A Heart-Lung Bypass Machine is a device used to oxygenate and circulate blood in a patient with no heart or lung function, essentially replacing the function of both of those organs. It is used primarily in open-heart surgery, including heart transplants, but may also be used in a patient with no heartbeat when the physician feels that the patient's life may still be saved if a problem with the heart or lungs can be repaired. A patient can be kept alive on bypass for a considerable period of time. However, setting up the machine requires that it be hooked up both to the aorta and the vena cava. This requires surgery and usually requires the patient to be under anasthetic.
In Human Error, the patient is put on a bypass machine when her heart stops beating, only to have it restart on its own when the machine is turned off.
In Wilson's Heart, Amber is places on bypass while the team attempts to determine why she suffered a cardiac arrest after trauma surgery.