The use of positive inotropic agents, such as sympathomimetics and phosphodiesterase inhibitors, in heart failure (HF) is limited by proarrhythmic and positive chronotropic effects. In the present study, we compared the hemodynamic effects of intravenous LY366634 (LY), a Na+ channel enhancer, with dobutamine (DOB), in eight dogs with HF produced by intracoronary microembolizations. We also determined whether intravenous LY has synergistic effects when combined with digoxin. After baseline measurements, infusion of DOB was initiated at a dose of 2 micrograms/kg/min and increased until an increase of heart rate (HR) 30% of baseline or ventricular arrhythmias developed. Once hemodynamics returned to baseline, LY was infused at a dose of 2 micrograms/kg/min and increased until the LV fractional area of shortening (FAS), determined echocardiographically, reached a similar level as with DOB. Both drugs increased FAS equivalently compared to baseline (DOB, 24 +/- 3 to 47 +/- 2; LY, 27 +/- 2 to 46 +/- 2%). DOB increased HR from 78 +/- 4 min-1 at baseline to 107 +/- 7 min-1 at maximal dose (p < 0.05) and provoked serious arrhythmias in one dog. In contrast, LY infusion did not increase HR (82 +/- 7 vs. 80 +/- 8 min-1) or elicit arrhythmias. After 1 week of oral digoxin, dogs were infused again with LY. A lower dose of LY was needed to achieve the same increase in FAS compared to LY alone, but this was not statistically significant. The combination of LY with digoxin did not increase HR or evoke arrhythmias. We conclude that in dogs with HF, intravenous LY improves LV function to the same extent as DOB without increasing HR or evoking ventricular arrhythmias. The combination of LY with digoxin elicits a safe positive inotropic response.