A change of heart

The Dodrill-GMR was for the first time providing mechanical assistive support by pumping blood around Henry’s body. This was the first instance of an operational mechanical heart at work and it allowed Forest Dodrill, a thoracic surgeon, to perform open heart surgery on Henry’s heart for 50 minutes. This innovation paved the way not only for open heart surgery but also for the development of implanted mechanical hearts.

As the development of devices to support patients during open heart surgery continued, a new question emerged: “Is it possible that a mechanical machine could replace the heart long term?” It wasn’t until 1963 that this question could be answered, and the first implantable mechanical heart device was fitted to a patient who survived for four days under mechanical assisted support. This device initiated the development of Left Ventricle Assist Devices (LVADs).

The key feature of an LVAD is that it supports rather than replaces the patient’s heart, which continues to pump while the LVAD siphons some blood from the left ventricle and pumps it directly into the aorta. This action relieves stress on the heart, giving it a chance to recover from trauma or to repair structural damage. The initial expectation was that LVADs would provide long-term cardiac support for patients on the transplant list while they waited for a donor heart to become available. The initial expectation was that LVADs would provide long-term cardiac support for patients on the transplant list while they waited for a donor heart to become available.

In 1966, a device developed by the leading thoracic surgeon Michael DeBakey was used to do just that. The patient was fitted with an LVAD which supported their heart for ten days until a donor heart was found and the LVAD could be removed and the new heart transplanted. Since then, LVAD development has moved on, and the first commercially available LVAD — the HeartMate IP LVAS — received FDA approval in 1994. It was subsequently improved and renamed the XVE, and was used extensively. This HeartMate device was an implantable pump that provided pulsatile flow (to mimic the pumping of the heart) and was driven by a pneumatic pump external to the body, and carried by the patient. These first generation devices allowed gravely ill patients to live a more normal life. You would be unlikely to see an LVAD patient running a marathon or climbing Everest, but they could live at home and leave the house for a few hours with the device running on battery power.

As other pulsatile flow LVADs entered the market, the number of devices implanted rose to several hundred, with some patients living with the device for up to six months before receiving a transplant.

The future was looking good for the LVAD market, but there were problems; the devices were large, required heavy pneumatic pumps to operate, and they had a high number of moving parts and a large internal surface area which made them prone to failure. Clearly, more innovation was needed.

The next big step was a complete rethink, and came out of a conversation in 1984 between NASA engineer Dave Saucier and Michael DeBakey (who implanted the first LVAD). Saucier and DeBakey hit upon the concept of developing a simple LVAD based upon axial flow pumps normally used for industrial applications. With the use of NASA’s engineering knowledge and supercomputers (normally reserved for modelling rocket flow) the Micromed DeBakey VAD was created and became commercially available in 1998. This new design completely changed the landscape; whilst previous LVADs were pneumatically driven pumps weighing around one kilo, and with a lifespan of a few months, this new device was driven by a tiny DC electric motor (housed within the device), weighed around 100 grams, and had a lifespan of years.

This completely new approach prompted a number of big LVAD manufacturers to go back to the drawing board, resulting in a large number of axial flow LVADs which became the second generation of the device. The most successful of these was the HeartMate II, which has been implanted into 5,000 patients worldwide. It’s around the size of a D-cell battery and has a service life of five years.

Second generation devices have quickly taken over the LVAD market as their simple design and low number of moving parts have made them dependable and compact. However, they are still far from perfect and some significant problems remain, not least the high shear stresses that axial pumps place on the patient’s blood. This shearing action damages red blood cells, which is not ideal for patients with a history of cardiac problems.

Since the launch of HeartMate II and similar devices, subsequent developments have been fairly incremental, such as improvements to the drive system design and the implantation operating procedure. The Circulite partial support device, for example, is sufficiently small (just larger than an AA battery) to be inserted using a minimally invasive procedure which can lead to greatly improved patient outcomes.

The market is now looking for the next innovation, and to find this we need to look away from industry developments and towards academic research. It is here that we see some interesting ideas which focus less on changes in pumping style and more on the potential of new materials. In this promising new area, a research team at Harvard (along with other teams) has made some encouraging progress; rather than build on the existing LVAD design that syphons blood from the heart, they have decided to wrap the damaged heart in a flexible membrane and squeeze it from the outside. The design uses flexible rubber tubes encased in a membrane which, when filled with air, contracts and acts like an artificial muscle.

The Harvard design is a novel idea, embraces new materials, and is a world away from the titanium and steel LVADs currently available. Innovations such as this look set to change the landscape of the market once again, and while the road to commercial availability is a long one, it is from this area of research — in my opinion — that the next major developments will emerge.

Ideas that are highly bio-inspired, and which work with the wealth of advanced materials currently in development, will lead the way in the future of cardiac support. All it needs is one manufacturer, or one start-up, to rise to the challenge and the LVAD market may be turned on its head again.