Relation between QT interval variability and cardiac sympathetic innervation in patients with diabetes mellitus

Elevated QT interval variability (QTV) has been associated with increased cardiac mortality, but the underlying mechanisms are incompletely understood. Sympathetic activity is thought to be a main contributor to QTV. The aim of this study was to investigate the relation between cardiac sympathetic integrity and QTV in 15 patients with type 2 diabetes mellitus and varying degrees of cardiac autonomic neuropathy. Cardiac sympathetic innervation was assessed by ¹²³I-mIBG scintigraphy based on heart-to-mediastinum ratio of ¹²³I-mIBG uptake 4 hours after infusion. To assess QTV high resolution ECGs (1000 Hz) were recorded during standing. Beat-to-beat QT intervals were calculated over a period of 5 minutes, using a template-stretching algorithm. QTV was quantified using time and frequency domain measures as well as non-linear approaches (symbolic dynamics, fractal dimension). The group mean and standard deviation of HMR values were 1.07 ± 0.48. Time and frequency domain QTV parameters were significantly increased in subjects with sympathetic dysinnervation and inversely correlated with HMR (r = -0.7, p <; 0.001). In conclusion, there is a clear link between sympathetic dysinnervation and elevated QTV in patients with type 2 diabetes mellitus during sympathetic activation. Sympathetic dysinnervation is associated with increased ventricular repolarization lability.