Introduction: The aging adult congenital heart disease (ACHD) population is faced to an increasing risk of both, the inborn heart defect and the development of acquired heart disease. In non-congenital heart disease diabetes is not only associated with atherosclerotic heart disease, but also with diabetic cardiomyopathy related heart failure (HF) and death.     
In ACHD the relation between impaired glucose metabolism and underlying cardiac and non-cardiac co-morbidities still needs to be determined.
We evaluated the evolution of HBA1c, related co-morbidities and the impact of prediabetes on heart failure (HF) and death in young adults with congenital heart disease. 

Methods:  In a contemporary cohort of 738 patients (mean age 35.15±11.2 years; 54.7% male, BMI 25±4.9kg/m²) clinical characteristics, biomarkers and exercise capacity were related to the upper quartile of baseline HBA1c. Binary logistic regression analysis calculated independent predictors.

Follow-up was complete in 656 patients (mean follow-up 53.3±38.7 months; mean age 35.1±11.1 years; 54.7% male) including 91 patients with prediabetes. Cox proportional survival analysis calculated the impact of prediabetes on outcome.

Results: The prevalence of prediabetes increased from 14% to 20.3% (p<0.0001). Multivariate predictors of the upper-quartile of HBA1c (≥5.5%) were BMI (p<0.0001), heart failure symptoms (NYHA-class [95% CI: 1.028-2.142]), congestive liver disease (gamma-glutamyl transferase [95% CI: 1.001-1.008]), hypoxia (oxygen-saturation [95% CI: 0.836-0.944]) and hypertension ([95% CI: 1.254-3.111]), even when adjusted for age, sex and heart disease complexity. Inclusion of exercise parameters identified maximum work rate and liver congestion as independent predictors of elevated HBA1c.

During follow-up fifty-six patients (8.5%) experienced HF/death, 46 patients HF (7.0%) and 22 (3.4%) died. In all models prediabetes and the upper quartile of NT-proBNP (≥237 ng/l) were independent predictors of HF and HF/death. In adjusted and unadjusted multivariate analysis the Hazard ratio (HR) of prediabetes varied between 2.42 and 3.02 for HF, and between 1.85 and 2.87 for HF/death. Prediabetes was not an independent predictor of all-cause mortality.

Conclusion: In ACHD incident impaired glucose metabolism is associated with the cardiac comorbidities HF-symptoms, liver congestion, hypoxemia and hypertension. Already prediabetes represents an independent risk factor for HF and HF/death.