The pop star talked about her tachycardia diagnosis in her autobiography, “Miles to Go.” The condition causes her resting heart rhythm to exceed the normal range. Cyrus has not disclosed her exact diagnosis but says the type of tachycardia that she has is not life threatening. Many people with harmless arrhythmias live healthy lives without medical intervention. Testing can determine the need for treatment. Cyrus added, however, that while it doesn’t hurt her, it does bother her. "There is never a time onstage when I'm not thinking about my heart,” she wrote.