Harvard Health Letter

Understanding the ECG: Reading the waves

The electrocardiogram (ECG) is one of the most common, enduring, and important tests in all of medicine. It's easy to perform, noninvasive, produces results right away, and is useful in diagnosing dozens of heart conditions. The ECG has taken on even more importance lately because a particular ECG pattern, called ST elevation, is a strong indication that a serious heart attack has occurred, and there's more emphasis than ever on treating heart attacks as soon as possible. An ECG isn't necessarily going to be part of a routine physical, but if you need medical attention because you have chest pain, sudden unexplained shortness of breath, or other symptoms that suggest a possible heart attack, you will almost certainly get an ECG.

The ECG is a reading of the electrical impulses in the heart that activate the heart muscle and its blood-pumping action. Twelve electrodes affixed to the skin on the chest, arms, and legs sense those impulses from various vantage points. Part of the reason the ECG has had such staying power is that the output is visual: a line graph with peaks and valleys, not a stream of numbers. As a result, reading an ECG is a matter of pattern recognition, not computation. There are many permutations, but someone can be trained to recognize the most common patterns relatively quickly.

During a normal heartbeat, the electrical activity starts in a small patch of pacemaker cells called the sinus node. When the impulse activates the atria, it produces a small blip called the P wave (see the illustration below). Next it activates the main pumping chambers, the ventricles, and produces the big up-and-down in the middle, the QRS complex. The final T wave is a recovery period as the impulse reverses and travels back over the ventricles.

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