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.