Modulation of Ventricular Repolarization and R‐R Interval Is Altered in Patients with Globally Impaired Cardiac 123I‐MIBG Uptake

BACKGROUND: Cardiac (123)I-metaiodobenzylguanidine (MIBG) imaging is widely used to assess cardiac sympathetic neuronal function. However, physiologic significance of impaired cardiac MIBG uptake is not fully elucidated. The purpose of the present study was to determine influences of abnormal cardiac sympathetic neuronal function on heart rate variability (HRV) and ventricular repolarization process. METHODS: Twenty-nine patients with prior myocardial infarction were divided into two groups by a heart-to-mediastinum ratio (H/M) of MIBG scintigraphy. Ten patients with globally decreased MIBG uptake (group I: H/M < 1.5), 19 patients with partially decreased MIBG uptake (group II: H/M >or= 1.5), and 17 control subjects with normal MIBG uptake (group III) were studied. Holter recording and a standard 12-lead electrocardiography were used for evaluation of HRV, QT-RR relation, and QT dispersion. RESULTS: Low, high, and total frequency components decreased in groups I and II, as compared to that of group III. The reduction of these frequency domain measures was more severe in group I than in group II, but the differences did not reach statistical significance. Circadian variation of frequency domain measures disappeared in group I. The slope of QT-RR relation was significantly greater in group I than in groups II and III. QT dispersion was also greater in group I (64 +/- 25 msec) than in group II (43 +/- 19 msec) and group III (28 +/- 9 msec). CONCLUSION: These results suggest that patients with sympathetic neuronal dysfunction inferred from globally impaired cardiac MIBG uptake have an altered modulation of ventricular repolarization process as well as decreased HRV.