Cardiology
From Tardis Wiki, the free Doctor Who reference
A cardiologist is a medical doctor dealing with disorders of the heart and circulatory system.
Dr. Grace Holloway was a cardiologist who operated on the Seventh Doctor in San Francisco after he was shot by gang members in 1999. Being unfamiliar with Gallifreyan physiology, the Doctor died on the operating table and regenerated into the Eighth Doctor. (TV: Doctor Who (TV story)