I have noticed a few times when I WILD, not every time though, that my heartrate and breathing speeds up. This could be from excitement when noticing first HH and realizing I'm close to a dream. But I think that most of the time when this happend was because I was already dreaming and it's normal for heartrate and breathing to speed up.

In any case, it's normal. It's most likely happening to us everytime we dream we just don't notice it because we are not WILDing. Try to stay calm, accept what's happening and see what's on the "other side".