Heart failure (HF) is a clinical condition characterized by typical symptoms (e.g. dyspnea, ankle swelling and fatigue) that may be accompanied by signs (e.g. elevated jugular venous pressure, pulmonary crackles and peripheral edema) due to a structural and/or functional cardiac abnormality, resulting in reduced cardiac output and/or elevated intracardiac pressures at rest or during stress.1
HF is a life-threatening disease and addressing it should be considered a global health priority. At present, approximately 26 million people worldwide are living with heart failure. The outlook for such patients is poor, with survival rates worse than those for bowel, breast or prostate cancer,2 and the deterioration of heart failure is difficult to be reversed by existing drugs or traditional medical instruments. Heart transplant is known as one kind of treatments, but it fails to benefit the general public because of extremely limited heart donors. Mechanical Circulation Support (MCS) therapy, especially Ventricular Assist Devices (VAD), has gradually become an important treatment for end-stage heart failure patients. This novel, practical and accessible treatment can enable many individuals with advanced heart failure to lead longer and more comfortable lives.
in the world
in United States
Currently, the number of patients with advanced heart failure is far greater than the number of donors available for heart transplants. Ventricular assist devices (VAD) not only save patients' lives, but also can possibly enable them to lead a normal life.
A ventricular assist device (VAD) is a mechatronics device, used as heart assist device to pump blood partly or totally in ventricular to ascending aorta, thus maintaining blood circulation of patients with heart failure. Its core component is a pump, pumping blood from the heart to the aorta after ascending blood pressure, to let natural heart rest by removing cardiac load, and address a natural heart’s insufficiency to pump blood.
After more than 50 years’ research and development in the United States and other developed countries, VAD was approved by FDA as an alternative treatment of heart transplant to treat severe heart failure in the early 21st Century. So far, nearly 100,000 cases of clinical use show that artificial heart can help patients with heart failure to improve quality of life significantly, prolong their lives, and even facilitate heart rehabilitation.
At present, a ventricular assist device (VAD) mainly has three clinical applications: one is to serve as a substitute for patients waiting for heart transplantation in the transition period, so they have more time to wait for suitable donors; The second is to provide short-term replacement support for patients with acute heart failure, it can be removed after the heart function is restored. The third is to provide long-term support for patients with end-stage heart failure. Long-term support therapy has become the mainstream clinical application of ventricular assist devices.
A VAD is a complex and sophisticated medical device, which has been developed from the first generation of pulsating flow VAD in the 1990s to the second generation of rotary assist device in the early 21st century. The fully magnetically levitated VAD as the newer generation product is characterized by magnetic levitation and bearingless, which has great clinical advantages of small size and good hemocompatibility.
The first generation of ventricular assist device is a mechanical ventricular circulation support device that uses pulsatile drive system, powered by electromagnetic or pneumatic device.
The second generation of ventricular assist device is rotary ventricular circulatory assist device. According to the existing bearing support methods, the new generation of rotary VAD has been divided into three categories, including axial flow pump, hydrodynamic suspended blood pump and totally magnetically suspended blood pump.
Compared with patients receiving medication alone, those who carried pulsatile VADs have more active and diverse lifestyle. However, the device features poor durability, severe blood damage, high thrombus incidence, stroke, and other serious adverse events. It is also too bulky to be carried conveniently, having high surgical invasiveness, and prone to cause driveline infection. The disadvantages explained why pulsatile VADs only served for the short term support as a transition prior to heart transplant.
The axial flow pump, which is completely immersed in the blood, transmits bearing force to the impeller rotor through mechanical contact. Compared with the first-generation of artificial heart, axial flow pump causes less surgical injuries due to smaller size and increased durability, having a lower incidence of bleeding complications. However, blood cells can be crushed and damaged once they contact the bearing to release thrombus formation stimulation materials.. Besides, the bearing generates heat high shear pressure, flow turbulence or “dead zones”, etc. Blood can be damaged, and thrombi can develop around bearing especially.
In Hydrodynamic suspended blood pump, surfaces of rotor and pump valute were designed specifically with different shapes, a thin layer of blood can enter the space between the two surfaces. When the rotor speed reaches a certain threshold, local high-pressure area will appear in the liquid films, and the surfaces on both sides will be split open to enable non-contact support. Although the hydraulic suspension has no mechanical contact, the liquid film thickness must be very thin to produce stable supporting force. The shear stress of blood in the liquid film is far higher than the normal physiological value, it is possible to cause greater blood damage. In a hydraulic suspension device, blood flow in the suspension gap produces suspension force and causes blood damage, so that the increased suspension force must be at the cost of greater blood damage, and vice versa, thus the hemocompatibility has limited room for improvement due to this restraining relationship at the basic principle level.
In a fully maglev suspension VAD, a series of magnets and electrically charged coils are arranged inside the rotor and pump valute to generate magnetic force (non-contact force), which keeps the rotor in the stable suspension. This support pattern can give rise to a much larger suspension gap than hydraulic suspension, preventing blood from damage caused by excessive shear stress from running through bearing. Moreover, because the magnetic force is not related to the blood flow in the suspension gap, when design the suspension gap-flow path, the sole consideration is given to best hemocompatibility to enable smoothest blood flow and blood flow scours the surface of each component without being hampered by the design requirements of the suspension force, thus to avoid thrombus formation.