Impedance Cardiography. Impedance cardiography will be carried out in a single centre by a cardiopneumology technician with Niccomo continuous cardiac output monitor (Medis, GmbH, Ilmenau, Germany). This equipment uses the technique of four electrodes, two of current application and two others that detect the changes of voltage. As the current amplitude is constant, the voltage detected is proportional to the tissue impedance. Patients must present with fasting of 6 hours but must take their antihypertensive drugs and the examination is carried out in 4 phases after an initial 5 minutes hemodynamic stabilization period: 1. supine position I - 20 minutes continuous recording; 2. 70º orthostatism -with the help of the tilting table for 10 min in continuous recording; 3. tilt-back at 0º; 4. supine position II - 10 min continuous recording. The examination is interrupted if there is syncope or pre-syncope; dizziness, nausea and malaise associated with poorly tolerated hypotension and / or bradycardia; pain / precordial discomfort; ECG ST segment changes; Systolic blood pressure > 210mmHg.