A previously healthy 58 years old man is admitted to the hospital because of an acute inferior myocardial infarction. Within several hours, he becomes oliguric and hypotensive (blood pressure is 90/60 mmHg). Insertion of a pulmonary artery (Swan-Ganz) catheter reveals the following pressures: pulmonary capillary wedge- 4 mmHg; pulmonary artery- 22/4 mmHg: and mean right atrial- 11 mmHg. This man would best be treated With -