Yes, they are indeed. As a medical student I had the privilege of seeing a heart valve being installed. They are a witness to how far medicine has progressed since the days of barber surgeons.
The blood is taken out of your body through the femoral vein (under the skin between your hip and your nether regions), taken through the perfusion machine and pumped back into the femoral artery. Because at this stage the heart will have been stopped, the high pressure in the femoral artery makes blood flow backwards up the aorta, where it supplies oxygenated blood from the machine to the whole body.
At one stage the flow has to be switched off in order to sew the metal valve onto the top of the aorta. In preparation for this, the blood leaving the perfusion machine is chilled (I'm not sure to what temperature). This slows the body's metabolism down and would cause hypothermic stupor if the patient had not already been under anaesthetic. I was told by the perfusionist that this gave the surgeons around an hour of extra operating time before the brain's oxygen content dropped to dangerous levels. All this is of course monitored by the anaesthetist and the perfusionist. The hypothermia will be reversed during the process of reviving the patient.
With the advent of endovascular procedures, this sort of surgery is becoming a rarity. It's much easier to pass a small tube up the aorta and inflate the narrowed artery from the inside than it is to open up the chest and sew a new blood vessel onto the heart. Of course, the medicine's more boring but it's all about getting a good outcome for the patient, isn't it?