A coronary artery must be narrowed to less than 30% of its original size before there is a serious reduction in the blood flow to the heart muscle served by that vessel. Generally, about 5% of the total cardiac output of blood goes through the coronary arteries; thus there is adequate coronary blood flow to meet normal demands at rest even if the vessels are 70 to 90% occluded.
If the coronary arteries are seriously blocked, however, blood flow may not be adequate for any increased demand, such as that of exercise or an emotional upset. If the heart muscle cannot get enough oxygen—a state known as myocardial ischemia—symptoms such as chest pain (angina) or shortness of breath may result.
A presumptive diagnosis of coronary disease is based on a review of symptoms, health history, an electrocardiogram, and an exercise stress test, perhaps with a thallium scan. A more definitive diagnosis requires cardiac catheterization and angiography.
During an exercise stress test, the patient is hooked up to an electrocardiographic monitor (an ECG or EKG machine) and then asked to walk on a treadmill, peddle a stationary bicycle, or climb steps. The ECG monitor will show whether the heart muscle is getting enough blood. An exercise test also detects silent ischemia, a condition with no symptoms in which heart muscle does not get enough blood.
If severe narrowing is suspected, a coronary angiogram may be needed. This examination entails threading a catheter through a blood vessel into the heart, and then injecting a dye into the coronary arteries to make them visible on x-rays.