In the late 1940s, thoracic surgeons were just beginning to enter previously off-limits territory: the heart and major blood vessels. These operations, however, were closed-heart procedures--more ambitious surgeries were not possible because there was no way to keep the body oxygenated while the heart was stopped. Just a dozen years later, thousands of open heart procedures had been done, and the field of cardiac surgery had grown enormously. Although development of heart-lung bypass machines is usually credited for this rapid progress, the first successful series of open-heart surgeries was done not with a machine, but with induced hypothermia--cooling patients to a very low body temperature. Denver surgeon Henry Swan II was the foremost developer of surgery using hypothermia, which made many types of cardiac operations possible, years before the first heart-lung bypass devices could be used routinely.

Swan was born in Denver, Colorado, on May 27, 1913, to Henry Swan, an investment banker, and Carla Denison Swan. He was the youngest of their three children and the only son. Both parents came from well-established Colorado families; his father's family owned the Swan Land and Cattle Company, and the Denisons had made their fortune in railroads. The Denison lineage also included many physicians: an uncle, Henry Denison; grandfather Charles Denison, and Charles's father and grandfather. Henry was inspired by this heritage, and would later recall that he had always wanted to become a doctor. His childhood was an active one; the family traveled frequently, and vacations often included horseback riding, camping, skiing, fishing and hunting (which would remain lifelong recreations). Henry also excelled at tennis and won several championships as a youngster.

Swan went to public school in Denver before attending Phillips Exeter Academy in New Hampshire. At Exeter he was an excellent student, wrote essays for the student literary magazine and skits for the drama club, and continued to play tennis and basketball. He then majored in history and English at Williams College, graduating in 1935 as class valedictorian. He received his MD from Harvard's School of Medicine in 1939, again the valedictorian. Following a one-year pathology internship at Colorado General Hospital in Denver, Swan returned to Boston for his surgical internship and residency at Peter Bent Brigham Hospital and Boston Children's Hospital. There he worked with Robert Gross, who was developing techniques to correct congenital defects in children.

In 1943, Swan was assigned to active duty in the U.S. Army's 4th Auxiliary Surgical Group (one of the first MASH units), which landed in France as part of the Normandy invasion in June 1944. He served in the 4th, and later in the 5th Auxiliary until the war's end, performing over 1600 surgeries. Returning home, he joined the faculty at the University of Colorado Medical School in 1946, becoming the first full-time faculty member in the department of surgery. There he helped to establish a surgical research laboratory and reorganize the school's curriculum, and began to distinguish himself as a surgical trailblazer. He served as chair of the department from 1950 to early 1961, and was a major force in its development as a first-class surgical program.

Like many surgeons of that era, Swan became interested in exploring ways to work on the vascular system and the heart. In 1949, after several years of experimental work with dogs, he did the first successful repair of an aortic aneurysm using a homograft (a section of stored cadaver aorta.) He also helped set up one of the first "artery banks" for collecting and preserving cadaver aortas for aneurysm repairs. In 1953, again after several years of laboratory research, he overcame the biggest single challenge facing would-be cardiac surgeons: stopping or slowing the heart without depriving the brain of oxygen. Swan found a way to safely induce hypothermia (very low body temperature) to reduce a patient's metabolic functions--and thus the need for oxygen--long enough to stop the heart and operate on its interior. He quickly established himself as the foremost expert on the new technique, performing hundreds of cardiac surgeries with hypothermia between 1953 and 1962. Surgeons from all over the world visited Denver during this period to learn more about Swan's technique, and he and his surgical team often traveled to demonstrate it to other institutions. (Swan was well-known as an avid small plane pilot; in early 1958 he logged 20,000 air miles traveling to Chile, Argentina, and Brazil, and later survived three plane crashes en route to medical meetings in the United States.) Eventually, hypothermia was eclipsed by improved heart-lung bypass devices, which allowed much longer operating times. In the meantime, however, Swan's successful methods afforded many surgeons their first open-heart experiences, and his intensive laboratory research greatly expanded medical understanding of the physiologic and metabolic changes that could occur during cardiac surgery. The bathtub Swan used for cooling his patients is now part of the Smithsonian Institution's collections.

Swan resigned from the University of Colorado in 1961, tired of an ever-growing burden of paperwork and committee meetings, and at odds with university administrators regarding their disposition of the income his department generated. Two years later he joined the faculty at the Colorado State University's School of Veterinary Medicine, where he introduced new standards in veterinary surgery and helped build a graduate program in veterinary research. He also pursued investigations into the relationship between body temperature and hibernation states, an interest that grew out of his earlier research on the physiological changes occurring during hypothermia. He started with a study of the African lung-fish, which estivates in dry river beds during the hot summer season. Swan eventually identified an active metabolic inhibitor in the brains of the lung-fish ("antabolone"), which he hoped might be isolated and synthesized to enable an artificial hibernation for surgery and other clinical uses. His research on hibernation, which included studies of ground squirrels and other animals, was incorporated into his book Thermoregulation and Bioenergetics: Patterns for Vertebrate Survival, published in 1974. Besides his hibernation studies, he continued work on an experimental model of shock due to hemorrhage, which he had started in the mid-1950s.

In 1982, Swan retired from his full-time position at CSU, partly due to a slowly progressing neuromuscular disorder. He continued to do hibernation research part-time, and to attend professional meetings. He also developed a synthetic buffer solution for use in transplant organ preservation, receiving a patent for this in 1993. A person of many interests, he was able during his retirement to devote more time to gardening, home improvements, and cooking.

Swan published over 250 medical articles and book chapters during his career. With a life-long interest in history, he also co-authored a series of articles on the history of surgery (with David Schechter). His professional achievements were recognized by the American Medical Association's Hektoen Gold Medal for Original Research in 1955; an honorary doctorate from Williams College in 1959; and the Academy of Surgical Research's Jacob Markowitz Award in 1996, among others. The University of Colorado established the Henry Swan Visiting Professorship Annual Lecture in 1982.

Swan married his college girlfriend, Mary Fletcher Wardwell, in 1936, after his first year of medical school. They had three daughters and a son before divorcing in 1964. Swan married Geraldine Morris Fairchild, a University of Colorado medical researcher, later that year.

Swan died of progressive neuromuscular disease at his Denver home on July 13, 1996, at the age of 83.