The heart contains four valves that open and close to maintain the flow of blood in one direction. If one or two of those valves become diseased, narrowed, or unable to close all the way, bloodflow can become limited and pressure can build up in the lungs. Sometimes a valve can be repaired, but often it needs to be replaced to keep the heart pumping smoothly.

In the early 1950s, scientists began replacing diseased heart valves with acrylic ones. Surgeons performed the procedure on several patients, but the valves failed to regulate bloodflow properly and clotting and swelling often occurred. Over the next few years, scientists attempted other artificial implants to replace the mitral and aortic valves—the two harder-working valves on the left side of the heart, which pumps blood through the body. Design criteria were important: The materials had to be compatible with human tissue, and the valves had to open and close rapidly and be able to function for many years.

The advantages of using human tissue to create heart valves became evident in 1962, when the English surgeon Donald Ross implanted the first heart valve harvested from a dead body: The immune system proved less likely to reject human tissue, and clotting was less likely to occur. The limited supply of cadavers, however, necessitated a search for other tissue substitutes, and in 1965, scientists first reported on five successful surgeries in which they’d implanted heart valves made from pig organs.

Today, patients have a choice between mechanical and biological valves. Mechanical valves are made from synthetic materials such as carbon, polyester, and titanium. They are durable and reliable and they last a long time, although patients need to take blood-thinning drugs to prevent clots from forming. Biological valves, made from cow or pig tissue (called a xenograft) or a donated human heart (called an allograft or homograft), must be replaced every 10 to 15 years. They are generally recommended for elderly patients or patients who cannot take blood-thinning medication. Sometimes a patient’s own tissue can be used for valve replacement (called an autograft or a Ross procedure).

ADDITIONAL FACTS

  1. The noise made by the acrylic heart valve replacements of the early 1950s was disconcerting to patients—reminiscent, some said, of a ticking time bomb.
  2. The average heart beats 60 to 90 times per minute, or more than 31 million times a year.
  3. Before the heart-lung machine was developed (see page 280), surgery on heart valves could attempt repair but not replacement. The surgery had to be done very rapidly on a still-beating heart