Cardiologist and the Mechanic

A cardiologist's car breaks down and he goes to a mechanic to get it fixed. After everything is done, the mechanic asks the cardiologist,

"Here's what I don't understand. I fix engines, and so do you, albeit human ones, so why do you get paid ten times more than I do?"

The cardiologist then turns the ignition on and says, "try it with the engine running."