Cardiovascular events with poorly controlled HbA1c
Several studies have analysed the relationship between HbA1c levels and the incidence of cardiovascular events such as peri-operative myocardial infarction (MI), atrial fibrillation (AF), and low cardiac output syndrome. The overwhelming body of evidence has demonstrated that HbA1c level alone cannot be used as a predictor for cardiovascular events29-33. By contrast, only one study showed a relationship between HbA1c and cardiovascular event which suggested that elevated pre-operative HbA1c may be protective for developing AF43. The main outcomes of these studies are summarised in Table 5.
The largest cohort study which demonstrated that there was no relationship between HbA1c level and cardiovascular events analysed outcomes in 1,461 patients undergoing CABG with or without valvular surgery 35. The study found that there was no difference in the incidence of cardiovascular events in patients with HbA1c <6.5% and >6.5%: AF (26.3% vs 26.6%;P= 0.90), acute MI (0.5% vs 0; P= 0.333) and cardiac tamponade (0 vs 0.2%; P= 0.313). Other studies similarly showed that there was no significant difference in the incidence of cardiovascular events between patients with raised HbA1c levels and those with normal levels38, 40, 44, 45.
Kinoshita et al 43 carried out a retrospective analysis of 912 patients who underwent isolated CABG. They found that median HbA1c was significantly lower in patients who developed AF post-operatively when compared to patients who did not ((5.8%, 95 % CI, 5.4-6.3) vs 6.1%, 95% CI, 5.5-7.2); P= 0.01). Additionally, results also showed that the incidence of post-operative AF showed a stepwise trend where the incidence decreased as HbA1c level decreased: 28.3% incidence for patients with HbA1c ≤5.6%, 17.4% for HbA1c 5.7-6.7% and 12.5% for HbA1c 6.8-11.4% (P= 0.01). These findings suggest that high HbA1c levels may be associated with a lower risk of post-operative AF but this is a single retrospective study carried out on Japanese patients thereby limiting the generality of findings and cannot be used as a sole evidence to support this clinical outcome in general population.