A Proof of Principle Study of Ondansetron for the Prevention of Vasovagal Syncope: The Eleventh Prevention of Syncope Trial (POST11)

About 20% of adults faint recurrently. These patients are often highly symptomatic, have problems with employment and driving, can be injured, and have poor quality of life. There are few therapies that have withstood the test of randomized clinical trials. the investigators will conduct a prospective, randomized, parallel, double-blind, proof-of-concept study that tests the hypothesis that serotonin 5HT3 receptor inhibition with ondansetron prevents tilt-induced vasovagal syncope (VVS) and pre-syncope in patients with clinical VVS. A total of 70 patients with quantitative clinical diagnostic criteria for VVS and at least 1 syncopal spell in the preceding year will be randomized in a double-blind acute phase 2 study to ondansetron 8 mg PO BID x 2 doses or matching placebo. The endpoint will be presyncope or syncope associated with diagnostic hemodynamic changes. These data should provide useful preliminary data as a foundation on which to conduct a subsequent randomized clinical trial.

Inclusion Criteria:(A) ≥1 syncopal spells in the year preceding enrolment (B) ≥-2 points on the Calgary Syncope Symptom Score( Accurate Calculation for Diagnosis of Vasovagal Syncope) (C) Age ≥18 years with informed consentExclusion Criteria:other causes of syncope, such as ventricular tachycardia, complete heart block, orthostatic hypotension or hypersensitive carotid sinus syndromean inability to give informed consentimportant valvular, coronary, myocardial or conduction abnormality or significant arrhythmiahypertrophic cardiomyopathya permanent pacemakera seizure disorderhypertension defined as >160/90 mm Hgpregnancylactating womenglaucomamedications with known effects on BPKnown hypersensitivity to ondansetron and related medicationsother factors which, in the investigator's opinion, would prevent the subject from completing the protocol.