 For the study of peripheral chemoreceptors will be used the single-breath or transient hypercapnia test. The patient makes a single deep breath of a gas mixture containing 85 O2 and 15 CO2 and then resume normal breathing. It is expected, therefore, a period of 3 seconds, required because the gases contained in the mixture may come from the pulmonary circulation to the peripheral circulation, at the level of peripheral chemoreceptors. The ventilation is recorded within the next 20-30 seconds, because in this time interval is evaluated only the sensitivity of the peripheral chemoreceptors, being the latency of response of the central chemoreceptors greater than 1 minute. 