How can I understand what I see on my electrocardiogram?

Posted May 17, 2013, 2:00 am

I recently had an electrocardiogram and my doctor gave me a copy of the tracing. Can you tell me what I’m looking at?

When the 20th century began, more than 100 years ago, doctors had no way of looking inside the body of a living person. Yet we knew from autopsies of people who had died that all of the normally invisible internal organs could become diseased. So the search was on for ways to “see” inside the body. The idea was simple: If you could spot a problem with an internal organ, you might be able to treat it and prevent future suffering.

The discovery of X-rays began what has become a dramatic improvement in our ability to make internal organs visible. X-rays could see how large the heart was. They also allowed doctors to draw some conclusions about how well the heart was working. For example, X-rays could see if blood was building up in the lungs (which happens in heart failure).

At about the same time as the discovery of X-rays, doctors invented another way of “seeing” the heart: the electrocardiogram (ECG or EKG). The heart works by producing, and responding to, electrical signals. The ECG measures those signals. It has become the most widely used test for detecting heart problems, as it’s easy to perform, noninvasive and produces results right away. If you seek medical attention because of chest pain, shortness of breath or other symptoms that suggest a possible heart attack, you’ll almost certainly get an ECG.

When you undergo an ECG, you lie down as a technician applies electrodes, or leads, to your chest, arms and legs. These leads pick up the electrical signals being given off by your heart. There are multiple leads in different positions, reading the signals from different parts of your heart. This enables doctors to find the location of possible heart damage. The ECG produces a reading, or tracing, of the electrical activity that occurs with each heartbeat. That tracing is a series of wavy black lines.

The four chambers of the heart need to beat in a coordinated fashion. They do so as the result of electrical signals caused and transmitted by special heart cells. If your heart is beating normally, the whole cycle takes about one second. (I’ve put an illustration below, showing how an ECG tracing corresponds with the phases of a heartbeat.)

An ECG produces a diagram, called a tracing, that corresponds with each phase of a heartbeat

By evaluating the ECG tracing, doctors can spot an irregular heartbeat (an arrhythmia), find out whether your heart is enlarged, identify a part of your heart that is not getting enough blood, and even detect the signs of damage from an old heart attack.

The ECG is crucial for evaluating chest pain. ECG abnormalities are often enough to diagnose a heart attack that’s in progress, allowing doctors to begin treatment. Thank goodness for the Dutch physician, Willem Einthoven, who developed the ECG; he richly deserved the Nobel Prize that he received in 1924.

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