What happens to the heart during a heart attack? To better understand a heart in extremis, two medical researchers have made a three- dimensional model of the electrical activity in a dog’s heart. (Dog hearts have been extensively dissected and probed, so researchers know much more about canine hearts than they do about human hearts.) In a normal heart (above), the electrical activity (shown in blue) sweeps across cardiac muscles in one quick wave. But during a heart attack, the electrical excitation spreads randomly through the heart, shown in the lower image as a splotchy muddle of relaxed (red) and excited cells. This chaotic activity reverberates all over the surface and within the heart. As a result, the heart no longer pumps blood, because it doesn’t contract and expand systematically, says Raimond Winslow, a biomedical engineer at Johns Hopkins who developed the model with Denis Noble of Oxford. The waves just circulate in a chaotic fashion within the heart, Winslow says. This model is just the first of a full range of simulations that he hopes researchers will be able to use to develop better drugs and pacemakers.