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State-of-the Art in Artificial Hearts

An interview with cardiothoracic surgeon, Mehmet Oz, MD,
New York-Presbyterian Hospital, NY, NY.

By Sean Henahan, Access Excellence
Copyright 2001 Info

A person dies of cardiovascular disease-related causes every 33 seconds. While the overall rate of heart disease in the US has shown a promising decline in recent years, one type of heart disease, heart failure, is on the increase. Patients with milder forms can be treated with a number of potent medications. However, patients with more advanced disease have few options. They must either receive a new human heart from a brain-dead donor or die. The outlook for these patients is now improving, as temporary implantable pumps called ventricular assist devices keep patients alive for longer periods as they await a heart transplant. Perhaps most promising of all, a new artificial heart is now ready for human trials. I asked noted transplant authority Dr. Mehmet Oz to provide an update on this exciting field of medical research.

Thank you for joining us Dr. Oz. To begin with, please review the status of heart transplantation today in order to help us understand why we need artificial hearts and ventricular assist devices.

A: One of the problems we have faced as we have improved the management of patients who would have died of heart disease is that these surviving patients develop heart failure, which is now the leading cause of death in this country. As these patients develop heart failure, we have to replace their hearts. The best biologic solution has been transplantation. But for the estimated 40-50,000 people per year who might need a heart transplant, we only have about 2,200 donors. You might ask why we don't push harder to get people to donate organs, . We do have about 7,000 potential donors per year. These are often tragic instances where young men and women have died in car accidents or from gun shot wounds and the parents aren't ready to make the decision, so it is understandable that we only get about one third of the potential candidates to provide their organs. So we have a big gap and the desire to fill that gap has led us to investigate mechanical solutions for pumping blood.

Let's begin with ventricular assist devices or VADs. What is a VAD? How does it work and when is it used?

A: A VAD is a ventricular assist device. LVADs assist the pumping on the left side of the heart, and RVADs assist pumping on the right side. Most patients are dying of left heart failure, the thicker chamber that pumps blood to the entire body. When the heart is unable to pump blood to the head, kidney, arms and legs, we find the muscle can be replaced best by leaving the heart in place and putting in a kind of piggy back heart, an assist device that can perform this role of the heart, without having to remove the heart.

LVAD use has become a fairly common procedure. Can you tell us how these devices have evolved?

A: One of the issues we have had to cope with as VADs were developed was that we knew we would not manage to create a perfect device the first time out. Remember, if these devices fail suddenly, you would have about five seconds to fix it. So we developed a model that works reproducibly well for months and years. We elected to keep part of the pump (the cable) that connects the pump to the power supply (either battery or wall socket) outside the body. It is also important for us to have access to the pump if we have to deal with an emergency. So all of these pumps have both electrical and air connections. If necessary, we can connect to a port and manually pump air through the device. There have been cases where a patient is literally pumping the device by hand en route to the hospital in order to keep the blood flowing.

Haven't newer VAD designs managed to get away from the need for external power and air venting?

A: Yes, there have been exciting advances in mechanical heart support devices. In a way it is very similar to the evolution of the automotive industry at the turn of the century. There are many parallels between replacing a heart and building an automobile. For example, both had competitors that were biologically based. With hearts you have the natural heart or a xenograft (animal) heart. For the car it was the horse. At first glance the heart and the horse seemed like better solutions. But they are limited in power. Ultimately the ability to reproducibly make a device that can save someone's life is easier to do with a pump than a biological organ. So these newer devices are based on lessons we've learned from the first generation devices. We have just started to implant these in humans in the past year. They are much smaller, about the size of your thumb. They work more like jet engines than propellers. The new heart pump devices are based on the use of something called an axial flow pump. These can swirl the blood forward at very high speeds. They still require electrical support, but cables are smaller and the battery packs are smaller.

The axial flow pumps are unusual in that the blood flow is continuous. Does this mean the patients do not have a pulse?

A: Yes. One of the big challenges we are going to have with these axial flow pumps is trying to figure out what happens to the human body when you loose the heartbeat. There are all kinds of romantic reasons why you want to have a heartbeat- you can tell if you are nervous or not, and so can the person you are with. The study of the heart has focused on this internal cadence or metronome. Yet it may be that we can more efficiently pump blood using these axial flow pumps. The problem for the medical staff is we don't know what your blood pressure is. We measure blood pressure by looking at pulsations. If you don't have a pulse you don't have blood pressure, so I don't even know if the device is pumping blood or not. You could be walking around feeling fine, and I would have no way to identify how your blood is flowing unless I put in a catheter, which I don't really want to do. So these devices monitor themselves and give us feedback so we can make intelligent decisions.

How are patients doing?

A: The majority of patients who have received these devices are still alive and most are doing quite well. Only recently have they been implanted in reasonable numbers. In US in particular, FDA approval is only now happening. You will hear more about these devices as they become more common.

One of the problems with early devices was that patients had wires coming out of their chest that needed to be plugged into a battery or other power source. Can you describe how the newer devices overcome this limitation?

A: We now have mechanisms for providing transcutaneous power transfer. We have a battery inside the body that can be charged externally, so the power supply never touches the pump. The benefit is that we avoid infection, the problem that has been the Achilles heel of the technology.

The earlier devices also had to equalize the air pressure by use of external venting tubes. How has the problem of venting been solved?

A: Venting has been a major concern for us. By venting we mean you have to place a reservoir in the body, because when the pump fills with blood, the air that was in the pump is displaced and has to go somewhere, either to the outside atmosphere as in first generation systems, or to reservoir inside the body. Those reservoirs unfortunately calcify and fibrose over time and become rigid and can no longer receive the air as they should, so the pumps lose efficiency. The virtue of the axial flow pump is that it does not require air transfer, so we are able to jump that big hurdle.

At the moment, VADs are used either until a patient's heart recovers from a temporary problem, or in other cases, until a donor can be found. What are the prospects for using these devices for longer periods, if not permanently?

A: We have been favorably impressed by the number of patients who can survive for a number of years with LVADs. To study this we embarked on large study called REMATCH trial, a $7.5 million NIH trial comparing how a group of patients with LVADs do in the long term compared to a matched group receiving optimal medical management. That trial is in its final stages. Enough patients are alive that there is considerable interest in following this approach. We may eventually evolve to a point where if you are younger than the age of 55 and you need a mechanical heart to keep you alive, you will receive one as a bridge for weeks or years. Ultimately you will be offered a transplant. But if you are over the age of 65 or whatever the cutoff will be, you will not be offered a transplant. Rather, if you require an LVAD, you will get an LVAD.

Another riddle you are dealing with is that some patients hearts recover sufficiently to let them get off the LVAD? Is there any way to predict who these patients might be?

A: It is fascinating to those of us who see heart failure every day to observe that there are patients whose hearts should be dead and should get mechanical heart support to stay alive, that somehow recover miraculously to near normal heart function. The problem is we can't predict whom that will happen with. More importantly, we cannot predict the duration of that recovery. We are not really interested in running a windsprint, we want a marathon running heart. If the heart recovers but only lasts another six months, I should have transplanted you. That is holding us back. On the other hand it has opened a huge vista for us. Now we have a model for predicting heart that should have been dead that are now being kept alive so we can study them meticulously to determine what happened at a chemical and hormonal level, but also at a genetic level, at the protein creation level in individual myocytes that have made this heart fail. When we tease these issues apart we may be able to offer genetic solutions to patients dying of heart failure.

Click here to continue to page 2 for a discussion with Dr. Oz of the Abiocor fully implantable artificial heart and related issues. You will also find extensive links to multimeda education resources on this topic.

This interview was conducted by Sean Henahan on February 27, 2001

 

Dr. Mehmet Oz

Dr. Mehmet Oz

"So Oz brought a pair of tinsmith's shears and cut a small, square hole in the left side of the Tin Woodman's breast. Then, going to a chest of drawers, he took out a pretty heart, made entirely of silk and stuffed with sawdust." "Isn't it a beauty?" he asked"

L Frank Baum
The Wizard of Oz

 

FACTOID
A normally functioning heart pumps about 3 ounces of blood with each beat, or about 5 or more quarts per minute.

Source- Mayo Clinic

 

Virtual Transplant
Click Image
New Window Will Open

 

LVAD in Action
(Image courtesy Abiomed)

 

 

A US First
On February 28, 2001, Walter Pae, Jr., M.D. and colleagues (pictured above) completed the first United States human implant of the LionHeart™ fully implantable Left Ventricular Assist System. The procedure was done at Penn State Milton S. Hershey Medical Center in Hershey, PA.

 

 

Above: Axial Flow Pump,
Experimental Micromed
Debakey VAD
(Courtesy Micromedtech)
click image to enlarge

 

FACTOID
Your heart pumps blood through a network of 60,000 miles of vessels.

Source- Mayo Clinic

 

Join a Discussion Forum on Artificial Hearts and LVADs

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