3. We will measure changes in cardiac output (heart's pumping capacity) by analyzing the air that is breathed ("rebreathing test", Innocor). This will be done through a mouthpiece connected to a bag full of air and small concentrations of the inactive gases SF6 and N2O. Subjects will breathe normally through a mouthpiece connected to a bag for about 5 minutes before and after the medication. Innocor is FDA approved for inert gases cardiac output measurement.