Discussion
In the early 1970s, Francis Fontan and Kreutzer independently used the
concept of the ‘dispensable RV’ to design a new circulation that had not
previously been witnessed in evolution.1,7 The ‘Fontan
operation’ as it popularly became known as launched the era of surgical
treatment of functionally univentricular hearts. The Fontan operation
itself has also undergone numerous modifications in the last few decades
with the extracardiac Fontan operation being the most widely used design
today.8 Although the Fontan operation can be performed
with very low mortality today, the morbidity associated with this unique
circulation has become an ever increasing problem.9Early on, Dr. Francis Fontan himself predicted the significant attrition
associated with the new operation over time.10 The
entire problem of the Fontan operation is related to the lack of the
final push to the venous blood before it enters the lungs. The Fontan
operation essentially creates a neo portal system where adequate LV
loading can happen only at the expense of raised central venous
pressure.2 Moreover, in the absence of a sub-pulmonary
pump any small changes in the resistance of this neo-portal system can
lead to significant alterations in cardiac output. The bottle neck of
the cardiac output is the resistance offered by this neo-portal system
rather than the pumping mechanism of the heart itself although in the
final stages this too may be affected and worsens the Fontan circulation
all the more. The term ‘Fontan failure’ is a loosely applied term with
some authors using it solely for exercise intolerance, while others
reserve it for late stages of overt failure such as ascites, protein
losing enteropathy and plastic bronchitis. Perfusion of the liver is
particularly endangered following Fontan operation. Decreased portal
blood flow coupled with chronic elevation in pulmonary capillary bed
pressure ultimately results in fibrosis progressing to cirrhosis in many
patients with Fontan circulation.11
Traditional heart failure therapy may not be as effective as medications
that can lower the resistance of the Fontan neo portal
system.12 Percutaneous creation of fenestration is
also not very effective as an intervention for failing Fontan patients.
Achieving a good balance between cyanosis, decreased congestion and
improved cardiac output can be challenging.13 Fontan
failure can sometimes be a consequence of structural issues that may be
amenable to surgical correction. Conversion of the atriopulmonary
connection to cavopulmonary connection using lateral tunnel technique or
extracardiac type of Fontan procedure reduces not only the energy loss
but also the transcoronary pressure gradient by placement of the
coronary sinus on the pulmonary venous side of the
circulation.14 Heart transplantation can provide
effective therapy for end stage Fontan failure. However, outcomes of
heart transplantation for the failing Fontan patient are not as
favorable as for other congenital heart diseases.15The continued shortage of donor organs with an ever increasing
population of Fontan patients and suboptimal outcomes of heart
transplantation have propelled the need for an effective mechanical
circulatory support device not only as a bridge to transplantation but
also as a destination device. Many patients with failing Fontan
physiology have preserved systemic ventricular function and hence
application of a systemic ventricular assist device is unlikely to be
effective. For such patients a right sided subpulmonary ventricular
assist device may afford a better solution. A variety of novel Fontan
specific devices have been proposed each with its own limitations (Table
5).3-5 Our study was an attempt towards the same end.
The purpose of this study was to test whether the idea of an oscillating
ball would work in the Fontan circuit. The mechanism seemed simple and
theoretically attractive. The device, if successful could be potentially
implemented in most adult Fontan patients as an oscillating ball placed
in a stent in the branch pulmonary arteries within the existing Fontan
configuration. Unfortunately, as our own studies in the lab proved, the
results were counter intuitive. Perhaps, testing the concept within a
closed circuit or using computational flow dynamics may have yielded
different results. We hope our findings are useful for other groups
making similar attempts to address the problem of Fontan failure.
Conclusion :
The proposed novel method for intervention is not a suitable option for
improving hemodynamics in failing Fontan circulation. Other intervention
modalities to improve hemodynamics remain to be explored.