So far, more than 40 patients in the United States and Brazil have received the implant. In the first phase of testing, two patients had a heart attack during the trial, both of which were detected by the device. Researchers are now enrolling patients for a larger trial of 600 to 800 patients. The trial will focus on high-risk patients, such as those who have already had a heart attack.

"I think there is lots of potential for these types of devices," says William Abraham, director of the division of cardiovascular medicine at Ohio State University, in Columbus. Abraham has done extensive testing with an implantable diagnostic device from Medtronic that monitors heart rhythms.

However, some experts are skeptical that the AngelMed device will make a significant difference in a patient's clinical outcome. "There's no question that the earlier you get treated, the better off you are," says William Maisel, director of the Medical Device Safety Institute at Beth Israel Deaconess Medical Center, who is not involved in the clinical trials. "But the vast majority of patients develop symptoms, such as chest pains or shortness of breath, when they have a problem. The idea that we need an early-warning system--I just don't see it."

Maisel adds that patients who have already had a heart attack--the most likely candidates for the device--are also the most likely to recognize and quickly respond to symptoms.

The issue of clinical impact has plagued previous cardiac diagnostic devices. In 2007, a Food and Drug Administration panel recommended against approving an implantable monitor developed by Medtronic to detect heart failure. While clinical trials showed that the device accurately measured intracardiac pressure, which signals when the heart isn't pumping enough blood, having that information did not significantly reduce the number of patient hospitalizations or ER visits due to heart failure.

However, Maisel says that two groups in particular may benefit from the AngelMed device: people who have heart attacks and don't feel them--a disorder called silent ischemia that can occur in some diabetics with nerve damage-- or "people who have frequent chest pain but not heart attacks, and go to the emergency room unnecessarily."