The standard treatment for cardiac arrest may make good television, but having up to 1000 volts pumped into your chest is a seriously painful treatment. Now scientists have discovered that a series of small electric shocks may be as effective at restoring the heart's beating as one large jolt. The new strategy has only been tested in beagles so far, but there's optimism it will also work in humans.
Under normal circumstances, heart muscle contracts in response to an electric impulse that starts around the upper chambers of the heart (the atria) and travels straight down the middle before rising up again slowly around the lower chambers (the ventricles) and dying away in time for the next beat.
However, occasionally these electrical impulses begin to travel around the heart in chaotic spirals called rotor waves—a process called fibrillation that can cause the heart to contract in a disorganized way and prevent it from pumping blood. The initial causes of fibrillation are variable and not fully understood, although they include blockages in the heart's blood supply. But just as a hurricane sustains itself once started, these rotor waves, which can affect either the atria or the ventricles, can keep swirling round and round the heart. Ventricular fibrillation is fatal if not treated within minutes, whereas atrial fibrillation, although not immediately life threatening, can lead to the formation of stroke-causing blood clots.
A defibrillator provides a reset button for the heart. The massive electric shock causes all of the cardiac tissue to contract and then relax at once, putting an end to the deadly rotor waves and allowing the heart's natural pacemaker to take over again. Unfortunately, it can also cause burns to skin and muscle tissue. Moreover, it's extremely painful, with some who have felt it describing it as like being kicked in the chest by a horse. Indeed, biophysicist Flavio Fenton of Cornell University says some patients with atrial fibrillation are reluctant to have miniature defibrillators implanted in their chests, choosing instead to depend on medications that are less effective and have many side effects.
So Fenton and fellow physicist Stefan Luther of the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany, together with a team of physicists and cardiologists on both sides of the Atlantic, set out to devise a subtler, less painful form of defibrillation. Mathematical modeling of the heart revealed that the rotor waves were driven from just a few key points in the heart, sites called vortex cores. Modeling showed that perturbing the electric field at these points would be enough to destroy the rotor waves and allow the heart to return to normal operation. All that would be necessary would be to get a small, repetitive electric pulse to these points. Today in Nature, the researchers describe testing their technique on live beagle dogs in which atrial fibrillation had been induced with internally implanted electrodes. They found that the traditional, single, huge shock could be replaced equally effectively by a rapid sequence of between five and 10 pulses, each delivering only one-seventh as much energy (which is thought to be below the pain threshold). Laboratory experiments on sections of dog ventricle indicate that the method would also be effective at halting ventricular fibrillation, and Luther and Fenton are now planning to test this in live dogs.
Cardiac electrophysiologist Richard Gray of the U.S. Food and Drug Administration, an expert on fibrillation, is impressed. "This is a very major advance in regard to two things: It increases our understanding of the mechanisms of defibrillation, and it helps reduce the amount of energy needed to defibrillate," he says. "The research was done in animals, but I see no reason why the idea itself wouldn't be of much interest to the clinical situation."