The Virtual Heart

April 2000

By Carl Zimmer

This detailed image and animation show muscle fibers of the heart wall, the mitral valve (purple) and the aortic valve (yellow). The animation, from an educational video produced by the PSC scientific visualization group, shows the heart beating while the viewpoint rotates. The mitral-valve structure extends upward, sealing off the opening to the left atrium, as the left ventricle, the heart's main pumping chamber, contracts. Click arrow to begin animation. In this animation of the beating heart, red markers show blood ejected into the aorta (left) as the left ventricle contracts. The markers on the right show blood forced upward against the sealed mitral valve. Click arrow to begin animation. You will need QuickTime to play these animations. Click icon to download.

A beating heart is not just a biological organ—it’s a puzzle for physicists to solve. The walls of a heart’s chambers are woven from thousands of elastic fibers, each of which is enclosed by a membrane. Electrical signals travel from membrane to membrane, causing the muscle cells to contract. As they squeeze and relax, the fibers move blood from one chamber to the next, then out of the heart altogether. To keep the blood from flowing the wrong way, the heart has valves designed to open when blood is moving in the right direction and to seal shut against backwash.

For the past twenty-two years, David McQueen and Charles Peskin, two researchers at New York University’s Courant Institute of Mathematical Sciences, have tried to simulate this choreography on a computer. Today their machine of choice is a supercomputer that can perform 8 billion operations per second; to simulate a single heart beat, they have to run it for an entire week. And yet the physics of the heart are so complex that Peskin and McQueen are still not satisfied. “It’s hard to get things right,” says Peskin. “The more we work on this, the more we respect the natural organ.”

Still, their long years of tinkering are paying off as their virtual heart starts behaving like a real one in some important ways. The blood swirls in the chambers properly, and the valves open and shut when they’re supposed to. This success has allowed Peskin and McQueen to study how valve shape influences the flow of blood. Based on what they’ve learned, they’ve even patented a design for an artificial valve.

Their simulation has also enabled them to study some fundamental questions about the heart. Why, for instance, is there a delay of one tenth of a second between the contraction of the atria and the contraction of the ventricles? “You’d expect nature to be optimal, but you don’t know what it’s optimized for,” says Peskin. The pause is just long enough for the atrium to squeeze its blood into the ventricle and for that blood to start swirling violently and pushing up against the valve separating the two chambers. Their swirling seals the valve shut, and the ventricle then contracts, pushing its blood out of the heart.

Peskin and McQueen hope that someday they can answer questions about the design of other animals’ hearts, but they are not ready to stop tinkering with the human one. “There’s always an infinity of next things to do,” says Peskin.

Web link:  HEART THROB: Modeling Cardiac Fluid Dynamics.

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