Future Directions
In this review we have discussed both retrospective and prospective
studies that have looked at HbA1c as predictor of various postoperative
adverse outcomes. However, the majority of these studies had small
sample sizes, thus from onset limited ability to draw statistically
meaningful conclusions about adverse outcomes of inherently low event
rates in any cohort; reduced statistical power and possible type II
statistical error. Moreover, studies have used different cut-off values
of preoperative HbA1c levels, such as 6.5%, 7%, 7.5%, and 8%.
Further research should be directed at determining a preoperative
cut-off of ‘suboptimal glycaemic control’ for pre‐operative optimisation
clinical guidelines of the surgical patient.
A recent retrospective study by Kim et al. of 703 patients with diabetes
mellitus who underwent off-pump coronary artery bypass surgery( OPCAB) provides strongest evidence to date of prognostic role
of HbA1c 25. The use of composite of postoperative
morbidity and mortality (CMM) endpoints (permanent stroke, prolonged
ventilation, deep sternal wound infection, renal failure, reoperation,
and 30-day mortality) attenuates the prospect of a misleading
statistical conclusion by combining adverse events of low incidence. Kim
et al. found that incidence of CMM endpoints was greater in patients
with HbA1c ≥7.0% (21% vs 15%, P = .041). Moreover, receiver
operator-characteristic curve analysis revealed HbA1c 7.85% as the
optimal threshold for CMM endpoints (area under the curve; 0.556, 95%
CI, 0.501-0.611, P = .048). This study has provided rationale for
future prospective studies with sufficient power to examine whether
postponing cardiac surgery in patients with high preoperative HbA1c
levels would improve postoperative outcomes.
Moreover, Kim et al. indicated that high preoperative HbA1c (≥7.0%)
level alone, and not the variables related to perioperative glycemic
control, was independently associated with adverse outcome in diabetic
patients undergoing OPCAB, although high HbA1c levels contributed to
greater perioperative glycemic variability25. However, a randomised controlled trial conducted
by Bláha et al. suggested that it is cardiac surgery patients with
previously undiagnosed diabetes who have the worst prognosis65. Comparable conclusion was suggested in non-cardiac
studies 66,67. Recent studies have shown that
perioperative intravenous insulin infusion is more frequently
administered in known diabetics due to more frequent monitoring of their
capillary glucose concentrations 25,68. However,
despite more frequently administered insulin in the high HbA1c group,
adverse outcomes remained more prevalent in this group compared to
normal HbA1c group, thereby further attenuating prognostic role of
HbA1c. Nevertheless, optimisation of pre‐operative HbA1c concentrations
with a combined intravenous and subcutaneous insulin glucose has been
shown to reduce surgical mortality and morbidity in diabetic patients
undergoing cardiac surgery69.
Future research should be directed at the determining the optimal level
of perioperative glycaemic management and the crucial perioperative
period to maintain this HbA1c level. Although there are current ongoing
outcome studies currently in this area (e.g. the Optimising Cardiac
Surgery outcomes in People with diabetes (OCTOPUS) trial – protocol
number HTA16/25/12), there remain few data on the outcomes and effects
of intervention on those not known to have diabetes70.