vasodepressor neurocardiogenic syncopeneurocardiogenic syncope diagnosisvasovagal syndromemalignant neurocardiogenic syncopeneurocardiogenic syncope syndromevasovagal triggersneurocardiogenic syncope medicationsdefine vasovagalvasovagal syncopal attackmalignant vasovagal syndromesyncope fainting episodedefine vasovagal response