Heather North -- the longtime voice of Daphne on 'Scooby-Doo' -- died of a heart attack brought on by respiratory disease ... TMZ has learned.
North's official cause of death is listed as cardiopulmonary arrest with an underlying cause of constrictive bronchiolitis ...
. She also had a very low level of oxygen in her blood when she died on November 29.
As we reported ... the actress passed away at her home in Studio City following a long illness.
Heather starred on 'Scooby' for more than 30 years, and also had on-camera roles on "Days of Our Lives," "The Barefoot Executive," "Mr. Novak," and "The Fugitive."
She was 71.