Discussion
In a US Agency for Healthcare Research and Quality database review of
5,234 patients who underwent TVS from 2003 to 2012, operative mortality
was 13.9%.9 The 9.8% operative mortality in our
series was similar to previously published studies.10–17
The key finding is that MELD-Na correlates well with operative mortality
after triple valve surgery and exhibits superior diagnostic accuracy
over EuroSCORE. The association remains significant after risk
adjustment. A possible explanation is that the key driver of operative
mortality was multiorgan failure in the valvular heart failure
population; this is in contradistinction to the CABG population, in whom
operative mortality was driven by low cardiac output
syndrome.18 Just as EuroSCORE is the quid pro
quo litmus test of ventricular recovery after cardiopulmonary bypass,
MELD-Na may serve the litmus test of organ reserve.
In polyvalvular heart failure, right ventricular function and
hepatorenal function are delicately intertwined. Both acute and chronic
dysfunctions in one organ can initiate and perpetuate dysfunction in the
others through complex neurohormonal feedback systems. This complex
system of crosstalks between failing organs is collectively referred to
as cardiorenal and cardiohepatic syndromes.19,20 A key
message for cardiac surgeon sis that treating the valvular dysfunction
may not necessarily lead to recovery in organ function. In fact, the
deleterious effects of prolonged cardiopulmonary bypass may precipitate
a potent immunogenic cascade leading to frank multiorgan failure and
death.
Due to a substantial portion of missing data regarding right ventricular
systolic function, the impact of chronic right ventricular dysfunction
on hepatorenal dysfunction cannot be quantified in this study. Using the
data available, right ventricular systolic dysfunction did correlate
with operative mortality on univariate analysis. Volumetric assessment
with MRI, right ventricular dimension, as well as strain imaging may
provide more accurate assessment of right ventricular function compared
with two-dimensional echocardiography. Nonetheless, we believe this is a
promising avenue for future research because strategies to mitigate
post-bypass right ventricular dysfunction and ensuing organ failure
remain limited.
We would like to highlight the importance of including hyponatremia in
the risk score. Hyponatremia in heart failure is mediated by increased
activity of anti-diuretic hormone and activation of the
renin-angiotensin-aldosterone system. Aggressive diuresis may also have
played a role. As demonstrated in the OPTIMISE-HF registry, hyponatremia
in patients hospitalized for heart failure was associated with higher
rates of in-hospital and follow-up mortality.21Although it is unclear whether hyponatremia contributes to poor
prognosis or serves simply as a marker of disease severity, the presence
and severity of hyponatremia should alert the surgeon to the severity of
the underlying heart failure.22
This study was retrospective in nature and suffered from weaknesses
inherent to all retrospective studies, including selection bias and
confounding. The number of subjects was small. The findings generated
were at best hypothesis-generating and would benefit from verification
in a sizable regional or national database.
In Papworth’s landmark publication regarding the EuroSCORE back in 1999,
the authors commented that “the true test of such a system is in its
widespread application in the field”, and that its use should be
routine for all cardiac surgical patients.2 In the
modern era, MELD-Na can be easily calculated by inputting the four
biochemical parameters into mobile applications on smartphones and
tablet computers. We believe that it is not a stretch to imagine that
the adoption of MELD-Na can become widespread and find its rightful
place in risk stratifying patients with valvular heart failure.