Objective: Cardiac uptake of [123I]metaiodobenzyl guanidine (MIBG) is reduced in patients with Parkinson's disease (PD). However, the cardiac sympathetic abnormality associated with this reduction is unclear. To unmask this abnormality in PD patients we examined the functional consequences of cardiac beta-receptor activation. Methods: Cardiovascular responses to stepwise administration of the beta1-receptor agonist, dobutamine (DOB), were assessed in 25 PD patients and 12 age-matched controls. Changes in blood pressure were compared to determine the optimal dose at which to detect denervation supersensitivity, and cardiac contractility was measured by DOB echocardiography, based on peak aortic flow velocity. The relations of these cardiovascular responses to the ratio of MIBG uptake into the heart vs. that into the mediastinum (H/M ratio) were analyzed. Results: At 4 μg/kg/min DOB, systolic blood pressure increased more in PD patients than in controls (PD, 17.5 ± 12.3 mm Hg; control, 7.2 ±6.2 mm Hg, p < 0.01), suggesting the presence of denervation supersensitivity. At this DOB dose cardiac contractility also increased more in PD than in controls (PD, 39.0 ± 15.7%; control, 23.5 ± 5.2%, p < 0.005) and this hyperdynamic response was significantly correlated with reduced H/M ratios (early: r = - 0.63, p < 0.01, delayed: r = - 0.66, p < 0.01). Conclusion: Low-dose DOB unmasks cardiac sympathetic denervation in PD patients, and decreased MIBG uptake indicates the presence of denervation supersensitivity within the heart, resulting in hyperdynamic cardiac contractility in response to a beta 1-stress condition.
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