Patients will be excluded if they have:

* other causes of syncope, such as ventricular tachycardia, complete heart block, postural hypotension or hypersensitive carotid sinus syndrome,

* an inability to give informed consent,

* important valvular, coronary, myocardial or conduction abnormality or significant arrhythmia,

* hypertrophic cardiomyopathy,

* a permanent pacemaker,

* a seizure disorder,

* urinary retention,

* hypertension defined as \>140/90 mm Hg,

* hepatic disease,

* glaucoma or

* a 5-minute stand test resulting in diagnoses of Postural Orthostatic Tachycardia Syndrome or Orthostatic Hypotension.

Assessment of Midodrine in the Prevention of Vasovagal Syncope: The Prevention of Syncope Trial IV