DISCUSSION
The main findings of study can be summarized as follow:
a) Individuals with MetS, according to the WHO definition, had
significant greater burdens of clinical comorbidities;
b) post-operative complications, including prolonged post-operative LOS,
were more frequent in the surgical strata MetS group; mortality rate was
significant higher in the MVS MetS group;
c) however, in the TAVR cohort, post-operative complications and
mortality rate did not differ between patients with and without MetS;
overall LOS was longer in the MetS group.
MetS may cause a number of effects on the myocardium and the circulatory
system, including myocardial fibrosis, activation on inflammatory and
proatherogenic pathways (macrophage infiltration and cytokine gene
expression), endothelial dysfunction and heart failure with either
preserved or reduced ejection function(13).
To the best of our knowledge, this is the first study that specifically
investigated the effect of MetS on post-operative complications after
isolated valve intervention.
It is important to notice that MetS in our study was a significant
independent predictor for post-operative mortality, however this was
largely driven by the mitral cohort. Notably, when controlling for other
confounders, systemic hypertension, atherogenic dyslipidaemia and
insulin resistance were not significantly associated to mortality. On
the contrary, BMI was inversely correlated with mortality. This latter
concept is known as ‘obesity paradox’ and has been described already by
others(13, 14).
Nevertheless, there are two important considerations to be made for
‘obesity’: the first is that not always obesity is synonymous of MetS
since there are so-called metabolically healthy obese (MHO) individuals
with high level of insulin sensitivity without systemic hypertension and
atherogenic dyslipidaemia and other features of MetS(12, 15). A survey
analysis, suggested that MHO may account for a significant percentage of
obese population(15). The second consideration is that waist
circumference rather than BMI is a more sensitive index for the
definition of obesity(16). An epidemiological study showed that, when
BMI and waist circumference were included in the same regression model,
the latter remained a positive predictor of risk of death while the
former was unrelated or inversely related to the risk of death(17).
Waist circumference is a more precise index for visceral adiposity /
central obesity(16). Visceral obesity causes a decrease in
insulin-mediated glucose uptake, insulin resistance and ultimately
endothelial dysfunction (3, 5). Nonetheless, waist circumference is
seldom measured in the cardiac surgery context, and most studies that
investigated the obesity paradox have considered the BMI as measure of
obesity, rather than waist circumference(14).
In our study cohort, individuals with MetS had significant greater
burdens of comorbidities that included COPD, peripheral vascular and
previously treated coronary disease, advanced age, reduced LVEF, renal
failure and prevalent female sex. The latter is a proven ‘condition of
risk’ associated to worst outcome in cardiac surgery(18). The
association of MetS with those comorbidities can explain the excess
mortality and complication rate in this group.
Post-operative LOS was significantly prolonged in the MetS group. The
presence of MetS was also independent predictor for increased LOS; also,
sensitivity analysis showed diabetes, systemic hypertension but not BMI
nor obesity (defined as BMI>30kg/m2) to
be independently associated to LOS.
Evaluating the feasibility and performance of the minimally invasive
surgical approach (both mitral and aortic) in individual with MetS is
beyond the scope of this research. However, there was no difference in
terms of number of patients approached with minimally invasive
techniques (mitral and aortic) among MetS and no-MetS.
With the advent of TAVR, many high-risk patients with symptomatic aortic
valve stenosis have been treated worldwide. Some studies investigated
the effect of specific risk factors (i.e., diabetes / obesity)(19) in
patients undergoing TAVR, but none focused on MetS. Two studies found
BMI as inversely associated to mortality, while visceral adiposity as
independent risk factor for post-operative mortality(20, 21). Those
findings confirm the suboptimal accuracy of BMI as trait for MetS.
Opposite to the surgical group, in our series we found that MetS was not
associated with worst post-operative outcomes in the TAVR subgroup.