Natalie Cole's cause of death was idiopathic pulmonary arterial hypertension, which led to heart failure, the singer's family said on Thursday.
Her family said in a statement to The Associated Press that after Cole received a kidney transplant in 2009, she was diagnosed with pulmonary arterial hypertension, a rare lung disease that causes shortness of breath, dizziness and sometimes chest pain. Because Cole's case was idiopathic, the exact cause is unknown.
The statement read: "[Cole] responded well to pulmonary arterial hypertension-specific agents over many years, during which she performed many concerts world-wide, but eventually succumbed to intractable right heart failure, an outcome that unfortunately commonly occurs in this progressive disorder."
Cole had battled drug problems and hepatitis that forced her to undergo a kidney transplant.
She died on Dec. 31 at age 65.