A 59-year-old man with a history of myocardial infarction 2 years ago undergoes an uneventful aoobifemoral bypass graft for aooiliac occlusive disease. Six hours later he develops ST segment depression, and a 12-lead electrocardiogram (ECG) shows anterolateral ischemia. His hemodynamic parameters are as follows: systemic BP 70/40 mm Hg, pulse 100 beats per minute, CVP 18 mm Hg, PCWP 25 mm Hg, cardiac output 1.5 L/min, and systemic vascular resistance 1000 (dyne * s)/cm5 . Which of the following is the single best pharmacologic intervention for this patient?