Are there any herbs I should avoid to prevent worsening my svt?

Not aware of any. As far as herbs, I am not aware of any that could worsen your svt. Definitely stay away from stimulants such as caffeine, pseudoefedrine (sudafed) and phenylephrine (pe) as they could trigger an episode of svt .
Long list. First of all, remember that many of our pharmaceutical drugs started as botanicals. Just because they are "natural" doesn't mean "safe" marijuana, heroin, and hemlock are "herbs" for svt, i recommend avoiding all herbal "stimulants." caffeine qualifies. Here is one list that i found on search engine: http://www.Motherherbs.Com/stimulant.Html by no means, exhaustive list.