[Firewire]

Heartbreak isn't the only thing Miley Cyrus is confessing in her new book: Miley, Miles to Go.

The tween queen reveals that she's suffering from a heart condition called tachycardia that often leaves her worrying for her health.

The condition causes her heart rate to exceed the range of a normal resting heart rate, so you can only imagine how worried she is when she's on stage putting on a full performance.

She writes, "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."

The important thing is that her condition isn't life threatening, but it's still something no one wants to deal with.

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