Performing cardiac stress tests that measure exercise capacity and heart rate recovery can improve dramatically on existing techniques that predict who is most likely to suffer a heart attack or die from coronary heart disease (CHD), the leading cause of death in the United States, a team of cardiologists at Johns Hopkins reports.
In the Sept. 13 edition of the journal Circulation, the Hopkins team reports that 90 percent of men and women with no early signs of CHD who, nevertheless, died from it had had below average results from their cardiac stress tests conducted 10 to 20 years earlier.
The team’s analysis showed these asymptomatic people were two to four times more likely to die from CHD within 10 to 20 years than people with average or better-than-average stress test results, even though traditional scoring for major risk factors for the disease, such as such as age, blood pressure, blood cholesterol levels and smoking status, had determined the asymptomatic people to be at low or intermediate risk of having heart problems.
According to the cardiologists, these exercise stress tests are easy to perform, lasting less than 20 minutes and requiring only that a person walk on a treadmill at progressively higher speeds and inclines every three minutes until they become markedly fatigued. During the test, people are hooked up to a heart monitor.
"This is the strongest evidence to date that selective use of cardiac stress testing improves prediction of who is really at high risk of suffering a fatal heart attack when traditional risk assessment suggests they are not at high risk of a heart attack within the next 10 years," says senior study author and cardiologist Roger S. Blumenthal, M.D., an associate professor and director of the Ciccarone Preventive Cardiology Center at The Johns Hopkins University School of Medicine and its Heart Institute.
The traditional risk factors combine to give a score called the Framingham Risk Score, or FRS, that was developed in the last 20 years. Considered the gold standard, the score is based on a summary estimate of the major risk factors for heart disease: age, blood pressure, blood cholesterol levels and smoking status. It consists of a percentage range of how likely a person is to suffer a fatal or nonfatal heart attack within 10 years.
However, Blumenthal says that many people, especially women, with cardiovascular problems go undetected despite use of the Framingham score, which does not factor in a person’s family history, weight or exercise habits. Blumenthal is also a spokesman for the American Heart Association, which estimates that 656,000 Americans died from CHD in 2002, the last year for which statistics are available.
More than 6,100 people took part in the study, conducted from 1972 to 1995, and part of a larger project known as the Lipid Research Clinics Prevalence Study. All participants in this smaller Hopkins study were age 30 to 70. None had early signs of heart disease, but every participant did have at least one major risk factor for it.
At 10 medical centers across the United States, study participants were given a physical examination, had blood tests performed and were scored on the FRS. Each participant also underwent cardiac stress testing, which included stress testing for exercise capacity and heart rate recovery, plus any changes in the heart’s electrical signaling that are typical of decreased blood flow to the heart muscle.
Those with a Framingham score of less than 10 percent were gauged to be at low risk for future CHD, while participants with a score between 10 percent and 20 percent were ranked at intermediate risk for future CHD, and those with a score higher than 20 percent were judged to be at high risk of CHD.
Once participants were ranked by Framingham score, the researchers monitored their health every six months until death or the end of the study to find out who did or did not die from a heart attack or CHD.
Cardiac stress testing is used to gauge how well the heart works when it has to pump harder and use more oxygen, for example, while walking on a treadmill. The exercise, sustained for five to 10 minutes, mimics the strain placed on the heart when arteries are blocked or narrowed.
The researchers goal, however, was to determine if more accurate prediction of whether or not a person will die from a heart attack could be made by adding exercise capacity and heart rate recovery to current assessment techniques that relied mostly on monitoring the heart’s electrical signaling.
During stress testing, a person’s breathing, blood pressure and heart rate are monitored while the intensity of their exercising is slowly increased to see how their heart responds. The amount, in number of beats per minute that the heart rate drops two minutes after exercise stops, is also recorded to determine heart rate recovery.
Using tables that take into account a person’s age, gender and weight, the results can be compared against average scores to see if a person is below, at or above the norm. There is very little risk of harm associated with the testing because participants are closely monitored.
The researchers report that 246 participants died from CHD even though they had initially been categorized by their FRS as at either low or intermediate risk of the disease. However, 225 of those who died also had below average test scores for exercise capacity and heart rate recovery.
"Our best means of preventing coronary heart disease is to identify those most likely to develop the condition and intervene before symptoms appear," says the study’s lead author, cardiologist Samia Mora, M.D., M.H.S., then a research fellow at Hopkins.
"Cardiac stress testing could significantly improve our abilities to find and aggressively treat these people so that they are much less likely to suffer a heart attack."
According to the researchers, these latest results support conclusions from earlier this year that traditional risk assessment with the FRS can be improved with selective use of cardiac CT scans to measure calcium scores in individuals with more than one risk factor, such as obesity, smoking, sedentary lifestyle or a family history of heart disease.
Funding for the study was provided by the Maryland Athletic Club Charitable Foundation in Lutherville, Md.
Other researchers involved were Rita Redberg, M.D., M.Sc., from the University of California in San Francisco; and A. Richey Sharrett, M.D., Dr.P.H., from Hopkins.
