Discussion
This study is the first to review current practice of UK national data
to compare sex-related differences in outcomes following surgical
coronary revascularisation and valvular cardiac procedures.
From our dataset of 210,155 patients (25.7% female), we found female
sex to be an important risk factor for 30 day mortality following CABG,
AVR and MVR. Following CABG, female sex was also associated with
increased post-operative need for dialysis, deep sternal wound
infections and length of hospital stay.
The present study supports the well reported claim that females
undergoing coronary revascularisation surgery are often older and with
more comorbidities than males (3). Furthermore, we found that women were
also more likely to need urgent, as opposed to elective,
revascularisation than men, which may be responsible for some of the
poorer outcomes reported.
It is also suggested that sex-related differences in operative strategy
decisions and techniques may explain sex-related differences in cardiac
surgery outcomes (14). For example, a higher proportion of males
compared with females received LIMA, RIMA or BIMA grafting in both our
cohort and other studies (15) which is suggested to predispose females
to incomplete myocardial revascularisation (16,17). Nevertheless, the
multivariable regression analysis used in our study adjusted for
differences in baseline and operative differences, including
revascularisation strategy and still a sex-related difference remained
in short-term mortality. These findings suggest that female sex is an
independent risk factor for short-term mortality following CABG which
supports the consensus of the current literature (6,18). The idea of
female sex being an independent risk factor for worse outcomes following
CABG is speculated to be related to the more challenging anatomy of
female patients, such as smaller coronary artery targets for grafting,
narrower conduits and more diffuse patterns of coronary disease (3,19).
Our study also evaluated other post-CABG outcomes. We did not find an
increased risk of stroke following CABG as other national studies have
reported (8, 20). This may be related to the fact that in our cohort of
females a significant 15% underwent off pump revascularisation which
has been reported to be particularly beneficial in women because of its
effect to reduce the risk of stroke (21).
Sternal wound complications were more common in females than males
following CABG in our study. A risk prediction tool developed in the UK
identifies female sex as one of six independent predictors of surgical
site infection following cardiac surgery (22). The B-SIR score also
includes raised body mass index >30, diabetes, left
ventricular ejection fraction <45% and peripheral vascular
disease; all of which were more common in our female patients. This
finding may indicate a complex multifactorial impact of female sex on
the risk of developing wound complications. This would suggest that
efforts to prevent SSI should aim to target all of the these modifiable
risk factors, especially in our female patients.
While the majority of patients who underwent CABG were male, single
valve surgery was more evenly distributed between the sexes. In contrast
to CABG, females were more likely to have a planned elective valve
procedure. Despite this, female sex was associated with significantly
higher short-term mortality following both isolated AVR and MVR
procedures.
Our finding of increased mortality following AVR in females is reflected
from other nationally representative databases such as United States of
America (23) and a previous UK database analysis (24). However, other
national studies did not report sex-related differences in AVR mortality
(12, 25).
In our study, men were more likely to receive a mechanical aortic valve
than women which may reflect the differences in age and comorbidities
between the sexes at time of surgery and their influence on the
management planning. It is known that women with severe aortic stenosis
are diagnosed at a later stage of the disease process (26) but even when
adjusting for pre-operative difference women are less likely to be
referred for surgical AVR than men (27, 28).
There is no clear explanation for why women have worse outcomes compared
to men following AVR but several mechanisms have been implicated. For
similar degrees of aortic stenosis, females tend to have higher
transvalvular pressure gradients, thicker ventricle walls and smaller
end-systolic and end-diastolic chamber sizes than males (29). Secondly,
females on average receive smaller valves than males, the outcomes of
patient-prosthesis mismatch (PPM) seem more severe in smaller size
valves (30) and therefore may effect women disproportionately.
Furthermore, females are also more likely to require additional aortic
annular enlargement than males leading to increased operative risk
associated with the annular enlargement procedure (5).
As with the other procedures, females in the UK experienced an increased
odds of 30-day mortality following MVR than males. A 2013 study of 3,761
patients found a difference in mitral pathology between males and
females undergoing mitral surgery; males were more likely to have mitral
valve leaflet prolapse whereas females were more likely to have
calcified mitral valve leaflets (31). This differences in pathology
explains why females are more likely to need a mitral valve replacement
whilst males are more likely to receive a mitral valve repair, a finding
reiterated in our study. A study of MV procedures from USA, 2000-2009,
also agreed men were more likely to receive a MV repair than women (32).
This difference in surgical management strategy is thought to contribute
to the poorer outcomes we see in females (33).
Interestingly, for both CABG and AVR surgery, female sex seemed to be
protective for post-operative bleeding resulting in returning to
theatre. Oestrogen has a pro-coagulant effect which may confer benefit
to limit post-operative bleeding (35). Despite females tending to have
lower rates of returning to theatre for bleeding, females have been
shown to receive more post-operative red blood cell transfusions with
males and this is associated with delayed recovery (10).