Mitral Valve Prolapse and burning calories

I am 21 and was diagnosed with Mitral Valve Prolapse last fall, my doctors said I've probably had MVP for quite a few years. Due to the MVP I have pretty regularly occurring tachycardia, my resting heart rate is usually between 100 - 120 ( I also have low blood pressure). The fast heart rate doesn't bother me though, I normally can't even tell when my heart is racing unless I actually feel for my pulse. My doctors gave me Toprol, but they said I only needed to take it if the tachycardia became something that bothered me.

My question is about burning calories. I wear a normal exercise heart monitor once or twice a month to keep a log of my heart rate. The monitor also measures calories burned. Now when I’m sitting at school or just slowly walking around ( my heart rate is between 140 - 170 when standing) the monitor says that I have burned around 300 calories in one hour by basically doing nothing. Now I'm a fairly small person, I'm only 5'2 and about 100lbs and I've always been thin, but is my body actually burning that many calories because of my increased heart rate? Or does the monitor just think that I'm burning calories because of the increased heart rate?

I'm sorry if this sounds confusing, but if anyone has any insight it would be greatly appreciated.


Thanks, Casey