This is a randomized, double-blind, placebo-controlled, cross-over study. Twenty five patients are going to be included. They are going to come to the laboratory to receive IV placebo or IV Mg sulfate at two distinct occasions about 48 hours apart. Half of the patients are going to receive Mg first and Placebo at the second day, while the other half are going to receive the treatments in an inverse order. Tests are going to be performed before and about 40 minutes after the IV infusions. Tests to be performed are: spirometry, arterial blood gases, Mg plasma level measurements, and a maximal exercise test protocol in cycloergometer.