Harvard Heart Letter

Ask the doctor: Does the length of the ST segment on an electrocardiogram matter?

Ask the doctor

Does the length of the ST segment on an electrocardiogram matter?

Q. I have an electrocardiogram as part of my yearly checkup. After the last one, my doctor mentioned that my ST segment was longer this year than it was last year. He recommended that I have a stress test to check this out. I passed with flying colors. When I asked the cardiologist who did the stress test about the ST segment, he said the length isn't really important, that the height and shape are what matter. Can you explain?

A. An electrocardiogram (ECG) measures the heart's electrical activity. The waves that appear on it are labeled P, QRS, and T. Each corresponds to a different part of the heartbeat. The ST segment represents the heart's electrical activity immediately after the right and left ventricles have contracted, pumping blood to the lungs and the rest of the body. Following this big effort, ventricular muscle cells relax and get ready for the next contraction. During this period, little or no electricity is flowing, so the ST segment is even with the baseline or sometimes slightly above it.

The faster the heart is beating during an ECG, the shorter all of the waves become. It is possible that your heart was beating faster during last year's ECG than it was during this year's test.

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