Six people in New Zealand have become the first to be implanted with a novel form of cardiac defibrillator that could radically change the way that people with life-threatening heart conditions are treated.

The new device, developed by Cameron Health, in San Clemente, CA, functions much as normal defibrillators do, shocking the heart to stop dangerous heart rhythms or to restart it if it stops beating. But unlike traditional devices--which are known as implantable cardioversion defibrillators, or ICDs--Cameron's device delivers a shock from outside the heart rather than from electrical leads inserted into it.

"We think there's a big advantage of not having to put the lead into the heart, because sooner or later that lead is going to have to come out," says Warren Smith, the cardiologist who carried out the implantations at Auckland City Hospital and Green Lane Hospital, in New Zealand.

According to Andrew Grace, a cardiologist at the Papworth Hospital, in Cambridge, England, who helped develop the device, patients with ICDs have a 20 percent chance of lead failure within 10 years. But leads are designed to embed themselves in the tissue of the heart, making them difficult to remove. If they don't come out easily, as happens in one in 50 cases, the only way to remove them is to perform open-heart surgery, says Smith. Lead replacement has a morbidity rate of between 2 and 5 percent, he says.

"It's unusual for the device itself to fail," says William McKenna, a cardiologist at the Heart Hospital, in London, England. "It's where the lead connects to the device or in the leads themselves that problems occur." Placing the leads can also be a problem, McKenna says, because if they are inserted into scar tissue caused by a previous heart attack, they may not deliver shocks effectively.

But until recently, placing the leads outside the heart just wasn't possible, says John Hunt, vice president of Cameron Health. "The technology wouldn't allow us to do it in the early days," he says. One reason is that shocking the heart from a greater distance requires more energy. But supplying that energy resulted in devices too bulky for surgical implantation.

Cameron's device, dubbed the subcutaneous-ICD, or S-ICD, uses leads placed just beneath the skin above the rib cage. Whereas a normal ICD would generate less than 30 joules per shock, the S-ICD generates 80 joules. Nonetheless, it's only marginally bigger than a traditional ICD, largely thanks to improvements in battery and capacitor technologies. The device itself sits beneath the skin below the armpit, instead of in the chest.