Open-heart surgery requires stopping the heart. While the surgeon works on the still organ, something else has to keep the patient alive — oxygenating blood and circulating it through the body. In the early 1950s, no machine could do this reliably for more than a few minutes. The first attempts at heart-lung machines, by John Gibbon and others, had high failure rates and were not yet clinically usable.

At the University of Minnesota, the surgeon C. Walton Lillehei solved the problem with a different kind of machine — a human one.

He called the technique cross-circulation. The procedure required two patients in the operating room at once. A healthy parent or close relative was anesthetized alongside the child. Tubes were placed into the donor's femoral artery and vein, and into the corresponding vessels of the child. The two circulatory systems were temporarily joined. The adult's heart and lungs now worked for both bodies. Their blood flowed into the child, oxygenated and pumped; the child's used blood returned to the adult to be cleaned and reoxygenated.

Between 1954 and 1955, Lillehei performed approximately 45 of these procedures, mostly on children with congenital heart defects who would otherwise have died young. Roughly half of the patients survived long term. None of the donors died, but several suffered serious complications. The technique remained the only operation in the history of surgery with a potential mortality rate of 200 percent — both patient and donor at risk on the same table. It was deeply controversial.

By 1955, the Mayo Clinic had developed a working heart-lung machine, and cross-circulation became obsolete almost overnight. But Lillehei had already proved that the human heart could be opened, repaired, and closed, with a patient surviving the operation. Modern cardiac surgery is built on those forty-five procedures.