Evaluation of QT Interval Length, QT Dispersion and Myocardial m-iodobenzylguanidine Uptake in Insulin-Dependent Diabetic Patients with and Without Autonomic Neuropathy

1. An association has been reported between QT interval abnormalities and cardiovascular autonomic neuropathy in diabetic patients. The QT interval abnormalities reflect local inhomogeneities of ventricular recovery time and may be related to an imbalance in cardiac sympathetic innervation. Sympathetic innervation of the heart can be visualized and quantified by single-photon emission-computed tomography with m-[¹²³I]iodobenzylguanidine. In this study we evaluated cardiac sympathetic integrity by m-[¹²³I]iodobenzylguanidine imaging and the relationship between both QT interval prolongation and QT dispersion from standard 12-lead ECG variables and m-[¹²³I]iodobenzylguanidine uptake in insulin-dependent diabetic patients.

2. Three patient groups were studied, comprising six healthy control subjects, nine diabetic patients without cardiovascular autonomic neuropathy (CAN-) and 12 diabetic patients with cardiovascular neuropathy (CAN +). Resting 12-lead ECG was recorded for measurement of maximal QT interval and QT dispersion. The QT interval was heart rate corrected using Bazett's formula (QTc) and the Karjalainen approach (QT_K). Quantitative measurement (in counts/min per g) and visual defect pattern of m-[¹²³I]iodobenzylguanidine uptake were performed using m-[¹²³I]iodobenzylguanidine single-photo emission-computed tomography.

3. Global myocardial m-[¹²³I]iodobenzylguanidine uptake was significantly reduced in both diabetic patient groups compared with control subjects. The visual defect score of m-[¹²³I]iodobenzylguanidine uptake was significantly higher in CAN + diabetic patients than in control subjects and in CAN — patients. This score was not significantly different between control subjects and CAN— patients. QT_c interval and QT dispersion were significantly increased in CAN+ diabetic patients as compared with control subjects (QT_c: 432 ± 15 ms versus 404 ± 19 ms, P < 0.05; QT dispersion: 42 ± 10 versus 28 ± 8 ms, P < 0.05). QT dispersion was also significantly longer in CAN- diabetic patients than in control subjects (41 ± 9 ms versus 28 ± 8 ms, P < 0.05). QT_c interval was significantly related to global myocardial m-[¹²³I]iodobenzylguanidine uptake and defect score in diabetic patients (r = −0.648, P < 0.01, and r = 0.527, P < 0.05, respectively). There was no correlation between QT dispersion and both m-[¹²³I]iodobenzylguanidine uptake measures.

4. In conclusion, these findings suggest that m-[¹²³I]iodobenzylguanidine imaging is a valuable tool for the detection of early alterations in myocardial sympathetic innervation in long-term diabetic patients without cardiovascular autonomic neuropathy. Insulin-dependent diabetic patients with cardiovascular autonomic neuropathy have a delayed cardiac repolarization and increased variability of ventricular refractoriness. The cardiac sympathetic nervous system seems to be one of the determinants of QT interval lengthening, but does not appear to be involved in dispersion of ventricular recovery time. It is assumed that QT dispersion is based on more complex electrophysiological mechanisms which remain to be elucidated.