The 16-year-old singer has tachycardia - meaning her heart rate exceeds the range of a normal resting rate - and admits she worries about the complaint when she performs live.

She says in her new autobiography Miles to Go: "The type of tachycardia I have isn't dangerous. It won't hurt me, but it does bother me. There is never a time onstage when I'm not thinking about my heart."

Health problems aren't the only subject Miley has spoken about in her tell-all tome.

An entire chapter is dedicated to 'Prince Charming', the nickname she uses to refer to her first love.

However, Miley is keen to play down reports that she is talking about ex-boyfriend Nick Jonas, a member of The Jonas Brothers.

The 'Hannah Montana' star said: "I didn't necessarily talk about 'dating a Jonas'. I talked about dating and who I really feel my first love is, and everyone wants to put a name with the code.

"But really I wouldn't use a code if I thought it was obvious. So no one really knows who he is."

The 7 Things I Hate About You singer wrote her autobiography to tell her story to her fans.