Head-up Tilt Test in Patients With Reflex Syncope and Asystolic Response Who Received a Dual-chamber Pacemaker With the Closed Loop Stimulation (CLS) and Participated in the BIOSync Trial

Inclusion criteria * Ability to understand the nature of the study. * Willingness to provide written informed consent. * Patients undergoing HUTT for monitoring of syncope, therapy adjustment, training or other medical reasons during ordinary follow-up, irrespective of study participation * Patients who participated in the BIOSync study\*. * Patients with a dual-chamber pacing system equipped with the CLS algorithm. \* If needed, inclusion of other patients who did not participate in the BIOSync study will be considered to complete study cohort, provided that they have already a CLS pacemaker system and fulfill the same inclusion/exclusion criteria of the BIOSync study. Exclusion criteria * Pregnant or breast feeding women. * Age less than 40 years. * Patients who after the BIOSync study participation have developed the following: * Any indication to pacemaker different from reflex syncope with positive HUTT response; or * Any classified indication to implantable defibrillator, cardiac resynchronization therapy according to current guidelines; or * Any cardiac dysfunctions likely leading to loss of consciousness (overt heart failure, ejection fraction \<40%, myocardial infarction, diagnosis of hypertrophic or dilated cardiomyopathy, clinically significant valvular disease, sinus bradycardia \<50 bpm or sinoatrial block, Mobitz I second degree atrioventricular block, Mobitz II second or third degree atrioventricular block, complete bundle-branch block).