Cardiac arrhythmias, electrical malfunctions that throw the heart out of rhythm, cause many of the 330,000 sudden cardiac deaths each year in the United States. Most fatal arrhythmias in the aging are caused when scar tissue left behind by heart attacks interferes with the heart's electrical system. As many as 1,000 of the deaths, however, are caused by a genetic disorder called Long QT Syndrome (LQTS), mostly in children and adolescents with otherwise healthy hearts.
The QT interval is one part of the heart's electrical signature as recorded by an electrocardiogram (ECG). The QT represents the time it takes for the heart's lower chambers to "reset" electrically after each heartbeat, and QTc is QT corrected for heart rate, a more accurate measure. In LQTS patients, the QTc reset time is prolonged, which makes the heart more susceptible to fatal arrhythmias. The condition may go unnoticed until sports, strong emotion or even loud noises knock the heart out of rhythm, causing loss of pulse and consciousness, and sudden death.
"LQTS impacts communities nationwide each year as young people die during football games and track meets, or just as tragically, in their sleep," said Ilan Goldenberg, M.D., Research Assistant Professor of Cardiology in the Department of Medicine at the University of Rochester Medical Center and first author of the JACC paper. "The condition runs in families and can remain undiagnosed until the victim's brother or sister dies as well. We believe our study will provide powerful new tools for protecting these patients. Beyond LQTS, the study shows that rethinking how we define risk has the potential to save the lives in several major heart diseases."
In 1979, Arthur Moss, Professor of Medicine in the Division of Cardiology at the medical center and co-author of the current study, helped to launch the International Long QT Syndrome (LQTS) Registry, a database of families with the LQTS trait. By following generations of sufferers, gene hunters used the registry to track down more than 300 genetic mutations involving seven genes that cause versions of LQTS.
In current study, researchers went back and looked at 375 patients from the registry for whom several consecutive QTc measurements had been recorded at yearly intervals before age 10, and who had experienced cardiac events through age 20. The study sought to determine whether several ECG-QTc measurements over time would improve risk assessment and better identify which children would face life-threatening heart rhythm disorders in their teens. Jehu Mathew, a medical student at the University of Rochester School of Medicine & Dentistry who helped conduct the research, presented the abstract today at the meeting.
By looking at QTc, researchers realized that this key measure of the heart's electrical activity varied over time. Subjects who had any one QTc measure that was especially prolonged were at increased risk for a dangerous heart rhythm disorder in the future. Given this variability, the likelihood of accurately detecting future risk of arrhythmia with a one-time ECG measurement on any given day is small, and only gets better the more times you measure, the analysis found.
Past studies had established that patients with a one-time QTc measure longer than a half a second (500 milliseconds) were at high risk for a future cardiac event. The new study revealed that many of the subjects, who were judged to be at low risk based on a one-time QTc measure below a half a second, were proved to be at high risk by later ECGs. Researchers found that the maximum QTc measure captured at any time in a series of ECGs was the most powerful predictor of a future heart trouble.
Based on these findings, researchers are calling for changes in standard medical procedure to ensure that decisions on risk for LQTS patients be made based on several ECG readings over time. Patients found to be at low risk by one-time ECG measures should be re-evaluated during follow-up to more accurately determine their risk, researchers said.
Preventive measures should be immediately put in place for any patient whose QTc interval duration exceeds a half a second at any time. Beta-blockers, widely used heart drugs, and implanted defibrillators that shock the heart back into rhythm when needed, are among the current treatments for LQTS, and will hopefully, based on the study results, become part of better preventive treatment to prevent sudden death, researchers said.
"It may seem obvious, but studies have not been done on whether follow-up ECGs have additional value in predicting risk in LQTS patients over time," said Moss. "Our research opens up the possibility that, in assessing risk for many cardiovascular disorders, serial measures of risk may be the best way to address the changeable nature of heart disease when trying to identify risk and prevent death in common conditions like coronary disease, hypertension, and stroke."
Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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