Total Artificial Heart
A study at the Cleveland Clinicfound that, "Approximately 2,200 Americans undergo heart transplants every year;while nearly 3,000 people with damaged hearts are placed on waiting lists to receive new donor hearts, and many are not lucky enough to live long enough to receive that donor heart" {12}.Total Artificial Hearts(TAH) have been developed to answer this need. It is the hope that these devices will help to ease the transplant process, as well as extend the lives of critical need patients. The newest model of TAH is a continuous-flow ventricular assist device {5}. With this remarkable device it will enable patients to improve their heart function as they await a heart transplant. And for those patients with advanced heart failure, who don't qualify for heart transplantation due to debilitating health, destination therapy with the device is now being studied {12}.
What is a Total Artificial Heart
The Total Artificial Heart (TAH) that is under development at theTexas Heart Instituteis known as the Continuous-Flow Pulseless Total Artificial Heart (TAH) which is a Ventricular Assist Device (VAD). This device is a novel continuous heart pump that has the ability to be a control system that emulates how the natural heart responds to physiological conditions within the body. The TAH is designed to perform the function of both the right and left ventricles {5}. This device was created by Matthew Franchek and Ralph Metcalfe, both mechanical engineering professors in the Cullen College of Engineeringat University of Houston, who are now working on a feedback controller that will allow two VADs to work together as a TAH {6}. Dr. Bud Frazier, the Texas Heart Institute's director of research and chief of cardiopulmonary transplantation, published papers on a continuous flow TAH back in 2006. His idea took a step closer to clinical reality this year, when the National Institute of Healthawarded a $2.8 million grant to the Texas Heart Institute to fund the development of the continuous flow heart design, which was Initially proposed in 2008 {5} {6}.
Pictured is a device similar to the TAH that is
being created that will perform the function of both
the right and left ventricles of the heart.
How it Works
VAD's are normally used individually as support for a remaining natural heart. In the case of the TAH, two VADs are used and have to operate in close coordination to emulate the balanced flow that a natural heart provides {6}.These two pumps that are working together in order to perform the function of both the right and left ventricles are what makes the TAH unique. One pump is dedicated to the pulmonary loop and carries oxygen-depleted blood away from the heart to the lungs. The pump then allows for the return of oxygenated blood back to the heart. The second pump that completes the TAH drives the systemic loop and carries oxygenated blood away from the heart to the body and returns deoxygenated blood back to the heart{5}. Franchek states, "Each pump's loading conditions and flow output affects the loading conditions and flow output of the other pump" {6}.
Research Behind the Product
Pictured are Doctors Franchek and Metcalfe holding a similar device.
Franchek and Metcalfe already have a head start in their programming and intelligent design of the TAH. MicroMed, which is a collaborator in this endeavor, has already produced VADs that contain their own controls. These feedback controllers can take an actual flow measurement using ultrasonic sensoring and then compare this measurement with a desired flow output in order to generate an appropriate control signal that allows the VAD to regulate impeller speed {6}. This initial VAD technology is helpful in producing the new control system in the TAH that emulates how the natural heart responds to physiological conditions within the body.
A large portion of the control development work funded by the NIH grant involves the creation of a lumped parameter mathematical model of the human circulatory system. This model will be incorporated into the control algorithms (6). According to Franchek, the mathematical model improves product integration with the patient. The majority of patients regulate cardiac output, but a small amount of people regulate pressure instead. The mathematical model will identify these changes, among many others, and identify how the vascular system is reacting with the pump. In this way, the TAH controller can sync these interrelated flow and loading conditions (including inlet pressure and outflow resistance)with the needs of the human body {6}. Franchek explains that everyday activities such as standing, sitting, or coughing all change flow and loading conditions and that the TAH has to account for these changes. Likewise, flow can be altered by hypertension, vascular restriction and changes in blood viscosity {6}. This mathematical model can then act as the control system and automatically adapt in order to enable the TAH to work properly in the patient’s body.
Franchek and Metcalfe are currently in the midst of creating the intelligent design system for the TAH. The “onboard intelligence”, as it is described by Franchek,is software that monitors the VAD performance. It is able to monitor whether blood is clotting, whether there are input/output blockages,and it can measure blood viscosity in real time. This intelligence will remarkably be able to notify physicians through wireless capabilities so that a doctor can notify a patient that they need to come in due to complications. Therefore, a patients needs and risks can be known without the patient ever having to go into the office for preliminary tests.
Recently, Franchek and colleagues have studied the behavior of the VAD by embedding it into a mock circulation loop {9}.The effects of the forces involved in the circulation of blood in the mock circulatory system were analyzed and the inducing of an artificial pulse in the TAH heart with two axial-flow pumps was examined. Amplitude, beat rate, and systolic duration were varied. This variation was either of the left pump, right pump, or both simultaneously in order to emulate what could happen if transplanted in the human body {9}. The loop consists of an axial flow pump that impels a water-glycerol mixture (used to correspond to blood's viscosity) from a liquid tank and back through plastic tubing {13}. Maximal flow pulsatility and an adequate reduction in left atrial pressure (which may be elevated in recipients of cardiac replacement devices) were found through this study. This experimental data demonstrated that there is in vitro feasibility of inducing the artificial pulse in a TAH that contains two axial-flow pumps {9}. There were several limitations to this study which need to be further investigated, including; exclusion of the contribution ventilation makes, relatively inelastic pump outflow tubing, and the effect that non-physiological augmentation of the tubing had on pulse pressure {9}.The assessment of pump synchronization modes requires further bench and in vivo experiments {9}.These modifications and further experiments are the next steps that will be taken toward creating a fully functional and usable TAH.
Schematic drawing of the mock circulatory system.
AoP, aortic pressure; LA, left atrium; PAP, pulmonary artery
pressure; PVR, peripheral vascular resistance; Qpul, pulmonary
blood flow; Qsys, systemic blood flow; RA, right atrium; SVR,
systemic vascular resistance {9}.
Market Potential
Metcalfe states, "With heart disease being the leading cause of death in the United States, this is crucial research that constantly needs fresh approaches and interaction across disciplines" {1}.This would give cause for a possible venue for market potential. It has been determined that only 2,210 heart transplants were performed in the U.S. in 2007 while 39,000 people die annually from heart failure {13}. With these staggering statistics, caused mainly by the lack of viable organ donors, the demand for the TAH will be extremely high. The patients who are most likely to be selected for this type of heart transplant would be the patients that are ineligible for traditional heart transplants due to age and medical complications. This new device allows for a permanent replacement option for those who need a new treatment option {4}. Potential buyers of this product are medical device companies as well as surgical hospitals, such as the Cleveland Clinic. The device also would be potentially appealing to insurance companies such as Medicare that will help pay for the devices.
Currently, there is a product which has been developed by SynCardia Systems Incorporated which is a slightly different version of this TAH. SynCardia hopes that this product will allow hospitals to receive two reimbursement payments ( one for the TAH and one for the Donor Heart transplant)as opposed to only receiving payment after a traditional heart transplant {11}. Because of these hopes, the Centers for Medicare and Medicaid Services has reversed their previous non-coverage policy against total artificial hearts. These products will now be covered in the same way as Ventricular Assist Devices ( VAD's){11}.
Advantage of TAH:
Advancements in technology are meant to enable this device to respond to the body’s changing need for blood {5}.Older versions of this device, which artificially mimic the pulsating pump action of the natural heart, were typically made from a combination of titanium and plastic that weighed roughly two pounds. The complexity of these devices makes their size nearly impossible for use in smaller adults and children. There is also some reliability concerns related to these former complex designs,the most important of which would be failure due to mechanical fatigue{5}. The new Continuous flow pumps will potentially require less power and cost less than the more complex pulsatile models {6}. This product would save money because of the fact that this new TAH device has no artificial pumps the cost is much less compared to older models {7}.
Pictured is a Thoratec IVAD (which provides biventricular
support) implanted in a patient. As can be seen, this older
version is very bulky and does not fit well in the body cavity. {10}
References
1. “The beat goes on: Artificial heart technology holds promise for alternatives.” Physorg.com, 2008. Retrieved February 23, 2010, from, http://www.physorg.com/news142792001.html
2. Frazier, O.H., Cohn,W.E., Tuzun, E. , Winkler, J. Gregoric, I.D. “Continuous-Flow Total Artificial Heart Supports Long-Term Survival of a Calf.” Texas Heart Institute Journal, 36(6), 2009.
3. Mattsson, Marc. “Take to Heart.” Texas Heart Institute, 2009. Retrieved February 25, 2010, from, http://www.texasheart.org/AboutUs/News/NoN8-19-09.cfm
4. Michel,F. (2010). FDA Approves for Permanent Treatment of Advanced Heart Failure Assist Device Pioneered by Texas Heart Institute at Saint Luke’s Episcopal Hospital. Retrieved February 25, 2010, from, http://www.texasheartinstitute.org/AboutUs/News/2010-01-21news_FDAapprove.cfm
5. “New Continuous Flow Heart Pump Under Construction.” MedGadget, October, 2008. Retrieved February 23, 2010, from, http://medgadget.com/archives/2008/10/new_continuous_flow_heart_pump_under_development.html
6. Ogando, Joseph. “Mechatronic Artificial Heart Doesn’t Beat: “Beatless” heart uses a clever control strategy to emulate the natural heart’s function minus the pulsation.” Design News, 2008. Retrieved February 23, 2010, from, http://www.designnews.com/article/48957-Mechatronic_Artificial_Heart_Doesn_t_Beat.php
7. Yoffee, L. “ New Artificial Heart May Have Continuous , Pulseless Blood Flow” Medical Device Daily,13(159), 2009.
8. Merkle, L. ( 2008). “The Beat Goes on: Artificial Heart Technology Holds Promise for Alternatives”. University of Houston. Retrieved March 30, 2010, from, http://www.uh.edu/news-events/newsrelease.php?releaseid_int=315.
9. Khalil, H.A., Kerr, D.T., Schusterman, M.A.,Cohn, W.E., Frazier, O.H., Radovancevic, B. "Induced Pulsation of a Continuous-Flow Total Artificial Heart in a Mock Circulatory System". Journal of Heart and Lung Transplantation,29(5), 2010.
10. Khalil, H.A., Kerr, D.T., Schusterman, M.A., Frazier, O.H., Radovancevic, B. "Artificial Pulse with a Continuous-Flow Total Artificial Heart". Journal of Heart and Lung Transplantation,27(2), 2008.
11. " Reimbursement :Better Patient Outcomes= Better U.S. Hospital Reimbursement" SynCardia Systems., 2010. Retrieved April 19,2010 from, http://www.syncardia.com/Medical-Professionals/reimbursement.html
12. " Top 10 Medical Innovations for 2010: #3 Continuous-Flow Ventricular Assist Device" Cleveland Clinic., 2010. Retrieved April 19, 2010 from, http://www.clevelandclinic.org/innovations/summit/topten10/three.html link title
13. Franchek, M.A., Metcalfe, R., Wassar, T., Jammoussi, H., Rafik, B., Benkowski, C., Benkowski, R., Frazier, O.H., Cohn, W.E., Tuzun, E., Parnis, S. "Modeling and Diagnostics of Heart Assisted Devices." [Peer review article sent by Franchek, not in print yet], 2010.