A 68-year-old hypeensive man undergoes successful repair of a ruptured abdominal aoic aneurysm. He receives 9 L Ringer lactate solution and 4 units of whole blood during the operation. Two hours after transfer to the surgical intensive care unit, the following hemodynamic parameters are obtained: systemic blood pressure (BP) 90/60 mm Hg, pulse 110 beats per minute, central venous pressure (CVP) 7 mm Hg, pulmonary aery pressure 28/10 mm Hg, pulmonary capillary wedge pressure (PCWP) 8 mm Hg, cardiac output 1.9 L/min, systemic vascular resistance 1400 (dyne*s)/cm5 (normal is 900-1300), PaO2 140 mm Hg (FiO2 : 0.45), urine output 15 mL/h (specific gravity: 1.029), and hematocrit 35%. Given this data, which of the following is the most appropriate next step in management?