Relationship between myocardial 123I-metaiodobenzylguanidine scintigraphic uptake and heart rate variability in patients with syndrome X

Background: We have recently demonstrated a striking impairment in cardiac uptake of ¹²³I-metaiodobenzylguanidine (MIBG) in most patients with syndrome X. In this study we investigated the relationship between cardiac MIBG defects and cardiac autonomic activity in these patients. Methods: MIBG myocardial scintigraphy and time-domain and frequency-domain heart rate variability (HRV) were compared in 11 syndrome X patients and 10 healthy controls. Cardiac MIBG uptake was assessed by the heart/mediastinum ratio and a cardiac MIBG uptake defect score (higher values = lower uptake). Results: The heart/mediastinum ratio was lower (1.71 ± 0.6 vs 2.19 ± 0.3, p = 0.03) and MIBG uptake score higher (37.1 ± 32 vs 4.0 ± 2.5, p = 0.005) in syndrome X patients, whereas average HRV values did not differ between the two groups. However, while there were no correlations between MIBG uptake and HRV in controls, in syndrome X patients both the heart/mediastinum ratio and MIBG uptake score correlated significantly with two HRV parameters, specific for vagal activity: the square root of the mean squared differences of consecutive RR intervals (r = 0.73, p = 0.01, and r = -0.67, p = 0.02, respectively), and high frequency (r = 0.64, p = 0.03, and r = -0.74, p = 0.009, respectively). Conclusions: In patients with syndrome X, the impairment in cardiac MIBG uptake was associated with a reduction in HRV indexes mainly reflecting vagal modulation of sinus node, thus suggesting that a predominance of cardiac adrenergic activity may be present in those with abnormal cardiac MIBG scintigraphy.