People who have coronary artery bypass graft surgery hope the procedure will prevent heart attacks and help them live longer. But bypass, which adds blood vessels to detour around blocked arteries, is not perfect: While it can greatly improve blood flow, it may not restore circulation to all of the heart areas starved of blood. If a patient has angina, or chest pain, after bypass surgery, cardiologists usually take this as a sign that blockages may remain. In these cases, patients frequently are given a thallium stress test to determine how well the surgery improved blood flow, and whether patients need further treatment.

But no one knows how likely it is that any remaining blockages will cause future heart attacks or death. And the risks are so unclear for patients without lingering angina that cardiologists rarely test them. Now, two large studies suggest that stress testing can help predict patients’ risks after bypass — even for those without angina.

In one study, published in the March 15 Journal of the American College of Cardiology, researchers tested 411 patients — a third of whom did not have angina — within two years after bypass. The thallium test measured patients’ ability to exercise and detected areas of good and bad blood flow, which usually indicate healthy and damaged heart tissue (for more on thallium stress testing, see the March/April Heart Watch).

Two abnormalities detected by the test were associated with a much higher risk of heart attack or heart-related death: angina brought on by the exercise, and moderately to severely blocked blood flow in multiple areas of the heart. The researchers found that nearly a third of people with both abnormalities died or had a nonfatal heart attack within six years of stress testing, compared with only 7 percent of patients without either abnormality. People with only one of the two anomalies fared somewhere in between (see graph).

Unlike the first study, the second, published in the February 28 Lancet, included only patients without angina after bypass. Still, among the nearly 900 patients tested about six years after bypass, two abnormalities similar to those in the first study determined patients’ risk of heart attack or cardiac death.

Nineteen percent of those who became winded before reaching their target heart rate or felt angina during exercise had a nonfatal heart attack or died of heart disease within three years of the test, compared with 5 percent of patients who exercised without pain or excessive effort. And 13 percent of patients with blood flow abnormalities had a heart attack or died of heart disease within three years, compared with 7 percent of patients with normal blood flow.

These studies provide new information that could help doctors decide who needs further treatment after bypass.

Thallium Stress Test Predicts Heart Health After Bypass

Patients with better thallium stress test results were less likely to have a nonfatal heart attack or die of heart disease than patients with poor test results. (Note that the survival scale begins at 30 percent.)