I suffer svt and it got MUCH worse while on mirena so I know what you are going through,I had heart rate of 176 bpm and had to have the injection to lower it(Impending Doom),its very scary when it happens all the time,it causes you to have panic attacks.I had mirena removed a few months ago and my svt is back under control again its such a relief not to have heart palpitations every day anymore.
I take metoprolol to control palpitaions(fast heartbeat)
I found that taking magnesium really helps to control svt.