Coronary artery disease (CAD) occurs when atherosclerotic plaque (hardening of the arteries) builds up in the wall of the arteries that supply the heart. This plaque is primarily made of cholesterol. Plaque accumulation can be accelerated by smoking, high blood pressure, elevated cholesterol, and diabetes. Patients are also at higher risk for plaque development if they are older (greater than 45 years for men and 55 years for women), or if they have a positive family history for early heart artery disease.
The atherosclerotic process causes significant narrowing in one or more coronary arteries. When coronary arteries narrow more than 50 to 70%, the blood supply beyond the plaque becomes inadequate to meet the increased oxygen demand during exercise. The heart muscle in the territory of these arteries becomes starved of oxygen (ischemic). Patients often experience chest pain (angina) when the blood oxygen supply cannot keep up with demand. Up to 25% of patients experience no chest pain at all despite documented lack of adequate blood and oxygen supply. These patients have "silent" angina, and have the same risk of heart attack as those with angina.
When a blood clot (thrombus) forms on top of this plaque, the artery becomes completely blocked causing a heart attack. Heart Attack illustration - Coronary Artery Bypass Graft Surgery
When arteries are narrowed in excess of 90 to 99%, patients often have accelerated angina or angina at rest (unstable angina). Unstable angina can also occur due to intermittent blockage of an artery by a thrombus that eventually is dissolved by the body's own protective clot-dissolving system.
The resting electrocardiogram (EKG) is a recording of the electrical activity of the heart, and can demonstrate signs of oxygen starvation of the heart (ischemia) or heart attack. Often, the resting EKG is normal in patients with coronary artery disease and angina. Exercise treadmill tests are useful screening tests for patients with a moderate likelihood of significant coronary artery disease (CAD) and a normal resting EKG. These stress tests are about 60 to 70% accurate in diagnosing significant CAD.
If the stress tests do not reveal the diagnosis, greater accuracy can be achieved by adding a nuclear agent (thallium or Cardiolite) intravenously during stress tests. Addition of thallium allows nuclear imaging of the blood flow to different regions of the heart, using an external camera. An area of the heart with reduced blood flow during exercise, but normal blood flow at rest, signifies significant artery narrowing in that region.
Combining echocardiography (ultrasound imaging of the heart muscle) with exercise stress testing (stress echocardiography) is also a very accurate technique to detect CAD. When a significant blockage exists, the heart muscle supplied by this artery does not contract as well as the rest of the heart muscle. Stress echocardiography and thallium stress tests are both at least 80% to 85% accurate in detecting significant coronary artery disease.
When a patient cannot undergo exercise stress test because of nervous system or joint problems, medications can be injected intravenously to simulate the stress on the heart due to exercise and imaging can be performed with a nuclear camera or ultrasound.