An electrocardiogram (ECG or EKG), is a measurement of the electrical activity of the heart. 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. Disorders that are not associated with heart conditions may also cause changes in the ECG.
To better understand the ECG, it is helpful to understand the heart's electrical conduction system.
The heart is, in the simplest terms, a pump made up of muscle tissue. Like all pumps, the heart requires a source of energy and oxygen in order to function. The heart's pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart.
An electrical stimulus is generated by the sinus node (also called the sinoatrial node, or SA node), which is a small mass of specialized tissue located in the right atrium (right upper chamber) of the heart. The sinus node generates an electrical stimulus regularly (60–100 times per minute under normal conditions) and is sometimes referred to as the "pacemaker" of the heart. This electrical stimulus travels down through the conduction pathways (similar to the way electricity flows through power lines from the power plant to your house) and causes the heart's lower chambers to contract and pump out blood. The right and left atria (the two upper chambers of the heart) are stimulated first and contract a short period of time before the right and left ventricles (the two lower chambers of the heart).
The electrical impulse travels from the sinus node to the atrioventricular node (also called AV node), where impulses are slowed down for a very short period, then continue down the conduction pathway via the bundle of His into the ventricles. The bundle of His divides into right and left pathways to provide electrical stimulation to the right and left ventricles.
Normally at rest, as the electrical impulse moves through the heart, the heart contracts about 60 to 100 times a minute. Each contraction of the ventricles represents one heartbeat. The atria contract a fraction of a second before the ventricles so their blood empties into the ventricles before the ventricles contract.
Almost all heart tissue, under certain conditions, is capable of starting a heartbeat, or becoming a pacemaker.
Almost everyone knows what a basic ECG tracing looks like. But what does it mean?
When your physician studies your ECG, he or 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.
Many conditions can cause changes to the ECG. Because the ECG is a fast, simple, painless and relatively inexpensive test, it may be used as a part of an initial examination to help the physician narrow the scope of the diagnostic process. ECGs are also done with routine physical examinations so that comparisons can be made with previous ECGs to determine if a hidden or undetected condition might be causing changes in the ECG. Some conditions which may cause changes in the ECG pattern may include, but are not limited to, the following:
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 done for the following reasons:
An ECG is one of the simplest and fastest procedures used to evaluate the heart. An ECG technician, nurse, or physician typically will place 12 separate electrodes (small plastic patches) at specific locations on your chest, arms, and legs. The electrodes are self-sticking and will adhere to the skin. The area where the electrodes are placed may be cleaned, or hair may need to be shaved or clipped so there is a better connection. You will be lying down on a stretcher or bed, and the leads (wires) will be connected to the electrodes. You will need to lie very still and not talk during the ECG procedure, as movement or talking may interfere with the tracing. The technician, nurse, or physician will start the tracing, which will take just a few minutes. You will not feel anything during the tracing. Once a clear tracing has been obtained, the leads and electrodes will be removed, and you will be free to continue on with your usual activities, unless directed otherwise by your physician. An ECG can indicate the presence of arrhythmias (an abnormal rhythm of the heart), damage to the heart caused by ischemia (lack of oxygen to the heart muscle) or myocardial infarction (MI, or heart attack), a problem with one or more of the heart valves, or other types of heart conditions.
There are additional ECG procedures which are more involved than the basic ECG. These procedures include the following:
Holter monitoring may be done when arrhythmia is suspected but not seen on a resting or signal-average ECG, since arrhythmias may be transient in nature and not seen during the shorter recording times of the resting or signal-average ECG.
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