If a significant rate drop (> 15%) is noted during head-up tilt test (HUT), either isolated or combined with vasoplegia, and associated with syncope or severe presyncope, patients will be eligible for inclusion in group A. If the HUT is negative and a pause > 3 seconds has been documented, patients will be assigned to group B. If both conditions are present, patients will be eligible for inclusion in either group but will be preferentially assigned to group A.