- Anatomy of the heart (wpclipart.com)
A standard ECG is called a "12-Lead" ECG, referring to the use of 12 carefully placed electrodes which measure electrical activity in different areas of the heart. By measuring electrical activity at these 12 points, a detailed and intricate view of the health and activity of the heart can be seen.
The rate at which your heart beats is set by what is often called the body's "natural pacemaker," the sinus node. A collection of specialized heart cells that operates without requiring control from the brain, the sinus node sends electrical signals to the muscles of the heart which cause it to beat. The ECG measures this activity as a series of waves, called P, QRS and T.
P, QRS, and T waves
- P, QRS and T waves in an ECG printout (Anthony Atkielski, public domain)
Most of the waves in an ECG describe the contraction of the four chambers of the heart: the left and right atria, and the left and right ventricles. Contractions of the left atrium and right atrium are displayed as the P wave. Contractions of the right and left ventricles make up a series of three waves that is called the QRS complex. The electrical activity that is produced when the ventricles are recharging for the next contraction is called the T wave. This process is also called repolarization. Each electrode records these same waves, but from a different point of view. By combining the information from all 12 electrodes, doctors can determine how the heart muscles are functioning and any potential problems.
Getting an ECG
- Locations of ECG electrodes (public domain)
The process of having an ECG is among the simpler medical procedures that you will ever undergo. The doctor or technician will have you lie down and will attach six electrode pads at different points along your chest and six pads at points on your arms and legs. This diagram shows the positioning of the different electrodes. As you lie still, the ECG machine will record the electrical activity from a few heartbeats, which is then printed on a chart like the one pictured below, or sometimes displayed on a computer. An additional reading with a single electrode may also be taken to show changes in the heart rhythm over a longer period of time. This reading is called a "rhythm strip."
Variations on the Standard ECG
- Example ECG printout
In cases where heart problems are intermittent or come and go, a standard ECG may be perfectly normal because it takes place over such a short period of time. When more extended monitoring is desired, an ambulatory ECG is done which can monitor a your heart constantly over a period of 24 hours or more, or can be used for shorter periods off and on over a period of several days. This long-term ECG has a much better chance of recording any intermittent abnormalities or problems, and enables your doctors to look at how your heart performs at different times and during different activities.
In other cases, the heart performs well at rest, and a standard ECG that is done while you are lying down and resting will appear normal. Your doctor may do a treadmill test, in which you are given a continuous ECG while walking or running on a treadmill. This test will identify any problems that appear only during exertion, when your heart has to work much harder to pump blood through your body, especially to your arms and legs. A treadmill test can identify problems like blockages in coronary arteries that can lead to a heart attack if not treated.
- For more information on ECG, the information generated during an ECG, and how the output of the ECG machine helps your doctor find problems with your heart, take a look at the links in Sources and Resources, at the end of this article. For an interactive look at the ECG process, be sure to check out the ECG game at Nobel Prize, linked below. This is a great way for both kids and adults curious about the ECG to see how the motion of the heart translates into the graph created during an ECG.