano shunt modification
was introduced by the author in 2002, mortality has
been less than 10%. A number of children have
progressed onto the second-stage bidirectional Glenn
shunt and all have done well. However, the Glenn shunt
generally needs to be inserted somewhat earlier than a
Blalock shunt. By three to four months the majority of
patients have an inadequate arterial oxygen saturation
and need to move ahead to the next stage. In the past,
the majority of children had their second-stage procedure
at approximately six months of age.
In the future it will be important to compare the
outcome for children after they have completed the
third-stage Fontan procedure following an initial Sano
shunt to children who have the traditional Blalock
shunt as part of their first-stage
procedure. However, early indications suggest that
there is every probability that children with the Sano
modification will do just as well if not better than those
with a Blalock shunt.
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In January 1983, William
Norwood, MD, of Children's Hospital
Boston astonished cardiologists and
surgeons working with children with
congenital heart disease by publishing
a report of successful surgical palliation
of hypoplastic left heart syndrome. Up
until that time, this condition, which is
not rare, had carried a 100% mortality
risk. In his original procedure, as
described in the New England Journal
of Medicine, Dr. Norwood divided the
main pulmonary artery and connected
it to the aorta so that the normally
developed right ventricle could take
over the work of the underdeveloped
left ventricle.
Dr. Norwood's recommendation
for supplying blood to the lungs was
through the use of a Blalock shunt.
The Blalock shunt was originally
described in the 1940s by William
Blalock, MD, at Johns Hopkins
Hospital. Dr. Blalock had developed
the shunt for treatment of cyanotic
babies with inadequate pulmonary
blood flow. By connecting the subclavian
artery directly to a pulmonary
artery, the shunt made it possible to
increase pulmonary blood flow. In the
late 1970s following the introduction
of the new biomaterial Gortex, it was
found that a small Gortex tube could
be connected between the subclavian
artery and the pulmonary artery. This
technique, first introduced by Marc
de Leval in London, came to be called
the "modified Blalock shunt" and was
adopted by Norwood and others for
supplying pulmonary blood flow as
part of the procedure.
Over the last two years, a number
of centers around the world, including
Children's Hospital Boston, have begun
to adopt a modification of the Norwood
procedure that involves a different type
of shunt. Introduced by Shunji Sano,
MD, who was trained in congenital
cardiac surgery in Melbourne, Australia,
and is now working in his home country
of Japan, this new modification indicates
further improvement in the survival of
newborn babies with hypoplastic left
heart syndrome.
