Hi bead, just to let you know that mine does that a lot too, for no reason!  I was told it was anxiety/panic, even though i don't feel like i'm anxious or panicky at the times that it does it.  It can also be regulated some by the way that we are breathing.  I don't like it when it does this and then it makes you feel more anxious and it goes even faster, vicious circle.  We are too tuned into our hearts and what it is doing and i really don't know of a way that i can stop being like that, but believe me i do try.  I think it does get better as we carry on and get more used to things as if our minds and bodies are getting used to these feelings, if that is possible.  Just wanted to let you know that i know what this is like and that i don't like it either, so you are not on your own hun.x

Add me to the list.  I posted here often back a few years ago.   Then was better since 04.  Now stressors in life have my anxiety at an all time high.   And all I think about is my heart rythmn.  I count the beats often.  They are sometimes 86 and then can go to 120.  I can tell all my muscles are as tight as a drum.
It all started with the anxiety.  So that has to be it.
I completely understand everyone who struggles with this and how the worry about the heart starts a vicious cycle that just feeds the symptoms.
I was just looking at an anxiety site I keep bookmarked to attempt to reassure myself of my symptoms.  I looked around the site more and noticed the books others here have mentioned.  Claire Weekes.

I think the Claire Weekes method is aimed at developing acceptance of it. Don't fight the feelings, panic, anxiety, let them wash over you. Easier said than done though, and if you are 'trying' to accept them, it's not really washing over you, you are working at it! I found it helpful though.

Supraventricular Tachycardia forms above the ventricals in the atrial chambers. The signal is initiated here and causes a rapid heart rate. Due to the atrial chambers being unable to fully relax and fill with the proper amount of blood during the event, it can cause dizziness or lightheadedness.

Although SVT has been a buzzword lately and has been observed in patients with underlying heart disease, it is just as commonly observed in persons without heart disease, even as young as adolescence and childhood. In of itself, SVT is not dangerous.

A heart rate of 145, while somewhate disconcerting, is rather mild on the scale of tachycardia. Nothing to worry about here.

Best regards and Good Health