During an open-heart surgery (such as valve or bypass surgery), the heart-lung machine takes over the functions of the heart and lungs. Also known as a cardiopulmonary bypass machine, it allows the heart to can be carefully stopped and drained of blood. Surgeons can then operate in a blood-free, quiet surgical field. The heart-lung machine allows surgeons to perform complicated surgeries without the interference of a beating heart.

The heart-lung machine basically consists of a pump (to replace the heart) and a bubble oxygenator (to replace the lungs). Acting as a mechanical heart and lungs, it keeps oxygen-rich blood flowing throughout the body after the patient’s heart has been carefully stopped. In a process called perfusion, the machine receives the patient’s blood, removes the carbon dioxide and other waste products, adds oxygen, warms (or cools) the blood and pumps it back through the body. Cooling the blood lowers body temperature. This helps protect the body's organs while the heart-lung machine is in use. After the surgery is completed, the heart is restarted, the heart-lung machine is stopped and the machine is disconnected from the patient.

The heart-lung machine can perform other tasks. For example, it can directly deliver medications into the recirculated blood. It can also minimize blood loss by vacuuming up and recirculating any blood that may get into the surgical field.

The first step in using a heart-lung machine during open-heart surgery is to give the patient a drug called heparin, which is a powerful anticoagulant. Heparin reduces the blood’s ability to clot, reducing the risk of clots forming in the heart-lung machine and within the tubes placed in the heart. Once the medication has taken effect, a tube (cannula) from the heart-lung machine is placed in the upper-right chamber of the heart (the right atrium), which receives oxygen-poor blood from the body. Another cannula is placed in the aorta, a large artery that carries oxygen-rich blood from the heart to the rest of the body. By setting up the machine in this way, oxygen-poor blood drains into the machine, receives fresh oxygen and is returned to the aorta to be carried to the rest of the body.

Once the machine is functioning, the surgeon can carefully stop the heart with medications in order to perform the necessary surgery. A patient may remain on the heart-lung machine for several hours, though surgeons typically try to limit the amount of time patients are on bypass.  When the surgery is complete, the surgeon will restart the heart. Once the surgical team is satisfied that the heart is beating strongly again, the tubes are removed from the right atrium and the aorta. To reverse the effects of the heparin given at the beginning of the process, the patient will be administered another medication called protamine.

Throughout this process, the heart-lung machine is operated by a perfusionist, one of several medical specialists on hand in the operating room during open-heart surgery. While the heart is stopped and the machine is working, the perfusionist continually monitors blood pressure, blood oxygen levels, carbon dioxide levels, blood temperature and breathing.