A coronary bypass was a surgical procedure performed on the heart of a lifeform. The process involved the grafting of arteries and veins to the heart's main artery in order to bypass blockages or improve blood circulation to and from the organ.
During The Doctor's holographic delusion in 2371, he was asked by Captain Kathryn Janeway to assist her in bypassing a power relay on the bridge of the USS Voyager. The Doctor asked if the process was similar to a coronary bypass, to which Janeway replied that it actually was. (VOY: "Projections")