This study is being done to determine the effects (good and bad) of intravenous infusion of a human brain natriuretic peptide (BNP), Natrecor (nesiritide), a hormone produced by the heart in persons who have just suffered a heart attack. The human BNP, Natrecor (nesiritide) has been approved by the United States Food and Drug Administration (FDA) to be given intravenously for the management of acute heart failure. It is unknown if human BNP may have good effects on the pumping function of the heart after a heart attack.