Expert Commentaries
Hypoplastic Left Heart Syndrome: Uni-ventricular or Bi-ventricular Repair
By Sitaram Emani, MD
Boston Children's Hospital
The approach of staged left ventricular recruitment allows the surgeon to avoid having to make a difficult decision regarding definitive biventricular repair or single ventricle palliation in a neonate. Taking the predictable outcomes of a single ventricle repair initially, the approach allows us to examine the response of the left heart to surgical maneuvers. Biventricular conversion can then be performed if there is improvement in left heart function and size over time. It is important to emphasize that the option of single ventricle palliation is not abandoned with this approach. It may be most applicable in patients who have risk factors for single ventricle palliation, yet are high risk for initial biventricular repair.
By Joseph M. Forbess, MD
UT Southwestern Medical Center - Children’s Health
Nasr and colleagues have presented a quite topical discussion on a surgical strategy, aggressively pursued at Children’s Hospital Boston, to “rehabilitate” left ventricles that might have been previously thought to be inadequate for the support of the systemic circulation. The impetus for undertaking such a strategy, which might be fraught with significant morbidity and mortality, are the concerning longer-term outcomes for Fontan patients.
Mammalian ventricles are bellows pumps. As such they require “adequate” inflow, outflow, as well as the systolic power and stroke volume required to produce a cardiac output. In addition to this systolic function, however, the diastolic function, or ventricular compliance, must be adequate enough to have a low enough left atrial pressure, for example, to avoid the development or persistence of pulmonary hypertension. In summary, the “adequacy” of a left ventricle is a function of the interaction of all of the above anatomic and physiologic variables. The surgical strategies employed to achieve an independently functional systemic left ventricle address any and all of these, usually in a staged fashion.
The surgical interventions on the inflow and outflow structures, while perhaps technically difficult, are conceptually straightforward. Valve repairs and replacements are designed to meet the “inflow and outflow” criteria mentioned above. The ability to truly “grow” the left ventricle is a different matter. Certain anatomic variants discussed by Nasr et al. respond to biventricular repair better than others. Unbalanced atrioventricular septal defects, for example, almost never have endocardial fibroelastosis (EFE). Quite unbalanced defects, wherein the left atrioventricular (AV) valve will be 30% or less of the total orifice area of the previously common valve can usually be partitioned successfully, and the resultant left ventricle size can become “normal” by echo dimensions within days to weeks. And this is with fairly normal left atrial pressures. Has this left ventricle “grown” during this brief time period? Unlikely. I would argue that the left ventricle, with a good amount of intrinsic compliance has successfully been stretched with increased preload.
This process is played out in reverse in hypoplastic left heart syndrome patients with mitral stenosis and aortic stenosis, with only a small patent foramen ovale. There the left ventricle is small or “borderline” despite the fact that the left atrial pressure is usually pathologically elevated—this is a much less compliant ventricle than the unbalanced AV canal. With the performance of a complete atrial septectomy as part of a typical Norwood procedure, a left ventricle in this setting usually becomes dramatically smaller than what was observed preoperatively almost immediately. Did “negative” growth take place? Again, I think not.
These two examples are given to reinforce the “bellows” principle that is unavoidable here. Many anatomic variants undergoing rehabilitation require high, possibly even pathologically high, left atrial pressures to maintain adequate ventricular dimensions and subsequent stroke volumes. Some of these ventricles might be growing, others, however might primarily be stretched to these dimensions by left atrial pressures that might—and this “might” is the crux for this entire enterprise—be considered undesirable.
Again, the valvar interventions are conceptually straightforward. The ventricular dimension and compliance variables in this “rehabilitation” function, however, are much less so. We in this field await longer-term studies in which we compare Fontans to these rehabilitated patients. The Fontan outcomes that are driving the rehabilitation effort really don’t come into play for a decade or more. At the current relatively short follow up periods reported for rehabilitation, for example, one might say that the Fontan patients are better off because they have usually required fewer surgeries and reinterventions. We will only know if ventricular rehabilitation is better for the noncompliant borderline left ventricle once comparable longer-term follow-up is available. It is my hope that such follow-up will allow us to select patients optimally in this “border zone”, and triage them appropriately to one or two ventricles.
