Like all first-generation cardiovascular surgeons, Swan had trained in general surgery, doing both thoracic and abdominal procedures, but not working on the heart or its major vessels. Cardiology had developed into a distinct specialty before World War II, greatly aided by tools such as the electrocardiograph (EKG) and cardiac catheters, which allowed physicians to measure pressures within the heart chambers. Radiographic techniques for studying arteries had also been developed. By contrast, cardiac surgery advanced slowly, owing largely to the difficulty of interrupting the heart's function long enough to repair it, without risking brain damage. But the boundaries of thoracic surgery were expanding rapidly during this period, often with dramatic results. In 1944 Alfred Blalock and Helen Taussig at Johns Hopkins had devised an arterial shunt operation that prolonged the lives of many "blue babies" born with heart defects. That same year, Robert Gross at Harvard and Clarence Crafoord and K.G.V. Nylin at Sweden's Karolinska Institut had each repaired a coarctation of the aorta (a congenital constriction of that large artery in the descending portion near the heart) by removing the defective section and suturing the aorta ends together. Dwight Harken's wartime success at getting pieces of shrapnel out of still-beating hearts inspired him to use the technique (after many animal trials) to correct mitral valve stenosis, a common outcome of rheumatic fever. (Prior to the advent of antibiotics, starting in the late 1930s, rheumatic fever often followed streptococcal infections such as strep throat and scarlet fever. This caused stenosis, i.e., scarring and narrowing of the heart valve openings, particularly the mitral valve through which blood flows from the left atrium to the left ventricle. With the flow of oxygenated blood from the lungs thus seriously reduced, patients often suffered from disabling shortness of breath. Over a million Americans suffered from rheumatic heart disease at mid-century.) In 1948, Harken, in Boston, Charles Bailey in Philadelphia, and Russell Brock in London independently carried out successful mitral commissurotomies to re-open the constricted valve. With the heart still beating, they made a small incision in the left atrium and, working by touch alone, opened the stenosed valves with a finger, sometimes fitted with a small blade. This was a "closed heart" procedure, carried out by opening the left chest cavity to gain access. Though risky, especially in the early years, there was soon great demand for the operation; hospitals everywhere were pressured to offer it.
By 1949, Gross had improved his technique for repairing coarctation of the aorta by using homografts--sections of preserved aorta from cadavers--to bridge the gap after excising the coarctation. The same year, Swan became the first to successfully repair an aortic coarctation with thoracic aortic aneurysm, using a section of preserved aorta. (An aneurysm is a ballooning of the arterial wall caused by the pressure of circulating blood against weak spots in the inner wall. Weak spots may be congenital or caused by diseases such as late-stage syphilis or, most commonly, by atherosclerotic plaque. Depending on the location and size of the weak spot, the aortic wall might bulge out into a pouch on one side, or it might be uniformly distended around its circumference. Under the continuous force of the heart's pumping, the aneurysm grows larger and the arterial walls thinner and weaker. If not treated, it will eventually rupture, causing a fatal hemorrhage.) Swan went on to do a series of peripheral artery repairs using homografts, and helped to establish Colorado's first artery bank. Swan would later recall that his reports of the work were not enthusiastically received at first; however, in late 1951, Michael DeBakey heard him speak at the Pan-Pacific Surgical Association, and questioned him closely afterwards about grafting. Several months later, DeBakey and Denton Cooley did their first grafted aneurysm repair and quickly became the leading experts at vascular repairs using homografts, and later synthetic grafts.
Meanwhile, Swan's investigations took a new direction in 1950, after he heard Canadian surgeon Wilfred Bigelow report on his research with hypothermia at the annual American Surgical Association meeting. Bigelow described cooling anesthetized dogs to 20-25°C, cutting off the blood flow into the heart for 15 minutes, then re-establishing circulation and warming them slowly to normal temperature. Only fifteen percent of the animals recovered from this process with no ill effects, but for Swan and others, the implications were obvious: with sufficient understanding of the physiological and metabolic effects of lowered body temperature, it would be possible to safely stop the heart long enough to perform open-heart surgeries.
Exposure to cold normally causes warm-blooded animals to respond with shivering and constriction of the peripheral circulation to increase heat production. These responses increase energy expenditure and general metabolic stress for a time, but once the body temperature falls to 5-6 degrees below normal, the metabolism and physiological functions slow down considerably. Further lowering of temperature will eventually lead to heart or kidney failure, coma, and death. In clinical hypothermia, the goal was to stabilize the patient in the second stage, so that circulation and respiration were greatly slowed, and body tissues required much less oxygen. For several years Swan and his associates worked in the surgical lab to better understand hypothermia and to eliminate the most serious hazards, especially ventricular fibrillation (in which nerve impulses to the heart muscle fibers become chaotic and the ventricles can no longer contract in a coordinated rhythm to pump blood.) They found that fibrillation could be avoided by keeping the patient hyperventilated, perfusing the heart with a drug, neostigmine, to slow it, and using intravenous glucose. On January 9, 1953, they performed their first heart surgery on a human patient using hypothermia, a closed-heart procedure to open a stenosed pulmonary valve. On February 19th, they successfully completed their first open-heart operation, five months after F. John Lewis did the first such procedure at the University of Minnesota.
Swan's operations began by immersing the anesthetized patient in a bathtub of lukewarm water. When the vital signs were stable, ice cubes were added to the tub, until the patient's internal temperature reached about 30°C (normal human body temperature is 37°C). The patient was removed from the tub and dried, and a diathermy heating device (which used high frequency electric current to generate radio or microwaves and warm the body from the inside out) was wrapped around the lower abdomen to stop the fall in body temperature and begin re-warming. After opening the chest, the surgeons gently clamped off the blood vessels to the heart. They then had only six minutes of intra-cardiac operating time; if more time was needed, the circulation could be started and then stopped again, for a total of about ten minutes. During this timeframe, a skilled surgeon could repair common uncomplicated heart conditions, such as stenosis of the pulmonary valves or atrial septal defects. When the procedure was completed, the patient was re-warmed to normal temperature and carefully monitored for complications.
Swan and his colleagues performed over 600 surgeries using hypothermia alone during the next ten years, including more than 400 open-heart procedures along with a number of closed-heart and non-cardiac surgeries. Many surgeons visited Denver during this period to learn more about Swan's technique, and he and his surgical team often traveled to demonstrate it for other institutions. In the surgical lab, they continued to extend their knowledge of the physiological processes involved, such as the tolerance of various body tissues to cold, the role played by acid-base balances, electrolyte changes, and shifts in plasma volume, blood chemistry, and so on, increasing the safety of such surgery even further.
The successful use of hypothermia in surgery was a major advance, but it had shortcomings: with its limited safe operating time, it required that the surgeon be skillful and fast, and that the cardiologist's diagnosis be accurate--if a more complicated disorder was revealed when the heart was opened, the surgeon might not have time for an effective repair. Using hypothermia also required that the surgical team have a sound understanding of the physiology of the "cold" patient. (As Swan often noted, his surgeries were usually successful, even in the first years, because he worked closely with cardiologist S. Gilbert Blount and anesthesiologist Robert Virtue.) While Swan was perfecting heart surgery with hypothermia, others, notably John Gibbon, Clarence Dennis, and John Kirklin were starting to use heart-lung bypass devices to enable open-heart surgery. Their initial mortality rates were fairly high, as were the risks of post-operative complications. And the first machines were expensive and required skilled technicians. However, they also gave surgeons a much larger window of operating time, thus allowing more extensive repairs than were possible with hypothermia. By the early 1960s, heart-lung bypass had become the accepted method for gaining access to the open heart. Although the era of hypothermia lasted only a decade, it was an important turning point in the evolution of cardiac surgery; it allowed surgeons to go beyond the closed-heart procedures of the 1940s, and provided the only means to do open-heart procedures during the years before the heart-lung machine became a safe and practical alternative for general use. Even after the transition to heart-lung bypass, surgeons would combine it with mild hypothermia because, as Swan had demonstrated, lowering the body temperature often helps lessen tissue damage and other trauma during operations.