Each year hundreds of thousands of Americans, including some 700,000 Medicare recipients, get on a treadmill not for exercise but to try to determine if their hearts are healthy.
Tim Russert, the NBC journalist, had such an exam, called an exercise or treadmill stress test, six weeks before he died of a heart attack in June at age 58. His results had been deemed normal, prompting people to question how worthwhile this test could be.
Two weeks before Russert died, Dr. Todd Miller, a cardiologist and co-director of the Mayo Clinic's Nuclear Cardiology Laboratory in Rochester, Minn., published an assessment of the test's ability to detect potentially life-threatening cardiac problems.
The test is meant to be used “almost exclusively” for people who have symptoms of heart disease, Miller said in an interview.
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“But in the real world,” he said, “it is often used as a screening test for people without symptoms who are worried about their risk. The accuracy of the test depends on whom it is used. It is most accurate among populations with a high prevalence of coronary disease.”
Limitations and advantages
In fact, this test is unable to detect the kind of problem that caused Russert's death – a plaque within the wall of a coronary artery that ruptured, resulting in a clot that set off a rapidly fatal heart rhythm abnormality. If not for the rhythm disturbance, Russert would have had a far greater chance of surviving his heart attack, said Miller, who was his doctor.
“Maybe three patients in 1,000 with a low-risk test will die from heart disease within a year,” he said. “Among those deemed at high risk, more than three patients in 100 would die within a year.”
The stress test's main advantages are its rapidity and low cost – one-fifth to one-quarter the cost of more definitive and often more time-consuming tests like a nuclear stress test, CT coronary angiogram or standard angiogram. Medicare pays about $150 for a standard stress test, though hospitals typically charge three to four times that when the test is done on younger patients.
The test has no value unless its findings are interpreted in the context of a person's other risk factors for heart disease: age, sex and heart disease symptoms, as well as smoking, being overweight, hypertension, high cholesterol, diabetes and family history.
Interpreting the results
During a treadmill test, patients are hooked up to an electrocardiogram machine (often abbreviated EKG, for the German spelling) that records the workings of the heart as the duration, speed and difficulty of the exercise increase.
Measurements taken during and immediately after the workout are indicators of cardiovascular fitness and how well a person's autonomic nervous system is functioning. Doctors used to rely mainly on an EKG finding to indicate heart trouble. But scores of studies have zeroed in on other, more reliable findings. Exercise duration has the strongest prognostic value, Miller said.
Even people who have three diseased coronary arteries can be expected to survive four years or more if they can stay on the treadmill for 12 or more minutes, a study in the 1980s of 4,083 patients with symptoms of heart disease showed.
But duration on the treadmill may be limited by lack of physical fitness, back problems or other unrelated disorders, leading to other, more expensive evaluations.
“When a middle-age couch potato is done in after only three minutes on the treadmill, you scratch your head,” Miller said. “Is this heart disease or just deconditioning?”