How surgeons stop a heart for hours and keep the patient alive
During many heart operations the organ is deliberately brought to a halt while a machine outside the body does its job and the lungs' job at once. A specialist called a perfusionist runs this heart-lung machine, which pumps blood, loads it with oxygen, strips out carbon dioxide and can cool the whole body to slow its needs.
The circuit has two core parts, a pump and an oxygenator, linked by tubes of silicone rubber or PVC. A venous cannula draws oxygen-poor blood out, and an arterial cannula returns it refreshed. Every surface inside is coated with heparin or a similar anticoagulant so the blood does not clot on the plastic. Older consoles use roller pumps that squeeze the tubing like a hand milking a hose; they are cheap but can build dangerous pressure if a line kinks, so many teams now prefer centrifugal pumps that spin blood along and harm it less.
To stop the heart, the team infuses a solution called cardioplegia into the coronary arteries or veins. Most versions block the fast sodium currents that carry each electrical impulse, so the muscle falls still and needs far less energy. Surgeons can then open its chambers, for instance to repair a valve, without air entering the circulation or blood hiding their view.
Cold is a powerful ally. Heat exchangers, fed by warm or ice water baths, typically hold the body at 28 to 32 degrees Celsius, which slows metabolism. With deeper cooling, the body can survive up to 45 minutes with no blood flow at all, whereas at normal temperature the brain begins to suffer permanent harm within three or four minutes. Run in reverse, the same machine can rewarm people with severe hypothermia, provided their core is above 16 degrees.
The machine is used only for the hours an operation takes, because it stirs up clotting and inflammation. Most oxygenators are rated for six hours. A simplified relative, ECMO, can support the heart or lungs for weeks; in one Taiwanese case it kept a patient going for 16 days until a heart transplant.
Source: Cardiopulmonary bypass
