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NewsletterThe Dawn of Artificial Hearts?SCA Newsletter -- October 2001K.W. Tim Park, MD Boston, MA
In recent months, there have been two reports of successful implantation of artificial hearts that can totally replace the functions of the diseased heart. First, on June 11, 2001, a Korean medical team led by Drs. BG Min and K Sun reported implanting an AnyHeart® (Biomedlab, Seoul, Korea), a totally implantable biventricular assist device that can supply up to 100 % of the cardiac output, in a patient with terminal heart failure.1 Shortly thereafter, on July 2, 2001, a team at the University of Louisville, led by Drs. LA Gray Jr. and RD Dowling, implanted an AbioCor™ (Abiomed, Danvers, MA), a heart-replacing artificial heart, in a terminally ill patient.2 Both of these events represented significant breakthroughs in the care of patients with terminal heart failure. Both hearts were totally implantable, thus reducing the risk of infection. Both could totally replace the diseased hearts, thus being end-therapies in themselves, rather than merely acting as bridge therapies for eventual heart transplantation. Yet the two hearts were based on two different philosophies in artificial heart implantation, as the AnyHeart® is a heart-saving electromechanical artificial heart, while the AbioCor™ is a heart-replacing artificial heart. In the United States, more than 700,000 patients die from heart failure each year.3 About half of these die sudden cardiac deaths and are not amenable to intervention with cardiac assist or replacement devices. For the other half, heart transplantation is an option. However, only about 2,000 people were actually able to receive donor hearts each year, due to a shortage in the supply of donor hearts. An implantable artificial heart may be an option to fill this gap in the supply of donor hearts. The AbioCor™ heart is made of titanium and Angioflex™, a proprietary polyurethane plastic that is durable and non-thrombogenic. This heart is inert and does not require immunosuppression. The thoracic unit consists of two ventricles and the associated valves and gets attached to the patient's aorta, main pulmonary artery, and the two atria via bridging grafts. A detailed figure is available on the web in Drs. Gray and Dowling's website.4 The thoracic unit is connected to a controller unit that provides electronic control of the heart and an internal rechargeable battery, both of which are implanted in the abdomen. The controller unit monitors the physiological needs of the patients and controls the pumping speed of the heart. The internal battery is a lithium battery that can last up to half an hour when standing alone and that is continually recharged from an external battery pack. The external battery pack can last a number of hours, depending on the number of batteries in the pack. There is no direct connection between the internal battery and the external battery pack, but energy transfer is achieved through a set of coils called Trans-cutaneous Energy Transmission (TET). Because of the internal battery, the patient can potentially engage in activities such as taking a shower, apart from the external battery pack. The operation that implants the AbioCor™ heart consists of intraabdominal placement of the internal controller, internal battery, and the TET coil, institution of cardiopulmonary bypass, and then orthotopic implantation of the thoracic unit.5 After many years of animal trials6 with AbioCor™, the device was approved for limited human trials by the Food and Drug Administration to implant the heart in at least 5 individuals at any of the following 5 locations: Jewish Hospital in Louisville, Brigham and Women's Hospital in association with the Massachusetts General Hospital in Boston, Hahnemann University Hospital in Philadelphia, the Texas Heart Institute in Houston, and the UCLA Medical Center in Los Angeles. For inclusion in the trials, the patient must be an adult patient (age > 18) with biventricular failure unresponsive to existing therapy, ineligible for cardiac transplantation, and with life expectancy less than 30 days. Pregnancy and serious noncardiac or psychiatric illnesses would exclude the patient. The main advantages of AbioCor™ include no need for immunosuppression, potential for patient mobility, and low thrombogenicity. In addition, unlike some of the traditional ventricular assist devices, the AbioCor™ heart can replace 100 % of the cardiac pumping function and provide a pulsatile flow. The patient who received the first AbioCor™ heart in July exceeded the original life expectancy and was reported to be walking and eating on his own on August 2.7 He has since suffered a "minor" setback, developing a lung infection and an intestinal bleeding by mid-August.3
The advantages of a heart-assisting artificial heart such as AnyHeart® over a heart-replacing artificial heart such as AbioCor™ are several fold, according to the developers of the former.12,2 The first advantage has to do with the ability to better balance the cardiac outputs of the two ventricles, as mentioned above. Second, in case of malfunction of the mechanical heart, the native heart may provide the critical survival period to replace the failed mechanical heart. Third, the native heart may recover to some extent in response to new treatment modalities being developed or to be developed in the future. Such modalities may include gene therapy. The inclusion criteria in the ongoing clinical trials of AnyHeart® are similar to those of AbioCor™. The progress of the patient who received the first AnyHeart® is to be reported during the November 2001 meeting of the International Society of Artificial Organs in Osaka, Japan. REFERENCES
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The AnyHeart® heart comes from a different philosophy about
the diseased heart, namely that it may actually be disadvantageous to remove
the diseased heart. This opinion was shared by such prominent figures as Drs.
DeBakey and Jarvik.2 At the same time, AnyHeart® overcomes the
shortcomings of earlier ventricular assist devices by being able to totally
replace the pumping function of both ventricles and to provide a pulsatile flow,
which is considered to be important for low thrombogenicity by the AbioCor™
developers. AnyHeart® is a moving-actuator pump, with the actuator situated
between two ventricular sacs, which are attached to the patient's own atria,
aorta, and pulmonary artery.8 The output from each ventricular sac
depends on the filling pressure of the sac, which in turn is related to the
filling pressure of the corresponding native atrium. In the most recent version
that was implanted in a man in June 2001, the AnyHeart® heart can pump up
to 7.5 L/min at a rate up to 150 bpm and can thus provide 30-100 % of the patient's
cardiac output. The ability to pump up to 100 % of the cardiac output sets it
apart from other ventricular assist devices and allows it to be more than just
a bridge to heart transplantation. Because of a dynamically variable interventricular
space between the ventricular sacs and because of the residual function of the
patient's native heart, any difference between the ejection volumes of the two
artificial ventricles can be accommodated, thus preventing pulmonary edema.9
The pump is powered by an external battery pack, with the power being transmitted
either via a wire or via a transcutaneous energy and information transmission
(TEIT) system.10 The battery pack worn as a band can last up to 8
hours, while a piggy bag system can be extended up to 24 hours. A solar recharging
system has also been developed.11 A schematic of the AnyHeart®
system is shown in the figure.12