I've a long history of super-ventricular tachycardia. starting from age 16 with a 4 hour bout @ over 280BPM.
From then on only marijuana induced further episodes (though first was naturally occuring).
Adenocard, Verapamil, and Tenormin I stopped taking after 6 months when 16.
I've naturally trained for over 10 years, and only had 3-4 episodes, and I think 1 from/during training.
I've tried all forms of caffiene/ephedrine products and not induced arythmia.
I am on my very first cycle. Testosterone, and Boldenone (Equipoise), and I have had NO experiences of any form of arythmia.
Contest prep time however, with fluid depleted and such, arythmia becomes common, and should be watched very carefully if you are naturally susceptible to tachycardia episodes.
Murmurs I have no information about.
I would think that hGH is something you would want to stay away from, as it enlarges your internal organs.
For further data I suggest Googling the steroids you are planning to take, combined with various names of pathology related to your unique biology.