Electrocardiogram (EKG) / Stress Test / Holter Monitor

What is an electrocardiogram?

An electrocardiogram (commonly referred to as EKG or ECG), 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 EKG from the normal tracing can indicate one or more of several heart-related conditions. Conditions that are not heart conditions may also cause changes in the EKG.

To better understand the EKG, it is helpful to understand the heart's electrical conduction system.

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The heart's electrical 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 in order to function. The heart's pumping energy comes from an intrinsic electrical conduction system.

How does the heart beat?

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 periodically (60-100 times per minute under normal conditions). 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 chambers to contract and pump out blood. The right and left atria (the 2 upper chambers of the heart) are stimulated first and contract a short period of time before the right and left ventricles (the 2 lower chambers of the heart). The electrical impulse travels from the sinus node to the atrioventricular (AV) node, where it stops for a very short period, then continues down the conduction pathways via the bundle of His into the ventricles. The bundle of His divides into right and left pathways to provide electrical stimulation to both ventricles.

Normally, as the electrical impulse moves through the heart, the heart contracts about 60 to 100 times a minute. Each contraction 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.

Under some conditions, almost all heart tissue is capable of starting a heartbeat, or becoming the pacemaker. An arrhythmia occurs when:

• the heart's natural pacemaker develops an abnormal rate or rhythm
• the normal conduction pathway is interrupted
• another part of the heart takes over as pacemaker

How does the physician know what an EKG means?

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Almost everyone knows what a basic EKG tracing looks like. But what does it mean?

• The first little upward notch of the EKG tracing is called the "P wave." The P wave indicates that the atria (the 2 upper chambers of the heart) are contracting to pump out blood.
  
  
• 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 2 lower chambers of the heart) are contracting to pump out blood.
  
  
• The next short upward 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 rest period before the ventricles begin to contract for the next beat.
  
  
• The next upward curve is called the "T wave." The T wave indicates the resting period of the ventricles.

When your physician studies your EKG, he/she looks at the size and length of each part of the EKG. Variations in size and length of the different parts of the tracing may be significant. The tracing for each lead of a 12-lead EKG 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.

Why is an EKG done?

Many conditions can cause changes to the EKG. Because the EKG 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. EKG's are also done with routine physical examinations so that comparisons can be made with previous EKG's to determine if a hidden or undetected condition might be causing changes in the EKG. Some conditions which may cause changes in the EKG pattern may include, but are not limited to, the following:

• ischemia - decreased flow of oxygenated blood to an organ due to obstruction in an artery.
  
  
• heart attack - also called myocardial infarction; damage to the heart muscle due to insufficient blood supply.
  
  
• conduction disorders - a dysfunction in the heart's electrical conduction system, which can make the heartbeat 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 of the sac (thin covering) that surrounds the heart.
  
  
• valvular heart disease - one or more of the heart's four valves becomes defective, or may be congenitally malformed.
  
  
• enlarged heart - a condition of the heart in which it is abnormally larger than normal; can be caused by various factors, such as valve disorders, high blood pressure, congestive heart failure, conduction disturbances, etc.
  
  
• chest trauma - blunt trauma to the chest, such as a motorist hitting the steering wheel in an automobile accident.

NOTE: 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 EKG pattern.

An EKG may also be done for the following reasons:

• to obtain a baseline tracing of the heart's function (during a physical examination). This baseline tracing may be used later as a comparison with future EKG's, to see if any changes have occurred.
  
  
• as part of a work-up prior to a procedure such as surgery to make sure a heart condition does not exist 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 an MI, or after a heart-related procedure such as a cardiac catheterization, heart surgery, electrophysiological studies, etc.

How is an EKG done?

An EKG is one of the simplest and fastest procedures used to evaluate the heart. An EKG technician, nurse, or physician will place 12 separate electrodes (small plastic patches) at specific locations on your chest, arms, and legs. Eight of the electrodes will be placed on your chest, and one electrode will be placed on each arm and leg. The electrodes may be self-sticking, or a gel may be applied to make the electrodes adhere to the skin. You will be lying down on a stretcher or bed, and the leads (wires) will be connected to the electrodes on your skin. You will need to lie very still and not talk during the EKG 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 EKG 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 EKG procedures which are more involved than the basic EKG. These procedures include the following:

• Exercise EKG, or stress test: The patient is attached to the EKG machine as described above. However, rather than lying down, the patient exercises by walking on a treadmill or pedaling a stationary bicycle while the EKG is recorded. This test is done to assess changes in the EKG during stress such as exercise.

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• Signal-average EKG: This procedure is done in the same manner as a resting EKG, except that the heart's electrical activity is recorded over a longer period of time, usually 15 to 20 minutes. Signal-average EKG is done when arrhythmia is suspected but not seen on a resting EKG, since arrhythmias may be transient in nature and not seen during the short recording time of the resting EKG.
  
  
• Holter monitor: A Holter monitor is an EKG recording done over a period of 24 or more hours. Three electrodes are attached to the patient's chest and connected to a small portable EKG recorder by lead wires. The patient goes about his/her usual daily activities (except for activities such as taking a shower, swimming, or any activity causing an excessive amount of sweating which would cause the electrodes to become loose or fall off) during this procedure. There are 2 types of Holter monitoring:
  
  
  • continuous recording - the EKG is recorded continuously during the entire testing period.
    
    
  • event monitor, or loop recording - the EKG is recorded only when the patient starts the recording, when symptoms are felt.

  Holter monitoring may be done when arrhythmia is suspected but not seen on a resting or signal-average EKG, since arrhythmias may be transient in nature and not seen during the shorter recording times of the resting or signal-average EKG.

Holter Monitor

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