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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.

If the heart is beating normally, the whole cycle takes about a second (roughly 60 heartbeats per minute).

illustration of heart rhythm


illustration of normal heart rhythm reading on ECG

The P wave, QRS complex, and T wave occur in sequence in a regular pattern.


illustration of heart rhythm reading on ECG showing angina

Angina is chest pain from the heart muscle working without getting enough blood and oxygen. The ST segment dips down instead of being flat (see arrow).

Serious heart attack

illustration of heart rhythm reading on ECG showing evidence of serious heart attack

The ST segment of the ECG is usually flat. If it has a humped, elevated appearance, a serious heart attack is probably occurring (see arrow). The medical shorthand for ST elevation is “STEMI,” so a STEMI heart attack is a serious one that needs immediate medical attention.

Atrial fibrillation

illustration of heart rhythm reading on ECG showing evidence of atrial fibrillation

Atrial fibrillation occurs when the two upper chambers of the heart move chaotically instead of pumping regularly. The P wave on the ECG disappears and is replaced by a jumpy baseline. The QRS complex occurs at “irregularly irregular” intervals.

Posted by: Dr.Health

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