Invasive tests require the insertion of needles and other instruments into the body. Both tests reveal how well blood flows through the heart. Coronary angiography is the insertion of a thin plastic catheter through an artery in the arm or leg and up into the coronary arteries of the heart. Angiograms reveal blockages and areas of restricted blood flow. Letterman underwent emergency surgery because his coronary angiography revealed severe blockages that were deemed life-threatening.
Cardiac catheterization is the same process as angiography, only no dye is used.
The cardiac catheter can measure blood pressure and blood oxygen levels as well as indicate the strength of the heart's pumping ability. A thallium stress test utilizes a treadmill. A special camera then takes images of the heart. If an area of the heart is damaged or blood flow is restricted, the thallium will not show up as expected. If no thallium is present, it means the heart muscle cells in that area are dead, possibly from a previous heart attack.