CARDIOVASCULAR MEDICINE Assessment of myocardial adrenergic innervation in patients with sick sinus syndrome: effect of asynchronous ventricular activation from ventricular apical stimulation

Objective: To investigate ventricular sympathetic innervation in patients with sick sinus syndrome and to detect regional deterioration of adrenergic innervation caused by asynchronous ventricular activation from right ventricular pacing. Design: Prospective controlled study. Setting: Tertiary cardiac referral centre. Patients: 22 patients with sick sinus syndrome and indications for permanent dual chamber pacing; 20 healthy individuals as controls. Interventions: All patients underwent myocardial imaging with planar and single photon emission computed tomography (SPECT) after an intravenous infusion of 5 mCi I-meta-iodobenzylguanidine (I-MIBG) before and after pacemaker implantation. A SPECT thallium myocardial study was done during the same week as the I-MIBG study in all patients. Main outcome measures: The heart to mediastinum (H/M) ratio and washout rate were calculated during the I-MIBG study to assess the global cardiac sympathetic activity; the aim of the SPECT study was to investigate the regional distribution of adrenergic innervation. Results: The H/M ratio was significantly smaller in the patients with sick sinus syndrome than in the controls (p < 0.001). In sick sinus syndrome there were regional adrenergic innervation defects, mostly in the inferior and apical walls. After a medium term pacing period, a redistribution of I-MIBG uptake was detected, with deterioration of adrenergic innervation in the inferior, apical, and posterior walls. The thallium myocardial perfusion study showed no change after three months of permanent pacing. Conclusions: Patients with sick sinus syndrome have global and regional disturbances of the adrenergic innervation of the left ventricular myocardium. These seem to deteriorate as a result of asynchronous electrical activation. The clinical significance of this finding requires further investigation.