| Artificial organs: direct cardiac compression heart assist device |
| Saturday, 01 October 2005 | |
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Replacement of diseased organs by artificial devices is an exciting challenge facing researchers with a diversity of scientific backgrounds. For instance, at the Biomedical Engineering Research Group in Leeds, mechanical and electrical engineers, material and computer scientists, physicists and biologists are joining forces to create joint replacements, synthetic tissues and devices to aid cardiovascular anomalies and disease.
One strategy to increase pumping power is to implant a motorised pump into the bloodstream within the chest cavity. These pumps use a rapidly spinning impeller (similar to a propeller) to increase the flow of blood and have been found to work in the few cases of human implantation so far. There are, however, some problems with this approach, which are currently being tackled. The rapidly spinning impeller can damage the fragile blood cells and potentially instigate blood clots. Additionally, the immune system must be repressed with drugs in order to prevent rejection of the device. These complications are the result of blood flowing through an artificial chamber, which may be recognised as foreign and attacked by the recipient's immune system. Alternatively, to avoid contact between the implant and the blood, the heart can be assisted with direct cardiac compression (DCC), providing a compressive pressure to the heart's outer surface. One method of achieving this is through a surgical procedure known as a cardiomyoplasty, which involves detaching the patient's own latissimus dorsi muscle (a sheet-like muscle across the shoulder blade), and wrapping this around the heart. This muscle wrap is then electrically stimulated so that it contracts in sync with the heartbeat. This technique was shown to be successful in that the heart can be briefly assisted. Stimulation must, however, be carefully controlled since skeletal muscle — unlike healthy heart muscle — fatigues over time. The difficulties associated with using human muscle have driven attempts to create a mechanical ‘artificial muscle’ assist device. Pneumatic pressure has been applied to hearts in different ways: by placing the heart within a pressure vessel, surrounding the heart with an inflatable cuff, or applying inflatable patches. The pneumatic actuators (cuffs, patches) have the benefit of high power to weight and power to volume ratios but the air hoses such a device requires pass through the skin and so are a potential source of infection, while attachment to a pneumatic pressure supply limits mobility. Several new ‘artificial muscle’ technologies are being developed, but these new technologies are at early stages of development and are not yet suitable for an implantable cardiac compression device. At the University of Leeds, we are developing a DCC device consisting of a series of bands to be placed circumferentially around the heart. These bands can contract in the same way a belt may be tightened around one’s waist. This contraction is powered by miniature motors, one per band, which are individually controlled.We can provide contraction in the form of a wave in order to squeeze blood up and out of the ventricles, and can apply varying levels of assistance to different areas of the heart. Power for the device is provided by battery packs. Control of the assistance force is crucial. If appropriate assistance is provided, allowing the heart muscle to rest, studies have shown that the heart muscle can start to regenerate. Assistance must be gentle to avoid damage to the coronary arteries that lie on the surface of the heart and supply blood to the heart muscle. After contracting to pump blood out of the heart, the device must relax quickly and not restrict the refilling of the heart. The time between heart-beats varies, and we are using a pacemaker as part of the system to sense when the heart beats and to synchronise the assist contraction. The bands are inelastic in their circumference, but flexible. If these were directly on the surface of the heart, the tissue and blood vessels could be at risk of abrasion, so we have used a structure which separates the contracting bands from the heart's surface, provides a protective cushion, and maintains the shape of the device. To minimise interference with the heart during surgery, the device is constructed as a ‘sock’, slipped onto the ventricles in one motion, and secured in place. The motors are sheathed from the body with a bio-compatible sheet. We have created a novel testing simulator that uses a computerised model of the heart and circulatory system, which is combined with a physical simulator representing a beating heart. This allows physcial testing of the mechanical performance of the assist device under realistic conditions. With the computer model we are also able to visualize how the assistance will affect the blood pressure and flow throughout the body; this can be repeated for many different states of heart failure and patient type. We are developing new motor technologies that will hopefully be used to create a flexible sheet of ‘artificial muscle’ to replace the motorized belts currently used. The way the body reacts to assistance in the long term is yet to be determined, although avoiding direct contact with the bloodstream should greatly reduce the likelihood of immune system rejection. Dr Ben Hanson is a Lecturer at the School of Mechanical Engineering, University College London, and a former Research Fellow at the University of Leeds. |
Heart attacks or viral diseases can weaken the heart muscle, reducing its pumping power. When this happens, the heart often still has sufficient power to sustain life, but is unable to increase output to cope with greater pumping demands during exercise. Where a heart muscle is weakened or failing, the treatment options available are limited and transplantable hearts are rarely available. Recently, efforts are increasingly being directed towards providing mechanical assistance to a weakened heart. | Next > |
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