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How a heart-lung machine works

Before World War II, open-heart surgery was a dream waiting to be realized. Experts wondered how blood could continue along its life-sustaining course while physicians operated on an open, stilled heart. Yet by 1953, the dream had become a reality with the invention of the heart-lung machine. Today, thousands of patients are living longer, healthier lives thanks in part to the device.

By pinch-hitting for the body during surgery, the machine allows heart valve replacements, coronary artery bypasses, heart transplants and other open-heart procedures to be performed every day.

To better understand how the device works, you have to appreciate how the heart and lungs function as a team. First, imagine your blood, depleted of oxygen after having visited your body’s many organs and tissues, returning to the heart. This “oxygen-poor” blood flows through the heart’s right atrium into the right ventricle. The right ventricle pumps the blood to the lungs, where the blood releases waste gases and picks up oxygen.

The now oxygen-rich blood returns to the heart and enters the left ventricle via the left atrium. The left ventricle pumps the oxygenated blood to all parts of the body, where it provides necessary oxygen and nutrients. Upon its return to the heart, the process starts all over again.

With every beat, the powerful heart muscle keeps the process going, beating about 100,000 times a day. But this workhorse needs to be stilled during open-heart surgery to give doctors time to perform their delicate tasks. While the patient lies peacefully unaware, the oxygen-depleted blood is diverted into the heart-lung machine before it enters the heart. In the machine, the blood is pumped into a chamber where it is spread thinly and exposed to a stream of oxygen, mimicking the action of a real lung. The machine then pumps the oxygenated blood back into the body’s circulatory system, in effect, taking over the heart’s job. While the operation is in progress, no blood passes through the heart or lungs.

Several innovations make today’s versions particularly safe and effective. Most models use roller pumps, which pump the blood smoothly and gently so as not to damage delicate blood cells. A special function allows doctors to cool the blood and lower the patient’s body temperature, a process that reduces the body’s demands on the heart. And, for good measure, the machine thins the blood to prevent clotting.