Ventricular assist device
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A Ventricular assist device, or VAD, is mechanical device that is used to partially or completely replace the function of a failing heart. The devices are generally designed to replace or assist cardiac function temporarily, but recently devices are becoming available that can be implanted permanently for so called "destination therapy". Most patients using the devices, however, are awaiting heart transplant.
VADs are designed to assist either the right (RVAD) or left (LVAD) ventricle, or both at once. The choice of device depends on the underlying heart disease and the pulmonary arterial resistance which determines the load on the right ventricle. Generally when pulmonary arterial resistance is high, right ventricular assist becomes necessary. Because the devices generally result in blood flowing over a non biologic surface, predisposing the blood to clotting, there is need for anticoagulation. There is one device, the Heartmate, which provides a biologic surface derived from fibrin and does not require long term anticoagulation; unfortunately, this biologic surface may predispose to infection.
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[edit] Complications
A test carried out in 2001 by Dr Eric Rose and the REMATCH study group using patients with Congestive Heart Failure and were ineligible for a transplant showed a survival at two years of 23% for those implanted with an LVAD compared with 8% for those who were given drug treatment. The two major complications of VAD implantation were infection and mechanical failure.
VAD-related infection can be caused by a large number of different organisms:[1]
- Gram positive bacteria (Staphylococci especially Staph. aureus, Enterococci)
- Gram negative bacteria (Pseudomonas aeruginosa, Enterobacter species, Klebsiella species)
- Fungi especially Candida sp.
Treatment of VAD-related infection is exceedingly difficult and many patients die of infection despite optimal treatment. Initial treatment should be with broad spectrum antibiotics, but every effort must be made to obtain appropriate samples for culture. A final decision regarding antibiotic therapy must be based on the results of microbiogical cultures.
Other problems include immunosuppression, clotting with resultant stroke, and bleeding secondary to anticoagulation. It is interesting to note that some of the polyurethane components used in the devices cause the deletion of a subset of immune cells when blood comes in contact with them. This predisposes the patient to fungal and some viral infections necessitating appropriate prophylactic therapy.
VADs need to be clearly distinguished from artificial hearts, which are designed to completely take over cardiac function and generally require the removal of the patient's heart. The longest surviving patient with an LVAD for permanent use has recently celebrated his 6th anniversary of extra life on the Novacor device. VADs not only extend the quantity of life, but also the quality of life. People who have had VADs implanted have had lower rates of depression than those people suffering from cardiac disease that have not undergone implantation surgery.
The majority of VADs on the market today are somewhat bulky, however one of the smallest devices available weighs only 20g.
One type of VAD which has CE Mark approval for use in the EU and is present in clinical trials in the US, relies on a varying current in electrical coils in the pump's housing to drive the rotor within the pump and on hydrodynamic forces to levitate the rotor. This results in a pump with only one moving part and with no contact between moving parts thus minimising wear. These pumps have been implanted in over 60 patients including a 10 year old girl. In one case the original heart recovered after the device had been implanted for a year and the device was able to be removed.
Another type of VAD currently being developed relies on magnetic levitation to drive the rotor within the pump, thereby minimizing the wear and reducing the size of the pump substantially. The first magnetically levitating pump has recently been implanted for clinical trials in Greece and is expected to begin US Clinical Trials in 2007. Magnetic levitation technologies are expected to result in small pediatric VADs within the next four years.
[edit] History
The early VADs emulated the heart by using a "pulsatile" action where blood is alternately sucked into the pump from the left ventricle then forced out into the aorta. Devices of this kind include the Heartmate, which was approved for use in the US by the FDA in October 1994. These devices are commonly referred to as first generation VADs
More recent work has concentrated on continuous flow pumps, which can be roughly categorized as either centrifugal pumps or axial flow impeller driven pumps. These pumps have the advantage of greater simplicity resulting in smaller size and greater reliability. These devices are referred to as second generation VADs. A side effect is that their users need to carry documentation saying that the lack of a pulse does not mean that they are dead.
Third generation VADs suspend the impeller in the pump using either hydrodynamic or electromagnetic suspension, thus removing the need for bearings and reducing the number of moving parts to one.
Another technology undergoing clinical trials is the use of trans cutaneous induction to power and control the device rather than using percutaneous cables. Apart from the obvious cosmetic advantage this reduces the risk of infection and the consequent need to take preventative action. A pulsatile pump using this technology has CE Mark approval and is in clinical trials for US FDA approval.
A very different approach in the early stages of development is the use of an inflatable cuff around the aorta. Inflating the cuff contracts the aorta and deflating the cuff allows the aorta to expand - in effect the aorta becomes a second left ventricle. A proposed refinement is to use the patient's skeletal muscle, driven by a pacemaker, to power this device which would make it truly self contained. In any case it has substantial potential advantages in avoiding the need to operate on the heart itself and in avoiding any contact between blood and the device. Interestingly this approach involves a return to a pulsatile flow.
[edit] In popular culture
- In ABC's television series Grey's Anatomy, an LVAD device was central to the storyline of Denny Duquette, a heart transplant patient who eventually died of complications related to his heart transplant. His intern doctor who had a crush on him, Dr. Izzie Stevens, cut his LVAD wires to worsen his condition and put him up higher on the transplant list. As a consequence she had her privileges revoked for a while, and she resumed practicing medicine after she heroically saved someone's life by drilling holes in his head when he was stuck underneath a car after a ferry accident.
[edit] References
- ^ Gordon RJ, Quagliarello B, Lowy FD (2006). "Ventricular assist device-related infections". Lancet Infect Dis 6 (7): 426–37.
