by Jeanne Erdmann
Most of us take a steady heartbeat -- about 100,000 beats per day in near-perfect rhythm -- for granted. But some people may suddenly develop a dangerous and irregular rhythm that could make the heart beat wildly out of control. Identifying people with this electrical glitch that puts them at risk, however, could allow them to receive implantable defibrillators, possibly saving thousands of lives each year.
A researcher at Washington University School of Medicine in St. Louis has invented the first noninvasive test that identifies this glitch in advance of a patient's first rhythm problem. The test is called the alternans test because it measures a tiny fluctuation in heartbeat called the T-wave alternans. Joseph M. Smith, M.D., Ph.D., associate professor of medicine and biomedical engineering, invented the test while he was at the Massachusetts Institute of Technology, and he hopes it will be used widely to identify those at risk of sudden death. Cambridge Heart Inc. developed the test for commercial use, and the FDA approved it in April 1999.
Smith and colleagues described their latest study at the North American Society of Pacing and Electrophysiology in Toronto, Canada, in May 1999. They determined how T-wave alternans changes as heart rate changes, which will allow for improvements in the test's accuracy.
"Because the consequence of a positive alternans test may be implantation of a defibrillator in some patients, we want to make the test as accurate as possible," Smith said.
Shortfalls of traditional testing
Most cardiac patients undergo a stress test to evaluate problems associated with insufficient blood supply to the heart -- a plumbing problem. Such tests are of little value in detecting the electrical problems responsible for sudden death, however. Studying the electrical system of the heart typically requires an invasive test in which electrode catheters are placed inside the heart.
While the majority of sudden deaths occur in patients with plumbing problems, only a small minority of patients with plumbing problems who also are suspected of having electrical problems go on to receive the invasive electrical test. This is due in part to the test's invasive nature, its cost and the associated risks. As a result, some people go about their daily lives without knowing they might suddenly die. In fact, one in three people who develop fatal arrhythmias have had no previous arrhythmia symptoms or known heart problems.
Some people with other types of heart disease, such as cardiomyopathy, also die suddenly, and doctors are frustrated by their inability to identify and treat such people before a first life-threatening event occurs. "Because sudden cardiac death continues to claim 250,000 people each year in the United States alone, more people at risk need to be identified and treated, especially now that we have such compelling data on the efficacy of internal defibrillators in preventing these sudden deaths," Smith said.
Automatic internal defibrillators are small devices that resemble pacemakers. They are implanted under the skin in same-day surgical procedures, and they correct the heart-rhythm abnormalities that otherwise would cause sudden death.
Getting the alternans test is similar to getting the standard stress test except that a patient is hooked up to a few extra leads. These additional leads help pick up subtle beat-by-beat abnormalities that are detected through computer analysis, much like a microscope picks up details that can't be seen by the naked eye.
By examining the relationship between the presence of these tiny signals and a patient's heart rate, doctors can identify people at risk of developing life-threatening arrhythmias. The cost of the alternans test is similar to that of a regular stress test.
"From studies done in the United States and abroad, we know that the presence of T-wave alternans is a marker of risk for sudden death in people with many different types of heart disease, though it's not the complete answer," Smith said.
At the November 1998 meeting of the American Heart Association, German investigators helping to evaluate Smith's test presented results from 107 patients with congestive heart failure. While fewer than half of these patients had a positive alternans test, all of the deaths and severe adverse events that occurred during the follow-up period were in patients in which the test had been positive. A negative alternans test in this group of patients with congestive heart failure predicted complete freedom from life-threatening consequences.
"One of the most depressing aspects of caring for patients with heart failure is that you work so hard to make them feel better but, despite doing well, they still die suddenly. Knowing in advance which patients are at risk will allow us to avoid these deaths, which will make a huge difference," Smith said.
In the latest set of studies at Washington University, Smith's group studied 60 patients extensively under controlled conditions. Thirty six of the patients had a clear risk of sudden death as defined by invasive electrical studies, while 24 were not at risk. This study demonstrated for the first time that T-wave alternans is a dynamic entity whose characteristic response to changes in heart rate reveals a patient's susceptibility to arrhythmias. The results also suggest several ways to further improve the accuracy of the test.
When Smith was a graduate student at the Massachusetts Institute of Technology, he read that sudden cardiac death was the major challenge facing contemporary cardiology. "I couldn't have asked for a more important problem to work on -- not that we're done," Smith said. "But the T-wave alternans test is a step in the right direction. For patients diagnosed by this test and subsequently saved by implantable defibrillators, this will be a huge advance."
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