What is an electrocardiogram (ECG)?
An electrocardiogram (ECG or EKG) is one of the simplest and fastest procedures used to evaluate the heart. Electrodes (small, plastic patches) are placed at certain locations on your chest, arms, and legs. When the electrodes are connected to the ECG machine by lead wires, the electrical activity of your heart is measured, interpreted, and printed out for the physician's information and further interpretation.
Why is an ECG performed?
The electrical activity of the heart is measured by an electrocardiogram. By placing electrodes at specific locations on the body (chest, arms, and legs), a graphic representation, or tracing, of the electrical activity can be obtained. Changes in an ECG from the normal tracing can indicate one, or more, of several heart-related conditions.
Some medical conditions which may cause changes in the ECG pattern include, but are not limited to, the following:
- conditions in which the heart is enlarged - these conditions can be caused by various factors, such as congenital (present at birth) heart defects, valve disorders, high blood pressure, congestive heart failure, or conduction disturbances.
- ischemia - decreased blood flow to the heart muscle due to clogged or partially-clogged arteries.
- conduction disorders - a dysfunction in the heart's electrical conduction system, which can make the heart beat too fast, too slow, or at an uneven rate.
- electrolyte disturbances - an imbalance in the level of electrolytes, or chemicals, in the blood, such as potassium, magnesium, or calcium.
- pericarditis - an inflammation or infection of the sac which surrounds the heart.
- valve disease - malfunction of one or more of the heart valves may cause an obstruction of the blood flow within the heart.
- chest trauma - blunt trauma to the chest, such as a motorist hitting the steering wheel in an automobile accident.
This list is presented as an example. It is not intended to be a comprehensive list of all conditions which may cause changes in the ECG pattern.
An ECG may also be performed for other reasons, including, but not limited to, the following:
- during a physical examination to obtain a baseline tracing of the heart's function (This baseline tracing may be used later as a comparison with future ECGs, to see if any changes have occurred.)
- as part of a work-up prior to a procedure such as surgery to make sure no heart condition exists that might cause complications during or after the procedure
- to check the function of an implanted pacemaker
- to check the effectiveness of certain heart medications
- to check the heart's status after a heart-related procedure such as a cardiac catheterization, heart surgery, or electrophysiological studies
How does the physician know what an ECG means?
Almost everyone knows what a basic ECG tracing looks like. But what does it mean?
- The first little upward notch of the ECG tracing is called the "P wave." The P wave indicates that the atria (the two upper chambers of the heart) are electrically stimulated to pump blood to the ventricles.
- The next short, flat segment is called the "PR interval." This represents the delay in the propagation of the electrical signal from the atria to the ventricles.
- The next part of the tracing is a short downward section connected to a tall upward section. This next part is called the "QRS complex." This part indicates that the ventricles (the two lower chambers of the heart) are electrically stimulated to pump out blood.
- The next short flat segment is called the "ST segment." The ST segment indicates the amount of time from the end of the contraction of the ventricles to the beginning of the "T wave."
- The next upward curve is the T wave. The T wave indicates the recovery period of the ventricles.
When your physician studies your ECG, he/she looks at the size and length of each part of the ECG. Variations in size and length of the different parts of the tracing may be significant. The tracing for each lead of a 12-lead ECG will look different, but will have the same basic components as described above. Each lead of the 12-lead is "looking" at a specific part of the heart, so variations in a lead may indicate a problem with the part of the heart associated with the lead.
What is the procedure for an ECG?
An ECG can be performed almost anywhere, as the equipment is very compact and portable. Thus, you may undergo an ECG in a physician's office, the ECG department of the hospital or clinic, in a procedure or testing area, in the emergency department, or even in the hospital room or bed. The equipment used includes the ECG machine, skin electrodes, and lead wires which attach the electrodes to the ECG machine.
An ECG normally takes approximately five to 10 minutes, including attaching and detaching electrodes. During an ECG:
- You will lie flat on a table or bed for the procedure.
- The ECG technician will need to have your chest uncovered in order to perform the test. Your privacy will be ensured by covering you with a sheet or gown and exposing only the necessary skin.
- Electrodes (small, plastic patches) will be attached to your chest and one electrode will be attached to each arm and leg.
- The lead wires will be attached to the skin electrodes.
- Once the leads are attached, the technician may key in identifying information such as your name and age into the machine's computer.
- The ECG is started. It will be important for you to lie still and not talk during the procedure, so as not to interfere with the tracing. At this point, it will take approximately five more minutes (or less) for the tracing to be completed.
- Once the tracing is completed, the technician will disconnect the leads and remove the skin electrodes.
Depending on the results of the ECG, additional tests or procedures may be scheduled to gather further diagnostic information.