HbA1c and mortality rates
As the incidence of Diabetes Mellitus (DM) increases, the proportion of
people with DM undergoing cardiac surgery has also increased17. There exists a large body of evidence which has
looked at the association between patient HbA1c levels and mortality
following cardiac surgery. The evidence on the effect of HbA1c levels on
mortality is contradictory with some studies reporting that increased
HbA1c levels are associated with increased mortality18–25. But by in large, most studies seem to show
that increased HbA1c is not predictive of increased mortality as solo
indicator 26–40. The findings from all these studies
are summarised in Table 1 and Table 2.
The study with the largest patient cohort looked at outcomes in 6,313
patients with type 2 DM who underwent CABG surgery between 2003 and 201318. The study found that HbA1c was associated with an
increased risk of death in patients with HbA1c level between 9.1-10.0%
(Hazard ratio (HR) 1.26; 95% CI 1.04-1.53), and this risk was even
greater in patients with HbA1c >10.0% (HR 1.33; 95% CI
1.05-1.69). Other studies similarly showed that elevated HbA1c was
associated with increased mortality 19–25. One study
showed that 30-day mortality was significantly higher in patients with
HbA1c greater than 6.5% compared to those with Hb1ac <6.5%
(4.22% vs 3.07%; P= 0.0035), however following multivariable
adjustment this association was lost and there was no significant
difference in the mortality rates 26.
All these studies, overall, demonstrates that increased HbA1c increases
mortality in cardiac surgery patients and all of them, with the
exception of one, had large sample sizes meaning they were all by in
large sufficiently statistically powered to make their findings
significant. However, all the aforementioned studies were retrospective
in nature meaning they were liable to biases inherent in the study
design and the studies do not demonstrate causality.
The largest cohort study demonstrating that there is no relationship
between HbA1c levels and mortality looked at outcomes in 6,393 patients
undergoing cardiac surgery 27. The study found that
HbA1c was not a significant predictor of post-operative mortality
(P= 0.88). Further studies similarly showed that there was no
significant difference in mortality following cardiac surgery according
to HbA1c levels, regardless of the HbA1c cut-off levels utilised28–32,34–40.
Overall the evidence showing that there is no significant difference
between higher HbA1c levels in the incidence of mortality outweighs
evidence showing that elevated HbA1c is associated with increased
mortality, suggesting that HbA1c alone may not be predictive of
mortality following cardiac surgery.