For many kinds of cardiac surgeries, it is necessary to have a still and empty heart. For this, the patient is placed on cardiopulmonary bypass (also known as the heart-lung machine). Cannulas (tubes) are placed into the venous and arterial circulation. The machine takes the blood before it can enter the heart, oxygenates it, removes carbon dioxide, and then reintroduces it into the aorta, so it can flow through the body and provide oxygen. At this point, the heart is often arrested and drained. This makes it easier for the surgeons for two reasons 1. a still heart is easier to operate on than a moving heart and 2. in valvular operations where you operate inside the heart, you don t want blood getting in the way. Doing so also protects the heart because it consumes much less oxygen when it s arrested and drained than when it s full. After the surgeons are done, the heart is allowed to start up again. The bypass machine is gradually cranked down. Often, the patient needs special blood pressure medicines to help support the heart as it comes off bypass. Hopefully, one can come off bypass easily. In some circumstances, if the heart wasn t well protected, the patient may need special support, such as an intra-aortic balloon bump or a left ventricular assist device to come off bypass. Bypass makes all sorts of surgeries possible, but has plenty of negative effects as well. It makes you much more prone to bleeding. As I said before, sometimes it is hard to wean the heart off bypass. The placement of the cannulas often knock off pieces of atherosclerotic plaques off the artery, leading to stroke. But, it s still an amazing technology.