Cardiac ¹²³I-MIBG reflects left ventricular functional reserve in patients with nonobstructive hypertrophic cardiomyopathy

Little is known about the relation between left ventricular (LV) functional reserve in response to exercise and cardiac sympathetic nervous function in patients with nonobstructive hypertrophic cardiomyopathy (HCM). We investigated whether an assessment of cardiac sympathetic nervous function by myocardial ¹²³I-metaiodobenzylguanidine (¹²³I-MIBG) scintigraphy might provide a sign of an abnormal LV functional reserve in response to exercise-induced β-adrenergic stimulation in patients with HCM. Methods: Thirty HCM patients underwent ¹²³I-MIBG scintigraphy and echocardiography at rest and subsequent biventricular cardiac catheterization at rest and during dynamic exercise. LV pressures were measured using a micromanometer-tipped catheter system. The early and delayed ¹²³I-MIBG images were quantified as a heart-to-mediastinum ratio (H/M). The plasma levels of brain natriuretic peptide (BNP) and norepinephrine (NE) were also measured. Results: Patients were divided into 2 groups according to the delayed ¹²³I-MIBG H/M: group I consisted of 12 patients with a delayed H/M of ≤ 1.8 and group II had 18 patients with a delayed H/M of > 1.8. Both the percentage increase from rest to exercise in LV isovolumic contraction (LV dP/dt_max) and the percentage shortening of LV pressure half-time (T_1/2) as an index of isovolumic relaxation were significantly less in group I than in group II (P < 0.05, respectively). A significant linear correlation was observed between the percentage increase in LV dP/dt_max and ¹²³I-MIBG H/Ms (early H/M: r = 0.49, P < 0.01; delayed H/M: r= 0.54, P < 0.005, respectively). A significant linear correlation was also observed between the percentage shortening in T _1/2 and ¹²³I-MIBG H/Ms (early H/M: r = 0.58, P < 0.001; delayed H/M: r = 0.64, P < 0.0005, respectively). The plasma NE levels were significantly higher in group I than in group II (P < 0.01), whereas the plasma BNP levels were comparable in the 2 HCM groups. Conclusion: β-Adrenergic enhancement of LV function during exercise may depend on the extent of cardiac sympathetic nervous innervation in HCM patients. Resting myocardial ¹²³I-MIBG scintigraphy can noninvasively evaluate LV functional reserve in response to exercise in patients with nonobstructive HCM.