When placement of catheter is completed, period 2 will begin with initiation of Study Drug infusion (IR nesiritide or IV D5W). Nesiritide will be diluted according to its labeled instructions in D5W to a final dilution of 6 mcg/mL. Placebo patients will receive IV D5W as a bolus and a continuous infusion of D5W at volumes equal to what they would have gotten had they been randomized to nesiritide.