The beneficial effects of VDD pacing (pacing the ventricle in response to sensed atrial activity) with a reduced PR interval as adjunctive therapy for the treatment of refractory congestive heart failure (CHF) secondary to dilated cardiomyopathy (DCM) has been reported in humans. Recent evidence suggests that this benefit arises largely from correction of severely abnormal conduction patterns primarily within the left ventricle (LV). This study evaluates the effects of pacing on LV performance in a naturally occurring model of canine dilated cardiomyopathy (DCM). Eight Doberman pinschers with stable CHF secondary to DCM were anesthetized with intravenous fentanyl and midazolam and instrumented with appropriate pacing leads, aortic and LV micromanometer catheters, and a LV conductance catheter for continuous LV volume determination. The acute hemodynamic effect of pacing the ventricles from one of three sites (LV, right ventricle, and left and right ventricle simultaneously) in response to right atrial sensed activity at one of 5 reduced PR intervals (15 pacing combinations) was evaluated by collection of continuous pressure and volume data. Each of the 15 combinations was randomly paced for 5 beats with 15 sinus beats between every pacing combination. This pacing sequence was performed 5 times. The pressure and volume data allowed the calculation of multiple indices of LV performance including; mean arterial pressure, aortic pulse pressure, LV end-diastolic pressure, maximum dP/dt, minimum dP/dt, tauL, stroke volume, and external stroke work. There was no significant hemodynamic improvement in any of the 8 patients with any pacing combination when compared to the preceding sinus beats. Therefore, this group of 8 Dobermans with symptomatic DCM did not demonstrate acute hemodynamic improvement with this form of pacing. Furthermore, we believe that given the homogeneity of any purebred dog population, it is unlikely that any Doberman pinscher with symptomatic DCM will benefit acutely from dual-chamber pacing.