Like the human heart, the PulsaCor has four valves that gate blood’s entrance and exit. But that’s where the similarity ends. This heart has only two chambers, instead of four-transparent hamburger-bun-shaped plastic domes clamped onto either side of a metal housing. This sealed, hockey-puck-sized core contains an electric motor that powers a spinning blade through hydraulic fluid. The fluid pushes out against two diaphragms that squeeze blood out of the chambers, through the valves, and into the arteries.
Unlike the Jarvik-7, Abiomed’s PulsaCor is designed to fit entirely inside the body, so that the patient can leave the hospital, and perhaps even return to work. To achieve this goal, the system includes an implantable battery and “controller package” containing the electronics that dictate the pump’s speed. Each is about the size of a small paperback novel, wired to the pump but implanted in the abdomen. Because the lithium-ion cell can only feed the pump’s 12- to 20-watt power demand for about an hour, it will have to recharge continually from a wearable external battery pack. A pair of spiral induction coils-one outside the body, one inside-spirit electrical energy across the skin. It’s an odd arrangement but, says Robert Kung, Abiomed’s chief of engineering, “any cable going through the skin is an invitation to infection.”
Several technical advances since the days of the Jarvik-7 have brought the goal of a totally implantable artificial heart within reach. Better batteries, for example, make it possible to eliminate the external power source that kept Clark and Schroeder tethered to their hospital beds. Faster computer processors have allowed engineers to model blood’s movement through artificial chambers and valves, and thereby eliminate spots where blood might pool and clot.
Still, creating a pump that is ultra-reliable, extremely power-efficient and small enough to fit in the 12-centimeter space between the backbone and rib cage has required some engineering stretches all around. Stresses on the flexing dia-phragms are high and unabating, but kept shy of the threshold at which cracks can form. The valves are clot-resistant, but not clot-proof-patients will still need to take blood-thinning drugs. The controller package and battery will leak heat into the body, but less than the 2.3 milliwatts per square centimeter that can damage the surrounding tissue. David Myerson, an electrical engineer turned cardiologist at Johns Hopkins University, compares the artificial heart with the Stealth bomber: “Both are engineering outliers. It flies, but it takes every trick you’ve got.”
The engineers who have worked on artificial hearts have had 35 years to learn some of these tricks. The National Institutes of Health (NIH) established the Artificial Heart Program Office in 1964 at the urging of Baylor College of Medicine heart surgeon Michael DeBakey. At that time, the advent of heart transplantation was three years away and patients whose hearts failed faced certain death. Many thought a mechanical replacement would take only a decade to develop-in the age of the Apollo mission, pushing blood through a pump looked eminently doable.
“The original expectation was that you could take existing components and put it together. That turned out to be a false assumption, ” says John Watson, director of the NIH’s National Heart, Lung and Blood Institute’s (NHLBI) office of bioengineering, which now funds the artificial heart. Up close, the project was a hydra, with unexpected materials, power and design challenges sprouting everywhere.