Rationale for the prevention of syncope trial IV: assessment of midodrine.

Background: Vasovagal syncope is a common problem associated with a poor quality of life, which improves when the frequency of syncope is reduced. Effective pharmacological therapies for vasovagal syncope have been elusive. Midodrine is a pro-drug whose primary metabolite is an alpha-1 adrenoreceptor agonist. A few studies have suggested that it may be beneficial in syncope, but all have had significant methodological limitations. A placebo-controlled clinical trial of midodrine for the prevention of vasovagal syncope is needed.
Structure of Study: The prevention of syncope trial IV (POST 4) is a multicenter, international, randomized, placebo-controlled study of midodrine in the prevention of vasovagal syncope. The primary end point is the time to first recurrence of syncope. Patients will be randomized 1:1 to receive midodrine 10-30 mg/day or matching placebo, and followed for 1 year. Secondary end points include syncope frequency, presyncope, and quality of life. Primary analysis will be performed with an intention-to-treat approach, with a secondary on-treatment analysis.
Power Calculations: A total sample size of 112, split equally between the two groups, achieves 85 % power to detect a 50 % relative risk reduction when the event rates are 55 and 27.5 % in the placebo and midodrine arms. Allowing for 20 % dropout, we propose to enroll 140 patients.
Registration: POST 4 is registered with http://www.clinicaltrials.gov (NCT01456481).
Implications: This study will be the first adequately powered trial to determine whether midodrine is effective in preventing vasovagal syncope. If it is effective, then midodrine may become the first-line pharmacological therapy for this condition.