The number of replacement parts for artificial prostheses that can be used by the human body is indeed considerable. From artificial eyes to artificial hip joints and knees, there are few parts of the human body that cannot be replaced with things that are at least as easy to use as the original, especially considering that the OEM part is in the first place.
But the heart has always been a human weakness, partly because it never rests, and partly because, considering all factors, we modern people have not really taken care of it. When the heart collapses to the point where medicine or surgery can help, we have far fewer options than people with knee burns. The fact is that the best we can hope for now is a mechanical heart, which allows patients to live long enough to find a donor heart. But even so, tragedy will inevitably happen. A young and healthy person must die so that others can survive.
A permanent implantable artificial heart has long been the goal of medicine. If the latest developments in materials science and electrical engineering have anything to say about it, such a device may soon become a reality. Heart replacement may one day be as simple as hip replacement, but to achieve this requires understanding the history of the mechanical heart and why it is not as simple as making a pump.
Although people’s understanding of the heart pump can be traced back to the 3rd century BC, it took nearly 1,000 years for medical science to make great progress. The early view of the Greek physician Galen that the heart provides heat to the body and that blood is transferred from the venous system to the arterial system through the hole in the heart diaphragm is unquestionable, at least in the Western medical tradition.
Once doctors can freely explore the human heart, its structure and function become clear. The heart is a four-chamber pump made of special muscle tissue. The upper chamber is called the atrium, and it enters the lower ventricle through a one-way valve to prevent backflow. The right atrium receives deoxygenated blood from the body through a large blood vessel called the vena cava; it moves to the right ventricle and then to the lungs, where it loses CO through a complex gas exchange process2 And get oxygen. The oxygenated blood returns to the left atrium of the heart, and then enters the powerful left ventricle, which provides oxygenated blood to the whole body through the aorta.
Looking at the heart from a mechanical point of view, it is easy to understand why attempts to make mechanical substitutes go back to the 1930s. Soviet scientist Vladimir Demikhov (Vladimir Demikhov) was the first person to try a mechanical heart, a device he designed himself, and he put it on a dog. This animal survived two hours after the operation.
Other surgeons used Demikhov’s dog experiment as the basis for a human artificial heart, first using an extracorporeal device such as Dodrill-GMR, a device made by the car manufacturer General Motors Research. Dodrill-GMR was used in 1952 to bypass the left ventricle of a 41-year-old patient during a mitral valve repair operation; he lived on the machine for 50 minutes, and finally lived for another 30 years.
The early experience of Dodrill-GMR and follow-up heart-lung machines provides doctors with valuable feedback on what works and what does not work in pumping blood. After all, blood is not a simple liquid; it is a complex liquid tissue composed of blood cells (red blood cells and white blood cells) and plasma. The heart is optimized to pump this liquid, introducing as little turbulence and shear as possible to keep the cells intact. Any mechanical substitute for the heart must not introduce this destructive force that may cause harm to the patient.
Bridge too far
Early efforts in mechanical circulation focused on completely extracorporeal devices, acknowledging that the engineering at the time was far from ready to provide a fully implantable artificial heart. But the dream of mechanical replacement of the heart still exists, mainly because the demand for a healthy human heart for transplantation continues to be in short supply. With the improvement of automotive engineering in decades, this situation has become more and more. The sad truth is that car accidents provide the most reliable source of donated hearts, and as cars become safer, the donor pool has become smaller.
The first so-called total artificial heart (TAH) that attracted widespread attention was the Jarvik-7 heart, which was first used clinically by Barney Clark, a retired dentist with severe congestive heart failure. Although Clark had to be tethered to a large air compressor 112 days after the operation, Jarvik-7 was the first artificial heart with a pumping mechanism entirely in the chest cavity. Even today, it sets the standard for total artificial hearts-pumping mechanisms are implantable, but due to engineering constraints, they must be powered from outside the body.
Both types of artificial hearts currently approved for patients rely on external power. SynCardia TAH is a direct descendant of the Jarvik-7 heart. It is made of better and more durable materials and is equipped with a valve that is less likely to damage blood cells. It is still powered by compressed air, although the drive unit is sufficiently miniaturized for the patient You can take one home. Another type of TAH is manufactured by the French company Carmat and uses a hydraulically driven flexible diaphragm to pump blood. The hydraulic power station is small enough to carry around and is powered by a lithium-ion battery.
Although they are mechanically different, the two devices have something in common: they are not used as a permanent replacement heart, but as a bridge to buy some time for the donated heart. The main reason for this is simple materials science: it is difficult to find materials that can be bent 70 to 80 times per minute for several years without tearing. The best thing Carmat managed to do was about 1,400 days. Although this was more than ten times better than Barney Clark’s experience, it was far from enough. Another problem is that connecting pneumatic or hydraulic lines to the chest cavity is clinically problematic, mainly through the risk of infection.
No pulse, no problem
Although it sounds like there is still a long way to go from permanent TAH, there are many things in this field that may shorten the journey.A family named BiVACOR A design that is very different from SynCardia or Carmat heart is being studied, and it contains components that are very familiar to any hardware hacker. BiVACOR TAH does not rely on a diaphragm to move the blood, but uses a centrifugal pump whose impeller blades are in direct contact with the blood and provide continuous flow. Usually, this will be an unacceptable source of shear force, tearing blood cells, not to mention the need for bearings that will eventually wear out. BiVACOR TAH avoids these problems by using a magnetically levitation impeller, leaving a large gap between the impeller and the pump casing. The gap distance is continuously monitored and adjusted, so there is no mechanical wear and shear force on the blood. The double-sided impeller is a single moving part in the pump, driven by a part very similar to the stator coil of a brushless DC motor.
One big advantage of BiVACOR TAH is that its output is much smaller than the current TAH. This is important because the space available in the chest cavity is very limited, even for large male patients. Female patients and children tend to have small hearts and often find it difficult to receive TAH. The BiVACOR heart is also suitable for a less invasive external power source; the cable enters the chest cavity more easily and is less likely to become an infected tube. In the near future, advances in battery technology are also likely to make implantable power sources possible. Coupled with percutaneous induction charging, the BiVACOR heart and similar hearts may be the first practical permanent total artificial heart.
There is obviously more work to be done here; some basic questions, such as whether the human body has the physiological cause of the pulse, remain to be answered. Fortunately, BiVACOR hearts can be programmed to provide pulsatile flow, so no matter what the answer is, they are in a good position. But the fact that we don’t even know the answer shows how far we are. Fortunately, engineering seems to be catching up to where the permanent artificial heart will eventually become a reality.