EFFECTS OF LONG-TERM RIGHT-VENTRICULAR APICAL PACING ON LEFT-VENTRICULAR PERFUSION, INNERVATION, FUNCTION AND HISTOLOGY

Objectives. The purpose of this study was to better understand the eff ects of long term right ventricular pacing on left ventricular perfusi on, innervation, function and histology. Background. Long-term right v entricular apical pacing is associated with increased congestive heart failure and mortality compared with atrial pacing. The exact mechanis m for these changes is unknown. In this study, left ventricular perfus ion, sympathetic innervation, function and histologic appearance after long term pacing were studied in dogs in an attempt to see whether ba sic changes might be present that might ultimately be associated with the adverse clinical outcome. Methods. A total of 24 dogs were studied . Sixteen underwent radiofrequency ablation of the atrioventricular (A V) junction to produce complete AV block. Seven of these underwent lon g term pacing from the right ventricular apex (ventricular paced group ), and nine had atrial and right ventricular apical pacing with AV syn chrony (dual-chamber paced group). A control group of eight dogs had s ham ablations with normal AV conduction. These dogs had atrial pacing only. Regional perfusion and sympathetic inner vation were studied in all dogs by imaging with thallium-201 and [I-123]metaiodobenzylguanidi ne, respectively. The degree of innervation was also determined by ass ay of tissue norepinephrine levels. Left ventricular function was asse ssed by radionuclide ventriculography, Cardiac histology was studied w ith both light and electron microscopy. Results. Mismatching of perfus ion and innervation in the ventricular paced group was noted, with per fusion abnormalities of both the septum and free wall. Regional [I-123 ]metaiodo- benzylguanidine distribution was homogeneous. Tissue norepi nephrine levels were elevated in both the ventricular and dual-chamber paced groups compared with the control group. No light or electron mi croscopic findings were noted in any groups. In the dual chamber paced group, diastolic dysfunction was noted, with normal systolic function . Conclusions. Ventricular pacing resulted in regional changes in tiss ue perfusion and heterogeneity between perfusion and sympathetic inner vation. Both ventricular and dual-chamber pacing were associated with an increase in tissue catecholamine activity. The abnormal activation of the ventricles via right ventricular apical pacing may result in mu ltiple abnormalities of cardiac function, which may ultimately affect clinical outcome.