 During study visits 2 and 3 you will be asked to again exercise at maximal exertion while breathing into a mouth piece. However, during these two visits you will be receiving an intrathecal injection of fentanyl one day and placebo the other day, randomly selected and you will be blinded as to which you are receiving. A catheter will also be placed in an artery in your arm and a vein in your leg, which will help us to measure blood flow, blood pressure and draw blood when need be. The investigators will also ask you to perform a brief chemosensitivity test, breathing in and out your own air, after you are done exercising. 