For the estimated 500,000 people who are too sick or too disabled to undergo an exercise stress test -- the gold standard of cardiac measurement -- there's a new option: a drug that simulates the effects of exercise on the heart.

The Food and Drug Administration approved the drug, dipyridamole in December for use in exercise testing. Stress or exercise tests monitor heart function as patients walk and run on a treadmill. During the last minute of a stress test, doctors often administer an injection of the drug thallium and then take a picture of the heart using a camera that measures thallium emission. Areas that lack thallium are not getting enough oxygen and often signal troublesome blockages.

Dipyridamole mimics the effect of exercise on the heart by increasing blood flow. By teaming the drug with thallium, doctors can detect abnormalities in blood flow.

"This adds an enormous new innovation for diagnostic cardiology," said Jeffrey A. Leppo, director of nuclear cardiology at the University of Massachusetts Medical Center in Worcester.

Without dipyridamole, Leppo said, the only option for disabled patients was angiography -- an expensive and invasive test that involves threading a tube near the heart and injecting dye to detect arterial blockages.