Cardiovascular Surgery, Pacemakers and Heart Valves

The living, pulsing heart has long awed, yet intimidated, physicians and surgeons. Until recently, a disrupted heartbeat or an occluded heart valve brought certain death for patients, while various kinds of congenital heart malformations killed thousands of children every year.

At the turn of the century, the development of techniques for suturing heart wounds created initial interest in the surgical treatment of mitral stenosis, a narrowing of the mitral valve opening. In 1902, the English physician, Sir Lauder Brunton observed:

''One is impressed by the hopelessness of ever finding a remedy [for mitral stenosis] which will enable the auricle to drive the blood in a sufficient stream through the small mitral orifice, and the wish that one could divide the constriction as easily during life as one can after death. The risk that such an operation would entail naturally makes one shrink from it.''

Braunton’s own attempt at a surgical cure (proposed in 1902, but not attempted until 1923) was unsuccessful as were most other early efforts. In 1925, however, surgeon Henry Souttar of the London Hospital, opened a mitral valve using only his gloved finger to separate the valve leaflets. The 19-year-old patient recovered. Souttar’s achievement was both rare and precious. By 1929, published reports of 12 patients surgically treated for mitral stenosis showed only two survivors.

Likewise, physiologists in the late nineteenth century understood the electrical nature of the heart’s contractions and developed rudimentary measuring devices using electrodes to create crude electrocardiograms that measured electrical activity in heart muscle. The new technology, however, required intricate interpretation before it could be successfully employed as a serious diagnostic tool.

By 1907, physiologists had a basic understanding of what caused the heart to beat—the electrical origin in a bundle of tissue in the upper right atrium (the sinus node) and its conduction through the ventricle. When this signaling went awry, clinical symptoms became apparent. Irregular heartbeats constituted one set of problems. Another was evident in patients who had either a partial or complete ‘‘block’’ in conductivity between the upper and lower chambers of their hearts.

Such patients had a clinical profile of a slow pulse accompanied by what were originally described as ‘‘fainting fits.’’ These ‘‘StokesAdams’’ attacks indicated that the ventricle was not providing an adequate supply of oxygenated blood to the brain. It was this condition, moreover, that inspired researchers to develop electric stimulators for the heart.

Before World War II, however, improvements in blood transfusion, anesthesia, and suturing techniques for blood vessels had begun to make surgery safer and led to the expansion of efforts to operate on the living heart. Combat itself finally dispelled the concept of the heart as a fragile organ, however, when it became possible to remove shrapnel and close the wound to a beating heart without killing the patient. The stimulus for an artificial heart-pacing device as well as advances in valve surgery came after World War II and the development of open heart surgery.

In 1947, Claude Beck of Western Reserve University successfully used an electrical defibrillator internally on a patient whose heart had stopped during routine surgery. In 1952, Paul M. Zoll announced that he had kept a Stokes-Adams patient alive through a bedside defibrillation device. Zoll’s most important insight was recognition that he should pace stimulate the patient’s ventricle, rather than the atrium, but his treatment, employing 130 to 150 volts was too painful for extended patient use. Implanting part of the pacemaker would lower the voltage requirement, and this idea was pursued at the University of Minnesota Medical School.

In operating on the hearts of small children, C. Walter Lillehei occasionally found that his sutures damaged conduction cells in the heart and produced a postsurgical heart block which killed some of his young patients. Lillehei commissioned an external pulse generator, devised by Earl Baaken, a Medtronics engineer, for use in his patients as a temporary assistance device he hoped would allow these conductive cells to heal naturally. When a myocardial pacing wire was inserted into the heart wall and connected to the external device, Lillehei discovered that ‘‘one or two volts drove the heart beautifully.’’

The wire lead could be removed without further surgery. Active children required a portable unit, and dependence on the hospital’s electrical system inspired the creation of a battery- powered portable unit. The Medtronic 5800, invented in 1958, became one of Medtronic’s earliest marketing successes.

Shortly after Beck’s success using internal defibrillation, Charles Philamore Bailey, a Philadelphia surgeon, performed the first deliberative and successful intracardiac operation on a 24- year-old young woman with severe mitral stenosis in 1948. Within two years, mitral valve surgery was being performed successfully around the world. Also during the 1940s, Charles Hufnagel, began the formidable task of developing a workable prosthetic aortic valve.

In September 1952, Hufnagel successfully implanted a prosthetic ‘‘ball’’ valve into a patient at Georgetown University Medical Center. Without a heart-lung machine, Hufnagel could not replace the faulty valve; he could only position the prosthetic as a ‘‘check valve’’ to correct for the effects of the diseased aortic valve. Nonetheless, the accomplishment was impressive as a first step in creating workable replacement valves.

Heart surgery had lagged technologically behind other surgical specialties in the 1940s. There were a few operations to correct specific abnormalities, but patients risked a mortality rate of 50 percent or more. Heart catheterization techniques, in which a small tube could be moved through a large vein and into the heart chambers, allowed steadily improving visions of diseased hearts, and became an ordinary hospital procedure and part of the standard workup for most patients scheduled for heart surgery during this time. In addition to better preoperative understanding of their patients’ heart abnormalities, surgeons required time to fix heart problems surgically.

Experiments with hypothermia in cardiac surgery proved that a patient’s heart could be stopped ‘‘cold’’ and then warmed up to resume its normal rhythm, allowing surgeons precious time to complete more complicated surgical procedures. Hypothermia and a riskier cross-circulation technique using a live donor were the only options for oxygenating blood during surgery until technological advances brought the first heart-lung bypass machines into the operating suite beginning in 1953. Such advances were critical in the subsequent development of artificial replacement heart valves. Cardiac pacing also remained closely associated with open heart surgery throughout the 1950s.

Between 1957 and 1960, at least eight research groups designed and tested fully or partially implantable cardiac pacemakers in humans. One patient who received one of the earliest pacemakers was still alive and on his twenty-sixth pacemaker in 2000. Between 1961 and 1963, a great number of implanted pacemakers failed and were replaced with improved models. According to one author, Jeffrey:

''these were inventions in the early stage of product development. The only justification for using them at all with human beings—a compelling one—was that the patients had little chance of survival without some electrical assist for their heartbeats.''

Although some nuclear powered pacemakers were implanted in the U.S. in the 1970s, smaller, more powerful lithium batteries and advanced circuitry made newer pacemakers more attractive. Moreover, by 1972, transvenous insertion of small pacemakers under local anesthesia virtually replaced the major chest surgery that had been required for implanting pacemakers in the 1960s.

The development of the heart-lung machine made it possible to remove damaged valves and replace them with mechanical prosthetic valves. The first truly successful artificial valve was developed by surgeon Albert Starr and engineer Lowell Edwards and used in a mitral valve replacement in 1960 at the University of Oregon Medical School. Many variations on this design were quickly tested both in the laboratory and in patients using improved materials and streamlined designs. Mechanical valves, however, do present ongoing challenges.

The perfect valve would produce no turbulence as blood moved through the valves, would close completely, and would be constructed of a material that discouraged the formation of blood clots. The danger of clot formation required that patients with mechanical valves take lifelong anticoagulants to prevent clots from forming. Although mechanical valves are extremely durable, they can fail, and when they do they often fail catastrophically. In 1968, Viking Bjork, a Swedish professor created a new mechanical valve with a tilting disk design held in place by welded struts.

The design was intended to reduce the risk of blood clots, a significant problem with previous implants. Manufactured by the Shiley Company, approximately 85,000 Bjork Shiley valves were sold worldwide between 1978 and 1986. An engineering flaw (strut failure) in hundreds of the valves, however, caused about 250 reported deaths and even more lawsuits. Nonetheless, for long-term use mechanical valves are still used in about 65 percent of the over 60,000 heart valve replacement surgeries every year.

In both the heart valve industry and the heart pacemaker industry, product failures in the late 1960s and early 1970s created concerns among consumers that led to major changes, both within each industry and within government circles. In 1976, Congress enacted the Medical Device Amendments of 1976 to clarify the scope of the Food and Drug Administration’s authority over medical devices.

Under the 1938 Food, Drug, and Cosmetic Act, the FDA had taken action against many quack medical devices, and had acted on occasion against medical devices under the drug provisions of the 1938 law, but regulatory actions under such conditions were cumbersome, risky, and expensive. Both industry and the FDA wanted clarification over the agency’s authority over the burgeoning medical device field. Dr. Ted Cooper, Director of the NIH National Heart and Lung Institute, chaired a committee that surveyed problems in the medical device industry.

Concluding that ‘‘many of the hazards seem to be related to problems of device design and manufacture,’’ the report cited over 10,000 patient injuries in the medical literature between 1963 and 1969. Signed into law in 1976, the new law governing medical devices classified them according to their perceived risk. Thus Class 3 devices would be the most tightly regulated, while Class 1 products including such ‘‘devices’’ as sterile bandages, would require less stringent regulation.

New Class 3 devices were also subject to premarket approval by the FDA: manufacturers had to demonstrate that they were ‘‘safe, reliable, and effective’’ before they could be put on the market. Debate continues over the long-term effects of government regulation on industry innovation in the medical device field, as in the pharmaceutical industry, but it remains clear that medical device firms, including manufacturers of heart valves and of cardiac pacemakers, have resisted any tendency to make valves and pacemakers mere medical commodities. Competing through innovation has kept prices high and competition in these technology driven industries alive and well into the twenty-first century.


Date added: 2023-10-02; views: 171; - Studedu - 2022-2024 year. The material is provided for informational and educational purposes. | Privacy Policy
Page generation: 0.018 sec.