Discussions of an artificial heart often spark visions of Dr. Barney Clark or William Schroeder – patients almost constantly bedridden with limited mobility for all but brief and infrequent periods. The pneumatically powered artificial hearts that kept each of these men alive for several months are truly amazing but have substantial limitations.
The most significant drawback is dependence on an external compressed air source. This dependence certainly limits the patient’s mobility, but, more significantly, requires large air hoses connecting the compressed air source to the imbedded heart. These hoses must enter the patient’s chest cavity through the skin. This entryway is vulnerable to infection and requires careful maintenance and monitoring.
Figure 1. Computer-generated illustration shows relative size and orientation of LVAS components. (Click image to enlarge.)
More recently, efforts have focused on developing a battery-powered artificial heart that requires no skin penetration. One such device is the electrohydraulic left ventricular assist system (LVAS) undergoing joint development by The Cleveland Clinic Foundation, Cleveland, Ohio, and Nimbus, Inc., Rancho Cordova, California. This LVAS, Figure 1, is not a total artificial heart but, rather, an implantable system that boosts performance of a damaged heart.
A human heart contains four chambers: left and right atria and ventricles. The ventricles provide most of the heart’s pumping power. The right pumps blood to the lungs, while the left pumps blood to the body. Because 80% to 90% of a heart’s work is performed pumping blood throughout the body, severe damage to the left ventricle can render a heart virtually useless.
If an artificial organ can help the left ventricle pump blood to the body, performance of the natural heart’s remaining three chambers often is adequate. So a totally implantable LVAS holds potential to improve the quality of life for a large portion of heart disease victims.
A critical component of the LVAS is as electrohydraulic energy converter. This device uses an electric motor-driven gear pump and spool valves to reciprocate a linear actuator. To hermetically seal the converter, the actuator’s piston is magnetically coupled to a follower magnet that drives a blood pump pusher plate. The reciprocating motion of the pusher plate allows the blood pump to alternatively fill with blood, then eject blood out to the body.
The IDEAL heart, of course, is a healthy natural organ. Through years of experimentation and development, researchers have learned many requirements of an ideal artificial heart. But aside from longevity and precise operation, the LVAS meets operational and performance requirements that challenge those of any industrial machine.
For example, in addition to not penetrating the skin, all LVAS components are located entirely within the chest area. This compact construction places all components close to the natural heart, so length of vascular connections are kept short. It also eliminates abdominal surgery and the need to penetrate the diaphragm. Moreover, placing all components within the chest does not compress abdominal organs nor interfere with body movements.
Overall system efficiencies on the order of 10% to 20% present a challenge in dealing with heat dissipation. At 10% efficiency, 2 W delivered to the blood (9 1/min at 100-mm Hg) represents a heat load of 18 W. Heat generating components — such as the hybrid motor-controller circuit, motor, gear pump and battery rectifier — have been packaged to dissipate heat through available surface area. Studies are being conducted to determine the safe level of local heat flux to lung and muscle tissue.
Noise and system vibrations are other concerns. These parameters can have a tremendous psychological impact on a patient. In addition, all materials are compatible with the high humidity, corrosive environment inside the body.
LVAS components include an externally worn battery pack, skin transformer, internal battery, variable-volume device, and integrated electrohydraulic energy convertor/blood pump, Figure 2. The external battery pack contains 12-V dc nickel-cadmium cells that can supply power for up to 10 hr to the pumping system. An inverter changes the external batteries’ dc into ac. The ac flows into a transformer with external primary windings and secondary windings just beneath the skin.
Figure 2. Simplified layout of left ventricle assist system during ejection cycle. (Click image to enlarge.)
From the skin transformer, the ac is directed to a rectifier, which converts the ac back into dc. The rectifier provides power for the system, or it can recharge an internal nickel-cadmium battery. This battery can power the LVAS for more than 1000 half-hour cycles to allow brief, totally untethered periods of operation.
Finally, power reaches the energy converter/blood pump combination, which is located in a space made available by removing a portion of one of the patient’s ribs. The energy converter case is a biologically inert, investment-cast titanium housing that resists corrosion in a body fluid environment.
The electrohydraulic energy converter itself, Figure 3, is driven by a 3-phase, permanent-magnet ac synchronous motor. Flow rate of the hydraulics is controlled electronically by varying motor speed. An inverter changes the dc back to ac and controls motor speed by varying ac frequency. Flow direction is governed by hydraulic logic: a 2-way spool valve, check and diaphragm valves, hydraulic switch, and an accumulator.
Hydraulic fluid floods rotating components of the pump-motor to provide cooling and hydrodynamic support of its bearings by eliminating rubbing contact and wear. Heat is dissipated by directing fluid to the outer surface of the energy converter, which lies just under the skin between two ribs. Because it is cooler than the body’s interior, skin acts as a heat sink to carry heat away from the energy converter. Such strategic placement also provides access to interior components of the energy converter without major surgery.
The piston of the energy converter’s double-acting cylinder is magnetically coupled to the blood pump. The magnetic coupling allows hermetically sealing the hydraulic system from the blood pump and offers advantages over a metal bellows seal. A compression spring contributes force to the piston’s thrust by storing energy when the blood pump fills and releasing the stored energy when the blood pump ejects.