In order to measure the pressure in your lungs, you will have a procedure called a "right heart catheterization". This is a procedure that may have been done as part of your standard of care if you were not enrolled in this study. For this procedure, a small tube will be placed in a vein in your neck. A longer tube will be inserted into the first tube and fed through the vein and into your heart. This tube will be used to measure the pressure in your heart and lungs. If the pressure in your lungs is above a certain level, you will receive treatment with nesiritide during the right heart catheterization procedure. Nesiritide will be given by vein over 30 minutes. After treatment, the pressure in your heart and lungs will be measured again. You will be awake during this procedure and lying flat on your back. An anesthetic will be used to numb the area of your neck where the tube is placed. The entire procedure (including treatment) should take no longer than 2 hours. When complete, the tubes will be removed.