Wednesday, February 22, 2006

Heart Tests: Stress Electrocardiography

This is part seventeen of the heart of the matter. It's an ongoing series which we hope readers find educational. In part 16 we covered the echocardiogram and it can be found here. Our disclaimer can be found here.

A stress ECG is probably the most popular and frequently used test to diagnose heart artery narrowing. It is reliable and reproducible, having stood the test of time. It's physiological principles are easily understood giving it a very wide appeal. It is relatively cheap and yet has a predictive accuracy of about 70-80%, when done for subjects at risk of coronary artery disease. The more coronary risk factors the subject has, the more accurate the test, reaching almost 90-95% in patients who has anginal type chest pains.

Since this test is time tested, the machines are cheaper and the test is widely available. It has almost no risk if the subjects are asymptomatic and the resting ECG normal. The test results are easily understood by many practitioners and there is almost no mystery to the test.

The stress test basically uses the ECG as an indicator of sufficiency of heart muscle blood flow. We have learned and validated the fact that insufficiency of heart muscle blood flow shows up in the ECG as ST segment change. The ST segment in an ECG is an indicator of heart muscle blood flow. A normal ST segment, signifies adequate blood flow, and a depressed ST segment, signifies inadequate blood flow. When there is blockage in a heart artery, the resting blood flow is sufficient for the resting heart, so the ST segment is normal. During stress, there is demand for more blood flow. Because of the narrowing, the supply of blood cannot meet the heart muscle demands, and the ST segment gets depressed. Simple and very straight forward.

However, if the area supplied by the narrowed artery is relatively small, the ECG detectors can miss. False positives and false negatives are occur with stress ECG test. But overall, for a cheap, clinically reliable, very low-risk cardiac test, with good predictive value (especially in the population at risk) the stress ECG is probably the best.

This picture illustrates how a normal ECG doesn't tell the whole story and the stress ECG can reveal more hidden facts.

In the next part we look at radionuclide scans.

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