 For the study of central chemoreceptors, investigator will use the steady-state hypercapnic test, which measures the ventilatory response to hypercapnia at a constant level of Oxygen partial pressure PaO2. The ventilatory response to hypercapnia is calculated by the method of linear regression and is represented by the angular coefficient of the straight line that describes the variation of ventilation for each variation of alveolar carbon dioxide partial pressure PCO2. 