Cleveland, OH--End-stage heart disease claims 350,000 American lives each year. Only 2,000 fortunate people receive transplants: There are far too few donor hearts. Fortunately, thousands with damaged or diseased hearts may live healthier lives within a decade without requiring an artificial replacement. Implantable ventricular-assist devices (VADs) promise to pick up the slack in weak hearts. Their potential market is estimated to be $3 billion by the year 2000.
At the Cleveland Clinic, a world leader in cardiac surgery and biomedical engineering, engineers may have devised the simplest, most elegant, and compact VAD to date. Their patented, continuous-flow, radial-output VAD fits in the palm of your hand and delivers about 5 liters/minute at 3,000 rpm. Its dual-impeller design has one moving part, uses a blood-lubricated hydrodynamic journal bearing, and employs a clever inverted, brushless dc-motor drive. The small secondary impeller prevents stagnant flow and clotting in the bearing. By using blood to lubricate the bearing, engineers see to it that no seals contact moving parts-a source of wear and eventual failure.
Lead engineer Bill Smith claims the Clinic's unique VAD design is simpler, cheaper and lighter than competing designs. He expects it to cost $10,000 to $20,000, about 40% less than other VADs. This VAD also solves a major design challenge facing VAD designers: "To prevent cell damage we want low shear, and for minimal clotting we want high shear," explains Smith.
The blood-lubricated journal bearing satisfies both requirements. It allows 10- to 20-micron-diameter blood cells to pass and keeps average shear between 2,000 and 4,000 dynes/sq-cm, well within the viscoelastic tolerance of red blood cells. Some cell death (hemolysis) is tolerable, because bone marrow constantly manufactures replacement cells.
Some competing VADs try to solve cell clotting and damage problems by employing more complex approaches. One method uses an external supply of saline to wipe cells off the pump's stagnant-flow foreign-material surfaces, where clots are likely to form.
Continuous flow also circumvents problems that plague pulsatile VADs which mimic the heartbeat. Pulsing pumps are bigger-and may not fit a child's chest-and require a more complex arrangement of one-way valves and timing mechanisms. Contrast that with the Clinic's 1.5-inch-long and 1.625-inch-diameter pump, which has one moving part. Pulse VADs prevent backflow if they fail. But they can experience backflow problems when operating.
Smith says the inversion of the magnet motor assembly and motor windings was inspired by inverted World War II aircraft engines. The design places the bearing on the inner diameter of the rotor rather than the outer diameter. This feature also reduces shear forces on the blood.
The motor's electrical input is about 6.5W and its mechanical output between 1W and 2W. These low figures reduce heat transfer to the blood, thus minimizing blood temperature rise to decrease the danger of a "coagulation cascade" that leads to clotting.
Batteries implanted in a patient's chest supply power to the VAD. They are recharged via transcutaneous electromagnetic coupling from a belt pack or other power source. Currently, a patient could theoretically spend as long as an hour, twice a day, without carrying the belt pack. Belt packs weigh between one and two pounds and last from two to four hours.
What's the future for this VAD? "We have a good design. With the resources at the clinic, we have a chance to bring it to fruition successfully," says Smith. He hopes to add a microprocessor to the system. It will use pacemaker sensors to shift the pump's output up or down-depending on activity level-as well as gather valuable diagnostic data. If things go Bill Smith's way, his design team may soon benefit from $5M in funding from the National Institute of Health. The team is a finalist in the competition. Of course, VADs must run the FDA's qualifying gauntlet before they find a place next to our human hearts.
Additional details...Contact Bill Smith, P.E., The Cleveland Clinic Foundation, 9500 Euclid Ave., Cleveland, OH 44195-5254.