Artificial pacemaker (AP) implantation is the mainstay of therapy for treatment of third degree atrioventricular block (3 AVB). Despite several reports detailing long-term survival for dogs with APs there is little data detailing the radiographic, echocardiographic or neurohormonal consequences in this patient population.

Complete baseline (3 AVB) and 4 weeks post fixed rate (100) ventricular demand (VVI) pacing were available for comparison from 9 dogs previously enrolled in a study of physiologic pacing. Compared to 3 AVB, VVI pacing reduced radiographic heart size (12.25+/-2.0 vs 10.92+/-1.1, p=0.01) and pulmonary parenchymal score (median 1, range 0-3 vs median 0, range 0-1, p<0.05). Echocardiographically determined left atrial size (3.21+/-0.5 vs 2.77+/-0.3 cm, p<0.01), left atrial to aorta ratio (1.50+/-0.4 vs 1.28+/-0.3, p=0.02), left ventricular (LV) diastolic dimension (4.41+/-0.6 vs 3.81+/-0.5 cm, p<0.001), fractional shortening (43.6+/-8 vs 22.2+/-7%, p<0.001), and LV posterior wall systolic dimension (1.56+/-0.2 vs 1.36+/-0.2 cm, p=0.03) were reduced during VVI pacing. LV systolic dimension (2.46+/-0.4 vs 3.00+/-0.5 cm, p<0.01) was significantly larger during VVI. Serum aldosterone levels were significantly lower during VVI pacing (158+/-127 vs 49+/-31 pg/ml, p=0.04). There were no significant differences between 3 AVB and VVI pacing in aortic dimension, interventricular septal dimensions, LV posterior wall diastolic dimension, NT-pro atrial natriuretic peptide, brain natriuretic peptide, plasma renin activity, endothelin-1, epinephrine or norepinephrine concentrations. These results suggest compared to 3 AVB, fixed rate VVI pacing at 100 beats per minute reduces radiographic and echocardiographic cardiac dimensions.