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: how long the person can continue as the treadmill’s speed and incline gradually increase; whether blood pressure drops instead of rising; whether the heart rate increases to an age-appropriate level and how fast it recovers when the test ends; and whether the pumping chambers of the heart develop abnormal beats.
Doctors used to rely mainly on an EKG finding called ST-segment depression 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. “The longer the patient can keep going on the treadmill, the less likely he or she is to die soon of coronary artery disease — or of any cause,” he wrote.
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?”
MEASUREMENTS AND DIAGNOSES
Duration on a treadmill is a measure of exercise capacity — the workload the body can achieve expressed as metabolic equivalents, or METs, a multiple of the amount of oxygen the body uses at rest. The average middle-age man can reach 10 METs to 12 METs, about 2 METs more than the average woman, Miller said.
Blood pressure that is lower during the treadmill exercise than when the person is standing at rest often indicates severe coronary disease — a heart unable to meet the demand — and has been linked to a threefold increase in the risk of a cardiac event within two years, a study of 2,036 patients showed.
A normal heart beats faster during exercise and slows as soon as exercise stops. Failure of the heart rate to increase as expected, a condition called chronotropic incompetence, is associated with an increased risk of death from heart disease, a study by the Cleveland Clinic Foundation showed. The risk of death within six years is also doubled if the person’s heart rate fails to fall quickly when the treadmill stops.
Finally, various heart rhythm abnormalities may occur during the treadmill test. Though most are benign, a rhythm disturbance after the test called ventricular ectopy is linked to a somewhat raised death rate over the next five years.
The bottom line? A stress test result is only one factor in estimating a person’s risk of dying soon of heart disease. All other risk factors must be taken into account and, if possible, treated to reduce or eliminate them. To this advice Miller added, “We’d all be a lot better off if we became more active.”
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