Independent Study Assignment
Research and development of artificial heart has been making tremendous progress throughout the years. Research areas now includes determining which types of cells can hold the most potential and finding the best way to grow those cells to form viable cardiac tissue that is strong, long-lasting and structured at a cellular level like natural tissue. It is now at a point where researchers can engineer first-generation prototypes of all cardiovascular structures: heart muscle, tri-leaflet valves, blood vessels, cell-based cardiac pumps Recently, tissue engineered ventricles published a paper describing their success in growing pulsing, three-dimensional patches of bioengineered heart muscle (BEHM). The research papers describes the use of an innovative technique, which is the usage of a fibrin hydrogel, that is faster than others, but still yield a tissue with significantly better properties. BEHM was capable of generating pulsating forces and reacting to stimulation more like real muscle than ever before.
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Patients who have some remaining heart function but who can no longer live normally may be potential candidates for ventricular assist devices (VAD) which do not replace the human heart. However, it complements the heart by taking up much of the function. The first Left Ventricular Assist Device (LVAD) system was created by Domingo Liotta at Baylor College of Medicine in Houston in 1962.
Another VAD, the Kantrowitz CardioVad, designed by Adrian Kantrowitz, MD boosts the native heart by taking up more than 50% of its function. Additionally, this VAD can help patients who are on the wait-list for a heart transplant. In a young person, this device could delay the need for a heart transplant by approximately 10-15 years.
The first heart assist device was approved by FDA in 1994. In 1998, another two more heart devices were being approved. While the original assist devices emulated the pulsating heart, newer versions, such as the Heartmate II which was developed by the Texas Heart Institute of Houston, Texas, provide continuous flow. These newer pumps (which may be centrifugal or axial flow) are smaller and potentially more durable than the current generation of total heart replacement pumps. Another major advantage of a VAD is that the patient can keep the natural heart, which can receive signals from the brain to increase and decrease the heart rate as needed. With the completely mechanical systems, the heart rate is fixed.
Several continuous flow ventricular assist devices have been approved for use in the European Union and as at August 2007 were undergoing clinical trials for FDA approval. Hence, in the following 2 paragraphs, we shall discuss about 2 kinds of artificial heart that was invented and approved.
The CardioWest temporary Total Artificial Heart (TAH-t) was the first FDA approved Total Artificial Heart. It received FDA approval on Oct. 15, 2004, following a 10-year pivotal clinical study. It was originally designed as a permanent replacement heart however, it is currently approved as a bridge to human heart transplant for patients dying because both sides of their hearts are failing (irreversible end stage biventricular failure).There have been more than 800 implants of the CardioWest artificial heart, accounting for more than 170 patient years of life on this device. In the 10-year pivotal clinical study of the CardioWest artificial heart, there are 79% of patients receiving the artificial heart survived to transplant. This is the highest bridge-to-transplant rate for any heart device in the world.
The AbioCor Replacement Heart by Abiomed is a fully implantable device. This means that no wires or tubes are needed to penetrate the skin and therefore there is less risk of infection occuring. The AbioCor is approved for use in severe biventricular end stage heart disease patients who are not eligible for heart transplant and have no other viable treatment options. To date, 15 patients have been implanted with the AbioCor, with one patient living for 512 days with the AbioCor. The first AbioCor to be surgically implanted in a patient was on July 3, 2001 and the first implant of the AbioCor since receiving FDA approval in 2006 took place on June 24, 2009 at Robert Wood Johnson University Hospital, New Jersey. The AbioCor is composed of titanium and plastic with a total weight of 2. A transduction device that sends power through the skin is used to recharge the internal battery. The internal battery can last up to half an hour and a wearable external battery pack lasts for 4 hours. However, there are limitations to the current AbioCor. One of the limitations is its size which only makes it suitable for only about 50% of the male population. Secondly, its useful life span is only up to 1 or 2 years. However, Abiomed has designed a smaller, more stable heart, the AbioCor II, by combining its valved ventricles with the control technology and roller screw. AbioCor II, which should be implantable in most men and 50% of women with a life span of up to 5 years, had animal trials in 2005, and the company hopes to get FDA approval for human use in 2008.
There are many problems faced by researchers when developing and improving heart valves, heart valves diseases are often solved by replacing the disfunctioning valve with a mechanical or biological prosthesis. Mechanical valves are entirely realised using stiff synthetic materials which, allowing a good consistency of production, guarantee a long durability. However because of their poor haemo-compatibility, they force the patient to undergo a chronic anticoagulant therapy. Hence, Bioprosthetic valves are produced. These valves tend to mime the behaviour of natural valves, so that the closure and opening are obtained by the spontaneous coaption and parting of biological membranes driven by the blood flow and pressure. ensure excellent hemodynamic and thrombogenic performances, that free the patient from ongoing anticoagulant therapy. However, on the down side, their durability is still poor compared to their mechanical counterparts.
Combining the advantages of mechanical and bioprosthetic valves, Polymeric heart valves are being manufactured using synthetic materials, hence, this provides an optimal solution to current prostheses. Currently, Dr Burriesci and his team, in collaboration with the team of Prof. Seifalian (Royal Free Hospital) has developed a synthetic leaflets heart valve with haemodynamic performances similar to biological valves and prolonged duration. The optimised valve design was defined using the finite element method to simulate the entire physiological cycle of the valve. The material selected for the leaflets was the synthetic nanocomposite polymer recently developed at UCL. This design was aimed at optimising the hydrodynamic performance and the durability
On October 27, 2008, a French professor and leading heart transplant specialist Alain F. Carpentier announced that a fully implantable artificial heart will be ready for clinical trial by 2011, and for alternative transplant in 2013. This fully implantable artificial heart was developed and will be manufactured by him, Biomedical firm Carmat, and venture capital firm Truffle. The prototype uses electronic sensors and is made from chemically treated animal tissues, called “biomaterials”, or a “pseudo-skin” of biosynthetic, microporous materials.
Some of the important design considerations in the development of the TAH system are:
Size: The TAH system must be able to fit in the cardiac cavity without interfering the functions of other organs in the region or obstructing the blood flow to and from the device. Also, the surgeon must also be able to close the thoracic cage after implantation of the heart device. During the developmental stages, the TAH system is implanted in a cast of the thoracic cage in order to ensure that it would fit in the available space without obstruction to the vessels.
Weight: Weight is another important issue since the TAH system should not exert too much stress on the surrounding organs while the patient is moving. More importantly, the patient should be comfortable in carrying the system.
Pumping system: The pumping system should not fail at all times and must be dependable. The current TAH systems are pneumatically driven even though electromechanical and magnetically coupled devices are under development.
Pump regulation: Another important consideration is the regulation of the pumping system and its effects on the nervous system and the distal vessels. The cardiac output with appropriate outflow pressures must be delivered while preventing inflow pressures from becoming excessive.
Valve selection: Valves should not fail during operation and optimized flow characteristics with reduced incidences of thrombus deposition are an important factor in valve selection along with durability.
Material selection for the components of the TAH system: The selected material must be of sufficient mechanical strength to avoid failure in the body. This material should be biocompatible in order to minimize foreign body response. The major problems with the implanted TAH systems have been infection and thrombus formation and the resulting embolic complications. Many of the long term patients with the implanted TAH system suffered from stroke. With growing clinical experience and better anticoagulation protocols, this complication has been substantially reduced.
Many improvements can be made before the TAH system can become a viable alternative for patients with cardiac failure. The first problem is encountered with the present system of thrombus and pannus formation. These problems seem to occur near the valves mounted in the system. Currently, a number of blood pumps, including pulsatile pump systems using pneumatic drives, electrical drives and centrifugal pumps are used in patients as bridge to transplant. Assuming that the discontinuities with the quick connect system may be causing the same, the valves sutured in place in the Philadelphia heart has been offered as an alternative. The permanent connection of atrial cuffs and arterial grafts eliminates the crevices between the male and female components of the quick connect system which also provides a compliant structure. An alternate quick connect system using precision machined components to permit ease of connection while eliminating surface discontinuities has been shown to substantially reduce valve and connector associated thrombus formation in animal trials. Other improvements that are possible includes the use of a variable dp/dt throughout ventricular systole which closes the mitral valve under similar conditions to the natural valve. In conjunction with the implantable inflow valve, a compliant atrial cuff reduces the shock transmitted to the valve body reducing the stresses on the valve. The ventricular housings, bases and air diaphragms are constructed out of vacuum formed polyurethane which results in a continuous intima and blood diaphragm without seams in the inner surface. The vacuum forming also lowers manufacturing costs. More testing is needed to evaluate the efficacy of these improvements on preventing thrombus and pannus formation.
References
http://en.wikipedia.org/wiki/Artificial_heart
http://engineeringtv.com/blogs/etv/archive/2007/03/15/episode-21-the-artificial-heart.aspx
http://www.egr.uh.edu/news/0708/?e=heart
http://www.mecheng.ucl.ac.uk/research/biomedical-engineering/cardiovascular-engineering/?project=design-artificial-heart-valves
http://www.future-science-group.com/news/3/11
http://www.engineering.uiowa.edu/~bme050/assist.html
http://www.nlm.nih.gov/medlineplus/ency/article/007268.htm
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