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Controversy exists regarding the importance of glycaemic control in patients with type 2 diabetes mellitus (T2DM) and chronic heart failure (CHF) based on conflicting reports using single baseline glycosyated haemoglobin (HbA1c). Using the time-weighted mean of serial HbA1c measurements has been found to be a better predictor of diabetic complications as it reflects the glycaemic burden for that individual over time. We therefore sought to confirm this in a large cohort of patients with T2DM and incident CHF.A time-weighted mean HbA1c was calculated using all HbA1c measurements following CHF diagnosis. Patients were grouped into five categories of HbA1c (≤6.0%, 6.1–7.0%, 7.1–8.0%, 8.1–9.0%, and >9.0%). The relationship between time-weighted mean HbA1c and all-cause death after CHF diagnosis was assessed. A total of 1447 patients with T2DM met the study criteria. During a median follow-up of 2.8 years, there were 826 (57.1%) deaths, with a crude death rate of 155 deaths per 1000 person-years [95% confidence interval (CI) 144–166]. A Cox regression model, adjusted for all significant predictors, with the middle HbA1c category (7.1–8.0%) as the reference, showed a U-shaped relationship between HbA1c and outcome [HbA1c <6.0%, hazard ratio (HR) 2.5, 95% CI 1.8–3.4; HbA1c 6.1–7.0%, HR 1.4, 95% 1.1–1.7; HbA1c 8.1–9.0%, HR 1.3, 95% CI 1.0–1.6; and HbA1c >9.0%, HR 1.8, 95% CI 1.4–2.3]. Further analysis revealed a protective effect of insulin sensitizers (i.e. metformin) (HR 0.7, 95% CI 0.61–0.93) but not other drug classes.In patients with T2DM and CHF, our study shows a U-shaped relationship between HbA1c and mortality, with the lowest risk in patients with modest glycaemic control (HbA1c 7.1–8.0%) and those treated with insulin sensitizers.