1.
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Does the bidirectional
cavopulmonary shunt lower the risk of a subsequent Fontan procedure (i.e., performed as a
staging procedure) as compared to a Fontan procedure with no previous bidirectional
cavopulmonary shunt?
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| The overwhelming feeling at present is that the bidirectional
cavopulmonary shunt (BCPS) is an excellent staging procedure for the infant with a single
functional ventricle and lowers the morbidity and the mortality of a subsequent Fontan
procedure. The BCPS reduces the volume overload on the functional single ventricle (when
extra sources of pulmonary blood flow are eliminated) and improves the efficiency of the
pulmonary circulation and oxygenation. It is felt that this reduction of volume overload
will allow regression of ventricular hypertrophy that is thought to contribute to the
diastolic dysfunction and morbidity of the Fontan procedure. In addition to this, the BCPS
can lower the mean pulmonary artery pressure over time, thereby lowering the risk of a
subsequent Fontan procedure. The BCPS can be applied in infants as young as 4-6 weeks of
age and often raises the saturation to about 85%. These factors make the BCPS an excellent
form of interim palliation in planning the long term strategy for the infant with a single
functional ventricle. One possibly detrimental factor of the BCPS is the observation that
pulmonary artery growth is often reduced following operation. This may have implications
for earlier conversion to a total cavopulmonary connection or Fontan procedure to negate
some of the negative effects of the BCPS on pulmonary artery growth. Despite these
numerous benefits, there are no randomized trials or hard data or to back up the clinical
impression that t he Fontan procedure or fenestrated Fontan procedure actually has a lower
mortality rate if preceded by a BCPS. |
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2.
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What age is optimal for
performance of the bidirectional cavopulmonary shunt? ?
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| There really is no correct answer. But, the general trend is toward
younger application of the BCPS. The BCPS can be performed as young as 3-4 weeks of age,
but there are no large series of patients in this age group and very little analysis of
late follow-up of patients done at this young age. There is some suggestion that earlier
application of the BCPS is associated with an increased formation of arteriovenous
fistulae, particularly in infants with heterotaxy syndrome. There is also some evidence
that early application of the BCPS is associated with a higher incidence of pulmonary
artery thrombosis. Both of these complications are indications of that pulmonary artery
resistance is more elevated in younger patients and a slightly higher complication rate
may be seen. The currently accepted optimal age for the BCPS is in the 3 to 9 month age
range. |
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3.
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What is the optimal timing
for conversion of a patient with a bidirectional cavopulmonary shunt to a Fontan
circulation (total avopulmonary connection)?
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| Again, there is really no correct answer. However, practical
experience tells us that this is usually 12 to 24 months following the BCPS. Several
factors are involved in this, but the most prominent is increasing cyanosis that occurs
after BCPS, forcing conversion to a total cavopulmonary connection. Cyanosis after BCPS is
partially due to the maturational changes seen in infants and small children as the amount
of superior caval flow drops in relation to inferior caval venous flow. Late cyanosis is
also contributed to by formation of venous collaterals that decompress the superior vena
cava into the inferior vena caval system and cause systemic desaturation. In addition,
pulmonary artery growth appears to be reduced following the BCPS, particularly after about
15 months following the BCPS. These factors would support conversion of a BCPS in this
12-24 month interval. However, there are distinct disadvantages of the Fontan that are
uncommonly seen with the BCPS, such as protein losing enteropathy, late development of
supraventricular arrhythmias, and reduction in exercise tolerance. The balancing of these
advantages and disadvantages are necessary to determining the optimal timing for
conversion of a patient with a BCPS to total cavopulmonary connection or Fontan
circulation. An excellent review of this question by Dr. Richard Jonas of Boston
Children's Hospital summarizes these concerns (J Thorac Cardiovasc Surg 1994;108:522-524). |
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4.
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Is the bidirectional
cavopulmonary shunt best performed with cardiopulmonary bypass or without extracorporeal
circulation, utilizing a superior cava to right atrial shunt?
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| Cardiopulmonary bypass does have some detrimental effects on lung
water and pulmonary resistance that negatively affect postoperative oxygenation. However,
in most instances cardiopulmonary bypass is necessary in order to address coexisting
problems that are best dealt with before a Fontan operation, such as pulmonary artery
distortion, a restrictive atrial septal defect or subaortic stenosis. Usually the drop in
oxygenation following a BCPS done utilizing cardiopulmonary bypass is transient and
resolves in 24-48 hours. The BCPS can be performed using a superior vena cava-right atrial
shunt and the transient pulmonary dysfunction can be avoided. With the move towards
younger patients with more complex problems, the number of patients that c an actually
have a BCPS without cardiopulmonary bypass is limited. |
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5.
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Is the bidirectional
cavopulmonary shunt best performed with or without an additional source of pulmonary blood
flow (i.e., leaving antegrade flow through a pulmonary artery band or leaving open a
systemic pulmonary shunt)?
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| This has been one of the most hotly debated topics in the literature
over the last several years. There are advantages and disadvantages of both approaches.
Advantages of leaving an extra source of pulmonary blood flow include improved oxygenation
and possible prevention of arteriovenous fistulae by preserving perfusion of the pulmonary
circulation by a "hepatic factor." Disadvantages include possible detrimental
effects of residual volume overload and possible prolongation of postoperative pleural
effusions. Since there is no correct answer to this question, the individual surgeon must
weigh these advantages and disadvantages for each individual patient and make a decision.
If a patient is marginally oxygenated immediately after completion of the BCPS and
superior vena caval pressure will allow it, the addition of some flow through a pulmonary
artery band or systemic-pulmonary artery shunt may well be advantageous. Conversely, if a
patient is adequately oxygenated following a BCPS (saturation >70-75%), then the
addition of an extra source of pulmonary blood flow is likely of little benefit. |
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6.
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What is the hemi-Fontan
procedure and how does it differ from the bidirectional cavopulmonary shunt?
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| The hemi-Fontan procedure is not, as the name implies, a
"half" Fontan procedure. The hemi-Fontan is physiologically equivalent to a BCPS
and is simply a method to perform a large connection between the superior vena cava/right
atrial junction and the pulmonary artery. The communication between the superior vena cava
and lower right atrium is then blocked by a patch, which is later removed when converting
to a Fontan-type circulation. Its proponents point out that the Fontan connection is
already made and conversion simply involves cutting out this patch and performing an
intracardiac tunnel of the IVC to the previously created opening. A possible disadvantage
of the hemi-Fontan includes the incision and suture lines across the cavo-atrial junction
and the increased possibility of supraventricular arrhythmias, similar to the regular
Fontan. The hemi-Fontan also does not allow "offsetting" of the superior and
inferior caval blood flow (thereby avoiding "colliding" blood flow pathways) and
the resulting theoretical benefits that a number of recent studies have demonstrated
inhydraulic fluid flow models of the Fontan procedure. With the recent advent of the
extracardiac-tube Fontan, the advantages of the hemi-Fontan have decreased. |
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