fast heart rate while at rest, sinus tachycardia?

I have been having a fast heart rate while I am resting or sleeping.  It has gone up to 170.  My cardiologist had me on a heart monitor for 2 weeks and when I went for my results she just told me that the fast rhythm were normal and I had sinus tachycardia.  My understanding of sinus tachycardia is that it happens when you are active doing something (not resting).  My resting heart rate is below 90, so this doesn't happen all the time.  I can be sitting and suddendly my heart starts racing and it stops on its own, or sometimes I am walking and my heart takes off as if I was running.  Again this doesn't happen all the time and it scares me a little.  What kind of sinus tachycardia is this?if anyone knows or has it themself.  If anyone has some insight to what is happening I would really appreciate it.

[itdood]

might be "inappropriate sinus tachycardia" or IST.  google for more info.

http://heartdisease.about.com/cs/arrhythmias/a/IST.htm

[josie172]

thank you for responding.  I check out some info on IST.  I am a bit confused because it says that people who have IST have a higher resting heart rate all the time which is not my case I have a normar resting heart rate with the ocasional high resting heart rate.  Has anyone experienced this.  I know is not a panic attack because I can control it.  the heart has a mind of it's own..