This study is seeking participants with Paroxysmal Supraventricular Tachycardia (PSVT) which is a specific type of fast heart rhythm, to participate in a randomized study using an investigational (not yet approved for commercial use or sale by the US Food and Drug Administration (FDA) medication versus placebo (looks like medication but there is no active ingredient). The study medication, known as Etripamil, is a nasal spray. Participants will receive a test dose of the medication to see its effect, then be taught to give themselves the medication so the treatment can be done in a home setting. Participants will then be randomized (assigned by chance) to study medication vs placebo to use at home when an episode of PSVT occurs. Participants will also be shown how to place a heart monitor on their chests to record while using the study medication. Participation in this study will last approximately 12-18 months and will include at least 4 clinic visits including screening/test dose visit, randomization treatment, randomization treatment follow up and final study visit. In addition to these 4 visits you will also have a monthly study call or clinic visit to check in with you. Physical exams, vital signs, blood work, urinalysis, ECGs (recording of electrical activity of the heart), placing and using a heart monitor and communicating with a telephone coach are all part of the study procedures.