The artificial heart
A patient heading home after being given an artificial heart has made news headlines. How do they work?
Earlier this month, 40-year-old Matthew Green left hospital and headed back home to his family after having his heart replaced with an artificial device made of plastic – the first UK patient to be discharged after having both sides of his heart replaced.
The development and operation of these life-saving devices requires understanding and application of a combination of biology, materials science and physics.
A natural human heart has four chambers – two atria and two ventricles.
The right atrium collects blood and the right ventricle then pumps it to the lungs where it is oxygenated. The blood is then picked up by the left atrium and distributed around the body and brain by the left ventricle. Each side of the heart has a pair of valves – one pair per lung – controlling the flow of blood.
Artificial hearts can now completely, if temporarily, replace the ventricles and valves with a device made of plastic or other man-made materials, which does the job of pumping blood around.
The type of artificial heart that was given to Green, made by Syncardia Systems, works by using a pump carried externally in a backpack – previously, patients would have to be connected to a large, immobile pump and would not have the freedom to move around.
The NHS Choices website explains that tubes connecting the heart to the pump “send pulses of air into two expandable, balloon-like sacs in the artificial ventricles, forcing out blood in much the same way that a beating heart would”.
Other models such as that produced by AbioMed use an internal pump and battery, which can be charged via transcutaneous energy transmission – a method of transferring power under the skin without having to penetrate it, thereby decreasing the chance of infection.
In the artificial hearts produced by AbioMed, an electronics package is implanted in the abdomen of the recipient of the transplant to monitor and control the pumping of the heart.
Power is supplied from an external source to components under the skin, without penetrating it, using inductive electromagnetic coupling – the same principle as used by transformers to transfer electricity between different circuits, as in the national grid.
At their simplest, systems of transcutaneous energy transmission will use an external power supply connected to an external coil of wire, generating a magnetic field in it. This, in turn, produces an induced voltage in a second coil implanted under the skin, and a rectifier is used to change this alternating current into direct current that can be used to power the electronics of the heart and its controller.
Though simple in theory, in practice there are complications that arise from the need to keep the two coils aligned correctly as the patient moves, in delivering the correct level of power so that there is no excess dissipated as heat to potentially damage surrounding tissue in the patient’s body, and in making the components small enough to be carried around without too much discomfort.
Monitoring blood flow
A replacement heart needs to be able to monitor the flow of blood to regulate its pumping and ensure that the correct amount of blood is delivered around the body.
Quicker pumping is required when the transplant recipient is more active, whereas the opposite is true while he or she is resting.
Blood-flow monitors make use of ultrasound – they bounce high-frequency sound waves off blood cells coming out of the heart, the volume and speed can be measured using similar basic principles to those behind radar.
Ultrasound is used because it can monitor the flow of blood without having to be in contact with it.
Artificial hearts need to be made of light but durable materials – the Syncardia version is plastic whereas that made by AbioMed is a combination of titanium and a specially developed polyurethane, called ‘Angioflex’.
Although the Abiomed heart is designed to have as few moving parts as possible, those that it does have are made from Angioflex and are tested to ensure that they are safe for contact with blood and capable of withstanding beating 100 000 times a day for years on end.
Materials scientists can develop substances with specific properties by manipulating the constituent elements and the way in which they are processed. Materials are characterised using various techniques from condensed-matter physics including electron microscopy, x-ray diffraction and neutron diffraction.
Because they were still quite large, the first devices produced were limited to around half the male population – those with the largest chest cavities. A newer, smaller, model is intended to extend their availability to smaller people.
An artificial heart being produced by the French medical company Carmat and expected to be available by 2013 will use chemically treated animal tissue to help avoid rejection by the host’s immune system. Aerospace engineers from Airbus were also involved in its development.
Artificial hearts combine, and improve upon, many existing physics ideas to produce a piece of technology that saves lives – although they are currently only approved as a stopgap until a donor heart can be found.