What is the normal fetal heart beat in the second trimester?

I was just curious if anyone knew what was considered as a "normal heart beat" in the second trimester. I went to the dr two weeks ago then i was 17 weeks 2 days and the hb was 155. Im now 19 weeks 2 days and had an apt. today and the hb was 160. The doctor said it was normal. I just forgot to ask her what was considered in the normal range. Thanks.