This article is an overview on using mechanical heart pumps in the treatment of end-stage heart failure. The article describes how technological advances are helping in extending patient lives with a good quality in the setting or organ shortage. Mechanical pumps can be a good alternative to heart transplantation in appropriate patients considering the current data on outcomes and survival.
The lack of adequate donor organs for end-stage heart failure (HF) patients has led to augmented use of mechanical circulatory support in recent years for extending life with a fairly good quality in an attempt to reduce morbidity and mortality. The early beginnings of mechanical circulatory support systems in non-human systems since the 1930s led to the initial implant of pneumatic pumps in patients to prolong life in the 1960s. The first ever reported use of a left ventricular assist device in a patient was in 1963 by Liotta and Crawford published in the American Journal of Cardiology (Liotta D, Hall CW, Henly WS, Cooley DA, Crawford ES, Debakey ME. Prolonged assisted Circulation during and after Cardiac or aortic surgery. Prolonged partial left ventricular bypass by means of intracorporeal circulation. Am J Cardiol. 1963; 12:399-405. doi: 10.1016/0002-9149(63)90235-2.) In 1964 the National Institutes of Health (NIH) started the Artificial Heart Program. In 1969 the first total artificial heart was used as a bridge to transplant (Denton Cooley and associates) and continued research and development in this area resulted in the first pulsatile pumps being placed since the 1980s as a bridge to transplant. Mechanical circulatory pumps have evolved rapidly as mainstream therapy over the decades with accelerated growth in the recent past.
Ventricular assist devices (VADs) were initially used only as a bridge to recovery (BTR) for patients who could not be separated from cardiopulmonary bypass in the operating room but today they have become the standard of care in patients with end-stage heart failure who are refractory to medical therapy.
A milestone in the development of ventricular assist devices was the development of the percutaneous heart pump at Penn State University by engineers in collaboration with industry to develop the technology for clinical use in patients. Dr William Pierce a cardiothoracic surgeon and a chemical engineer by training spearheaded the development of the first pneumatic heart assist pump called the Pierce-Donachy Ventricular Assist Device, also known as the Penn State Assist Pump which was designated an International Historic Mechanical Engineering Landmark by the American Society of Mechanical Engineers. Since then, VADs have been in different phases of development to make them more compact, less noisy, and fully internalized. Today’s most advanced heart pumps, though implanted inside the body, are still tethered to an external power source.
The development of pumps for assisted circulation requires engineering technology to match human cardiovascular and circulatory physiology. The collaboration between engineers, physicians, and scientists has yielded today’s devices. Expertise in hydrodynamics to design, test, and model the devices to be physiologically compatible is a task that also requires collaborations with material scientists and physiologists. It has therefore been a truly multidisciplinary marvel to save the lives of patients with failing hearts.
The use of ventricular assist devices (VADs) has slowly and steadily evolved from being temporary support to life-long use as “destination therapy”. Initially, VADs were indicated as a bridge-to-transplant, but steady improvements in medical management and technology with newer generation devices have helped with patient outcomes, and increased use as destination therapy. Adverse events after implantation of VADs remain a significant problem and limit its use as a complete alternative to heart transplantation. All long-term pumps currently available only support the left ventricle and hence patients with biventricular failure do not have long-term devices as a suitable alternative to heart transplant.
VADs are now being implanted globally. From 2010 to 2019 a staggering 25000 VADS were placed across the world. The evolution of VADS from large, loud, pulsatile pumps housed outside the body to small miniaturized noiseless pumps placed in the heart remains a marvel of technology. The basic structure of the VAD is shown in Figure 1. It has a simple design with a motor connected on one side to the inflow cannula and on the other side connected to an outflow graft that connects to the aorta. Hence blood flow is conducted from the left ventricle via the inflow canula, the motor, and the outflow graft into the aorta therefore unloading and by passing the entire left heart. As the field evolved the pump technology focused on continuous flow (CF)pumps. These pumps have different physiology and can be divided into axial and centrifugal pumps. The CF pumps have now become the mainstay with the latest being a fully magnetically levitated centrifugal CF pump which has a wider blood-flow shaft and intrinsic pulsatility. By programming the device to rhythmically decelerate and accelerate, this pump attempts to partially restore the native pulsatility to reduce bleeding and clotting-related complications.
VADs come with their own set of problems in addition to advantages. One of the major problems is driveline infection which contributes to increased healthcare costs, lower quality of life, and poor outcomes. Additionally, due to the present-day continuous flow pumps lacking pulsatility, there is an incompatibility between the physiological beating of the heart and the circulatory support provided by these artificial pumps. Such incompatibility can lead to the formation of arteriovenous malformations which can cause bleeding. There is also a shear force on the blood due to the VAD which can destroy the formed elements in the blood including the von Willibrand factor (vWF). vWF is a high molecular weight multimeric protein, which binds to factor VIII to modulate hemostasis. Shear stress on blood elements speeding through the rotor causes massive degradation of vWF to yield inactive fragments of low molecular weight causing the patient to be more susceptible to bleeding especially most commonly from the arteriovenous malformations formed in the GI tract. Hence close follow-ups and surveillance are required throughout support.
The management of patients on long-term support involves arriving at a fine balance between bleeding and clot formation. High VAD speeds will cause narrower pulse pressures (the difference between systolic and diastolic pressures) which will predispose to the formation of arteriovenous malformations. The patients supported on VADS require lifelong anticoagulation to prevent clot formation and therefore this has to be finely tailored to prevent excessive clotting and bleeding. The formation of a clot can be detrimental in these patients as it will predispose them to strokes. Though strokes and neurological issues are beginning to reduce in the newer generation of VADs, gastrointestinal bleeding remains a problem in some patients most often early after implant though it can occur at any time during VAD support.
Despite all the post-implant complications, the survival on VADs has increased and is comparable to that of heart transplants at 2 years. More recently 5-year survival on the newer fully magnetically levitated pump is shown to be close to 60% with a good quality of life.
Quality of life on a VAD is comparable to that with a transplanted heart except for some inconveniences such as not being able to swim while supported on a VAD. Additionally, women of childbearing age may be advised against pregnancy and childbirth on the pumps because this has been successful in some cases but carries a very high risk of maternal and fetal death. Pregnancy on VAD support includes hemodynamic stress, hypercoagulable events, teratogenicity due to medications, and impingement of the uterus. The use of a multidisciplinary approach highlights the rate of success in these highly complicated patients. Implementation of shared decision-making, cautious anticoagulation, VAD speed adjustments, and medications will help maintain hemodynamic support during pregnancy. Pregnancy brings about an increase in heart rate, blood volume, and decrease in systemic vascular resistance which can influence the efficiency of the VAD detrimentally.
Some ideal characteristics of future pumps would be internalized and surface chargeable, which can be used to support both right and left ventricles. The current total artificial heart pumps are large and have drivelines. Considering the large amount of metal in the pump which has blood flowing over it, these pumps require high levels of anticoagulation. Patients on these devices are therefore prone to both hemorrhagic and ischemic strokes. Hence future pumps are moving in the direction of small, electromechanically driven; self-contained, fully implantable, with very few moving parts. The surface of these pumps which remain in contact with blood requires to be coated with pericardial or bovine tissue to optimize biocompatibility.
The future holds promise in further development of these devices to support the right and left ventricles and be fully internalized with surface rechargeability and better biocompatibility. Striving to create biomaterial surfaces that effectively prevent the formation of blood clots on surfaces in contact with blood would be an optimal pursuit. Many suggested modalities are the use of physical/chemical treatment, incorporation of anti-thrombotic drugs, / promoting endothelialization. Hemocompatibility is an important aspect of improving pump durability. Mimicking and adopting the properties of endothelial cells will have to be achieved possibly by cell and tissue engineering using chemical adsorption, surface grafting, plasma treatment, and control of protein adsorption. Achieving such a surface topography can be very challenging when applied to VADS due to the complexity of the design of the pumps and the hemodynamics. Therefore, innovations to speed up endothelialization under different levels of shear stress could be another strategy to improve biocompatibility.
In summary, mechanical circulatory support has paved the way for a new way of life. The future holds promise for more improved pumps which could someday become a complete alternative to heart transplantation with comparable survivability and quality of life for patients with end-stage heart failure.
