The Beat Goes On – artificial heart transplants
After decades of deadly prototypes, engineers can finally re-create the body’s most amazing muscle
WHEN ROBERT KUNG LAYS HIS HEART ON THE table, waiters come to chat about it and his coffee is on the house. Kung’s man-made heart is slightly bigger than a real one, and it looks like a fat hamburger with legs: a titanium patty in a hard plastic bun. A normal heart beats 70 to 80 times a minute for 70 to 80 years–a couple of billion beats in all. But Kung, research director at a company called Abiomed, will be happy if someone gets 175 million beats–or about five years–out of his device. It sits gleaming in the center of the table now, clearly artificial, and yet its oddly angled legs–Dacron tubes that will one day connect to living atria and arteries–give it the lumpy look of life. “Is that actually the heart itself?” asks Jorge, the waiter. For a moment he seems to think it’s real.
Soon–perhaps as soon as next year–Kung’s artificial heart will become very real indeed to a human patient; it has already kept calves alive, and it is now undergoing final tests in Abiomed’s lab in Danvers, Massachusetts. At Penn State’s College of Medicine in Hershey, Pennsylvania, a second version of an artificial heart is at about the same stage of development, pumping away in a vat of salt water. Alan Snyder, a bioengineer at Penn State, has worked on that device since 1976, and his coworkers have been at it since 1971. But artificial hearts of various sorts have been around since the 1950s. The most famous of them, the Jarvik-7, was implanted in a patient named Barney Clark in 1982. For Kung, who began work on his heart that same year, the coincidence must have been sobering: As Clark and then a second man, William Schroeder, died lingering deaths, tethered to air compressors and visited by repeated infections and strokes, the Jarvik heart became a public relations disaster. No one has performed a complete heart replacement since.
Why is the human heart so hard for researchers to imitate? “There’s one very simple problem,” says Steven Vogel, a biomechanic at Duke University and author of a book on the circulatory system called Vital Circuits. “It’s that the engines we have available, whatever their power output or efficiency work so differently Muscle is a soft, wet, contractile engine, and that’s just unlike anything in our technological armamentarium. So you can’t imitate a heart–you’ve got to start from square one.”
A natural heart consists of two pumps, each made of two chambers. The right atrium squirts oxygen-depleted blood from the body into the right ventricle, which pumps it to the lungs; the left atrium squirts aerated blood from the lungs into the left ventricle, which pumps it out to the body. With each heartbeat, the two small atria contract together, then the two large ventricles, making the heart go ba-BUM. Kung’s and Snyder’s hearts will leave the patient’s atria in place (they’re usually not the problem) and just replace the ventricles.
Kung’s and Snyder’s artificial hearts have two ventricles, just like the real thing, but there the similarity ends. “In the natural heart, you’re using muscle as a container,” Snyder says, “and the container pumps on its own.” Not knowing how to build a self-contracting container, he and Kung had to sandwich a separate engine between the two ventricles. The engine pushes against the inside wall of each ventricle–they’re made of a flexible polyurethane–squeezing blood out of the heart and into the arteries. In the Penn State heart, a battery-powered motor drives a piston that moves back and forth. In the Abiomed heart, the motor drives a fan blade that sits immersed in a hydraulic fluid; a rotary valve shoots the fluid alternately left and right.
Both artificial hearts seem to work, at least in calves. (The calf studies lasted a few months–after which the animals outgrew their hearts–but researchers found no signs of rejection and few problems with infection.) And both look to be enormous improvements over the Jarvik-7. The recipient won’t be chained to cumbersome equipment, because the control system and even the working battery will be small enough–like an oversize pack of cards–to implant in a patient’s abdomen. To reduce the risk of infection, nothing will penetrate the patient’s skin after the initial surgery, not even an electric wire; the battery will be recharged continuously by radio waves from a larger, rechargeable battery pack patients will wear in a harness.
Such is the dream, at least. But artificial hearts still fall far short of their living models, and no one knows that better than their designers. The thing that impresses Kung most about the natural heart is its dynamic range–its ability to pump a little or a lot of blood, as the occasion requires. A resting heart pumps out five liters of blood (about as much as a typical human body contains) every minute. But exercise boosts the output dramatically, mostly because the heart beats faster. A typical athlete’s heart churns out from 25 to 30 liters per minute. “That’s a challenge that currently no mechanical device can meet,” Kung says.
The Abiomed heart can pump more than 10 liters per minute–plenty for most activities but not enough for others. You will be able to have sex, for instance, but not a very athletic kind. “You certainly won’t be able to play squash,” says Kung. For weekend athletes and aging lovers, there’s a silver lining: An artificial heart won’t suddenly fail if you ask too much of it. You just won’t be able to execute the desired motion. And you won’t be able to work your way up to it, either. Whereas a living heart, like any muscle, gets stronger over time if you exercise it, an artificial heart will never be stronger than on the day it was implanted, precisely because it is not alive.
Indeed, an artificial heart needs to be as inert as possible to prevent blood clots–which, after migrating from the Jarvik heart, caused the strokes partly responsible for killing Clark and Schroeder. Clots are Kung’s and Snyder’s biggest worry They’ve tried to minimize the danger in two ways. First, they’ve designed their hearts’ ventricles so that blood flows through them in a swirling vortex, without forming any slow-moving or stagnant pools that would encourage it to clot. Second, they’ve coated every surface the blood will touch with their special polyurethane, which sheds blood cells and platelets the way Teflon sheds grease.
A living heart has a more vigorous strategy for preventing clots, sending out chemical signals that stop them from forming. And like other tissues, the heart extracts fuel and nutrients from the blood and uses them to rebuild itself continuously “You think the heart is the same year to year; same organ, same place,” says Vogel. “In fact, it has a half-life of less than a year, in terms of replacing its molecules.”
Such feats are still beyond the dreams of engineers. A self-repairing pump sustained by the fluid it circulates and capable of strengthening over time to meet demand, your heart is an extraordinary kind of pump–one with a normal service life that no artificial heart has even approached. “That’s probably the biggest challenge,” says Kung. “There isn’t anything man-made that can go 50 years without maintenance.” The titanium-and-plastic gizmo on the cafe table in front of him begins to lose a bit of its gleam.
And yet, natural hearts fail. They grow diseased and weak and distended, which is symptomatic of congestive heart failure. They suffer sudden attacks of oxygen starvation that destroy most of the muscle. Decades from now, practitioners of the brave new art of tissue engineering may have learned how to grow new hearts of muscle to replace diseased ones. But for now, the only hope for people with severe afflictions are transplants–and there are not nearly enough donor hearts to go around. Moreover, the typical heart transplant patient has only a 60 percent chance of surviving longer than five years. The demand for Kung’s and Snyder’s contraptions, in other words, promises to remain high.
“I won’t interrupt you further,” yet another waiter says to Kung, after being drawn almost irresistibly to the strange organ at the center of the conversation. “I’ll probably be needing one of those myself someday.”
COPYRIGHT 2000 Discover
COPYRIGHT 2000 Gale Group