With heart disease on the rise and a chronic shortage of donor organs for transplant, can technology come to the rescue to build an artificial replacement heart?
Our hearts beat approximately 35 million times a year, yet this incredible feat is often taken for granted. But what happens if your heart fails? With a shortage of donor hearts available for transplantation, can we use technology to build an artificial heart to replace our own biological one?
The short answer appears to be: yes. Mostly. Artificial heart devices are available for implantation into humans as a ‘bridge to transplantation’ until a suitable donor heart is available, or even as a longer-term option or ‘destination therapy’ for some patients. These devices are already saving lives, but there are still some technological and physiological challenges. Reducing damage to blood cells through improving biocompatibility, maintaining appropriate blood flow, device durability, and power supply all need to be addressed before artificial hearts can become a permanent heart replacement solution. Recently, collaborative projects in Australia and around the world have yielded new and innovative ways to overcome some of these limitations, refining and advancing existing technologies to push us ever closer to the goal of engineering a total artificial heart that can take the place of our own human organ.
The idea of an artificial heart is not new. The first artificial heart device, the Liotta heart, was implanted into a human in 1969. The patient survived only a few days after the surgery, and the case was shrouded with controversy around human experimentation and the ethics of consent. The next few decades were peppered with further attempts at artificial heart implantation. Whilst improvements were seen in survival, significant problems remained.
In 1982, amid a media frenzy, 62 year old dentist Barney Clark survived 112 days with his new heart: a model called the Jarvik-7. To many people, Clark was a hero — bravely volunteering to undergo such an experimental medical procedure. But his life post-implantation was not a happy one. He suffered severe complications, including blood clotting, seizures and confusion, and was reported to experience suicidal thoughts in the weeks before his death. The artificial heart had kept him alive for a few extra months but at a great cost.
The goal since then has been to improve not only survival but the quality of life following implantation. As John Fraser, Professor of ICU and Anaesthesia at the University of Queensland explained, “What we're trying to do [is] make the devices better, make the devices cheaper, make them easier to use, reduce the risk to the patient, reduce the cost to society and get a better outcome. [It’s] not just about adding years to their life, but life to their years.”
Today, only one total artificial heart, the SynCardia heart, has FDA approval in the US for use as a bridge to transplantation, and is currently being trialled as a destination therapy. While a few Syncardia hearts have been implanted into patients in Australia, most patients are fitted with Ventricular Assist Devices, which contain internal rotors and are implanted to support either one or both ventricles by assisting with blood pumping.
So what is the difficulty with making a new heart? At first glance, the human heart is just a simple pump. And yet, the heart is a beautiful, elegant example of evolutionary engineering, deceptively simple but hard to replicate. The action of the human heart has been the inspiration for many artificial heart models to date. These utilise a ‘volume displacement pump’ with chambers that fill with blood which are then squeezed using air-pressure to pump blood out of the heart into the circulation. Whilst physiologically similar, this action is complex to engineer, requiring a flexible material to expand and contract, and a mechanical means to make it move. Such reliance on multiple moving components results in durability problems; moving parts are much more susceptible to wear and tear and this can lead to mechanical failures.
An alternative approach has been developed based on a similar premise to the rotary pump technology in existing ventricular assist devices. Within the BiVACOR heart, designed and developed by Brisbane engineer Dr Daniel Timms, is a small rotor with two impellers that propel blood to the lungs and the systemic circulation. With only a single internal moving part, the BiVACOR device is simpler and at less risk of failure than the more complex volume displacement models. Although not yet available for use in humans, initial non-human trials have been very promising and work is ongoing to bring the device into human clinical use.
An interesting consequence of using a rotary pump is that there is no pulse — the pump is always whirring, in contrast to the pulsating action of our own hearts. While we know people can survive with no pulse (as many patients fitted with ventricular assist devices already do), recent studies suggest that there is some benefit in maintaining a physiological pulse.
“The blood vessel lining is actually very active, it's like the policeman that keeps your blood from being not too clotty but not too runny," said Fraser, who was Chief Medical Officer for the BiVACOR project. "So we think pulse plays a part in keeping that membrane producing the right number of proteins." Recent work has investigated ways in which ventricular assist devices could generate a physiological pulse, by modifying the speeds of the rotor within the device, to more accurately mimic the natural blood flow through our circulatory system.
Problems with red blood cell damage and dysfunctional blood clotting are still a major issue for patients with artificial heart devices. The cause lies within the heart itself. Replicating the complex forces and interactions between the biological components (the blood cells) and the mechanical components that make up the device is extremely difficult. The forces and stresses on the blood cells as they pass through a rotary pump device sound like something from a horror movie when described by Fraser: “So these spinning blades are rotating at maybe three, to three and a half thousand times a minute. If you imagine that your blood cells are like little bubbles from fairy liquid, they're very very fragile, and we're hitting them with a titanium blade at several hundred miles an hour.”
Progress has been made since the early days of artificial heart development, and damage to blood cells is now lessened through improved design and understanding of the dynamics of blood flowing through the devices. The BiVACOR heart, whilst still comprising a mechanical interior of man-made materials, employs large gaps through which the blood flows in order to minimise high stress interactions between the blood cells and the inner workings of the heart.
In an alternative approach, French cardiac surgeon Professor Alain Carpentier and his company Carmatsa have designed the CARMAT heart to mimic our own. The interior surfaces of the heart have been optimised to improve blood flow through the device and are lined with tissue taken from a cow heart to help provide a more ‘biomimetic’ environment, improve biocompatibility and reduce complications. Initial human trials have shown promise and currently the device is still undergoing further clinical testing.
The balance of power
The human heart is a master at balancing the rate and pressure of blood flow to different parts of our circulatory system — low pressure to the lungs, and higher pressure to the rest of our body. This is a challenge for artificial heart devices, which must be engineered to efficiently balance blood flow from the left and right sides of the heart. In Australia, where patients are fitted with two ventricular assist devices (one for the left and one for the right ventricle), this is a major issue. “It is like driving from Brisbane to Sydney on two motorbikes, and trying to keep the front wheel exactly aligned the whole way down there...for 20 years,” said Fraser. “If one of them goes out of kilter, it causes either too much blood to go to the lungs so you start to get the lungs congested and full of blood and the patient can't breathe. Or if the left side is going too fast then it sucks too hard and there's not enough blood coming from the lungs...and that's when it does damage to the red cells.”
Providing power to artificial heart devices is an additional challenge. Current devices rely on an external power supply, receiving power via a ‘driveline’ that passes through the skin from the external unit to the device implanted within the chest. Having a conduit passing from the outside of the body to the chest cavity dramatically increases infection risk, which is particularly problematic for long-term use of such devices. No matter how good the surgery and nursing care is, infection is still highly likely. Once an infection starts, it can traverse the driveline deep into the chest to where the device is implanted. At best, this requires treatment and further surgery; at worst, the consequences are dire.
With these numerous and diverse challenges to overcome, what is the best way forward? Fraser is a keen proponent of collaboration and patient-centred research. As Chief Investigator at the NHMRC Centre for Research Excellence in Advanced Cardio-Respiratory Therapies Improving OrgaN Support (ACTIONS), he has been striving to bring together the best and brightest researchers, clinicians and engineers to work on improving the technology for artificial heart and lung devices. He described the artificial heart and lung area as being similar to an adolescent boy: “It’s just starting to grow and it’s going to grow quickly, and there's a whole pile of things that we don't understand perfectly…our job is to scientifically parent this device technology. If we do it properly with all the best and the brightest people, we reduce costs and we improve outcomes”.
The lives of those early artificial heart recipients were fairly grim — unable to leave the hospital, attached to large machines to power the devices, and suffering multiple medical complications, infections and mechanical failures. Artificial heart devices have come a long way since those early patients, with heart failure patients now able to return home and live relatively normal lives with their implanted devices. Clearly, there is still work to do. But how close are we? Will we ever get there?
John Fraser is optimistic. “I think it'll come, you know, I think if you look at pacemaker technology 20 years ago or 30 years ago, it was this big massive thing that would sort of hold your left shoulder down. Today pacemakers are about the size of 50 cents.” In the face of rising rates of heart disease, collaboration is key. Extensive multidisciplinary collaborative efforts such as that headed by Fraser are pushing the boundaries of the technology, improving devices and providing hope of a new lease of life for the many thousands of heart patients not only in Australia, but around the globe.
Edited by Tessa Evans and Bryonie Scott