Cardiology was a medical science dealing with disorders of the heart and circulatory system. A cardiologist was a medical doctor who worked in this science.
Dr Grace Holloway was a cardiologist who operated on the Seventh Doctor in San Francisco after he was shot by gang members in 1999. Because she was unfamiliar with Gallifreyan physiology, the Doctor died on the operating table and regenerated into the Eighth Doctor. (TV: Doctor Who)