Electrocardiography is used to detect heart-function abnormalities. It indirectly detects the heart's electrical activity over time by measuring the electrical potential at the body's surface. If the heart's activity varies from normal, signs of this are seen in the surface electrical potentials. The electrocardiography unit produces a visual representation of the electrical potential, called an electrocardiogram (EKG or ECG), that is often recorded as a continuous line along a strip or special graph paper. When used to diagnose a heart ailment or check the effectiveness of a heart treatment, doctors, nurses, and other technicians read the strips, looking for telltale signs of various cardiac problems.
Because the electrical activity of the heart is the basis for its workings, many heart problems show up in an EKG tracing. The machine can detect coronary artery disease, where the blood vessels carrying blood to the heart have hardened and no longer work effectively; a heart attack, either current or previous; and arrhythmias, a heart beating at an abnormal speed or rhythm.
The electrocardiography unit is a machine that transfers the very faint electrical signals of the heart into a visual representation of that activity. The unit commonly includes multiple electrodes and leads (often 12, but as few as three), a galvanometer to measure the electrical signal, an amplifier and filter to convert the faint electrical signal to one that can be seen, a computer screen or oscilloscope to display the output, and an ink-and-paper arrangement to produce hard copies of the signal.
In the simplest arrangement, three pairs of bipolar electrodes, where one is positively charged and the other negatively charged, are placed on particular areas of the patient's body. The electrodes are adhesive pads filled with conductive gel that are attached to the patient's skin. Wires called leads connect the electrodes to the unit. The electrical signal measured by each group of electrodes is also called a Lead. When used to identify the signal, the term is often capitalized.
The first pair of electrodes has the positive electrode placed on the left arm and the negative on the right arm; this produces Lead I. The second pair has the positive electrode placed on the left leg and the negative on the right arm; this produces Lead II. Lead III comes from the
Finer measurements of electrical potential can be made if additional sets of electrodes are used. The first additional set of electrodes is known as the augmented connections because the signals are significantly weaker than Leads I–III and have to be additionally increased, or augmented, by the machine. These Leads are unipolar, meaning that they measure the electrical difference between that electrode and a group of others. Lead aVR measures the signal between the right arm (the positive electrode) and the average of the signals from the left arm and the left leg (two negative electrodes). Leads aVL and aVF are similarly arranged, with the left arm and the left leg having the positive electrode, respectively. All six of these limb leads measure electrical activity in the frontal plane of the heart, through the middle from top to bottom.
A second additional set of electrodes are the unipolar chest Leads, also known collectively as the modified chest Lead (MCL). These Leads measure electrical potentials across the horizontal plane of the heart. They are unipolar, measuring the electrical difference between the positive electrode and the average of the collective signal from the right arm, left arm, and left leg. There are six leads placed across the chest, numbered V1–V6 from the patient's right to left. V1 and V2 are put on either side of the sternum, in the fourth intercostal space (the space between the fourth and fifth rib). V4 is placed in the fifth intercostal space (between the fifth and sixth rib) on the line that divides the clavicle in half. V3 is placed halfway between V2 and V4. Both V5 and V6 are placed horizontal to V4, with V5 on the line that runs down the body from the inner armpit and V6 on the line that divides the armpit.
When the electrodes are placed as described, the electrical activity of the heart is printed in line patterns known as waves or waveforms. Waves come in two types—positive deflection (movement above the baseline or isoelectric line) and negative deflection (movement below the isoelectric line). Positive deflection is created when electrical activity flows toward the positive electrode; a negative deflection is produced when current flows away from the positive electrode (toward the negative). No heart activity produces a baseline or isoelectric waveform. The isoelectric line is normally the beginning and ending of all waveforms.
EKGs are recorded on strips of graph paper that are fed through the machine at a constant rate (25 mm/sec or 1 in/sec) to allow for easy estimates of beats per time period and for points of comparison between the isoelectric line and the wave. At this standard feed rate, each small block of the graph paper represents 0.04 seconds, each larger dark box (having a 5 × 5 group of small boxes within it) is 0.2 seconds.
The normal heartbeat begins with an electrical impulse in the part of the heart with the fastest innate beat, the sinoatrial (SA) node. The electrical activity travels through the heart tissue, in a process known as depolarization, from the upper right of the organ to the lower left. Five major waves are produced: the P wave; the Q, R, and S waves (known as the QRS complex); and the T wave.
The P wave results from the depolarization of both atria and is a rounded, upward deflection that usually lasts about 0.10 seconds (about two small blocks of graph paper). The PR interval (PRI) is the time needed for the electrical impulse to travel from the atria to the ventricles. Normally, this lasts about three to five small squares (or 0.12 to 0.20 seconds).
The QRS complex has three recognized events and is the conduction of the impulse through the bundle of His and throughout the ventricles and atrial repolarization. The first downward deflection after PRI is the Q wave. It is followed by the largest deflection seen, the upward deflection of the R wave. Immediately after the R wave is a downward deflection called the S wave. The QRS complex generally happens in less than 0.12 seconds (three small squares) and all three waves are not always present, even in people with normal heart function.
The time interval between ventricular depolarization and repolarization is known as the ST segment and it is normally isoelectric (baseline). The full cycle is completed with the T wave, which is the result of the ventricles repolarizing. This wave is often a slightly asymmetrical, rounded positive deflection that finishes at the baseline.
To perform a resting EKG, the patient is placed on a table and the 12 electrodes are attached as described above. Sometimes, to improve connection, the areas of the skin where the electrodes will be placed are shaved or have conductive gel applied. Because some types of heart conditions are only evident when the heart is under stress, EKG analysis can also be performed with the patient on a treadmill.
Sometimes a patient's symptoms occur at unpredictable intervals and are not exercise related. Heart activity can then be followed by a special portable EKG machine known as a Holter monitor. This EKG has three electrodes and stores the information for the monitoring period (generally from 24 hours to five days).
Diagnosing heart problems
In general, there are five aspects of the EKG that can reveal potential or present heart abnormalities: the heart rate, the heart rhythm, the P wave, the PR interval, and the QRS complex.
The heart rate is determined by counting the number of QRS complexes (for ventricular rate) or P waves (for atrial rate) over six seconds (30 large boxes on the graph paper). Normal is between 60 to 100 beats per minute (bpm). Less than 60 bpm is considered a slow or brady-cardic rate and greater than 100 bpm is considered a fast, or tachycardic rate.
The heart rhythm, as revealed by the waveform pattern, can be classified either regular or irregular. To determine whether the ventricular rhythm is regular, a measurement is made from R-to-R wave. A measurement from P-to-P wave determines the regularity of the atrial rhythm. If the interval is the same between waves, the rhythm is regular, if different, the rhythm is irregular.
Numerous changes in the P wave, PR interval, and QRS complex are possible. They depend on the actual damage to the heart, such as those that accompany a heart attack (myocardial infarction or MI). First, as the heart becomes ischemic, or starved for oxygenated blood, repolarization of the ventricles becomes abnormal; this depresses the ST segment more than 1 mm below baseline and the T wave becomes inverted. Next, if there is no treatment of the ischemia, actual damage to the heart tissue will occur. This can be seen through an elevation of the ST segment of more than 1 mm above baseline.
If the heart attack actually occurs (one or more coronary arteries becomes completely blocked), at least three possible indications can appear on the EKG. First, if it hasn't already happened, the ST segment will become elevated and the T wave will invert. Changes in the ST segment will remain for up to four weeks after the attack and the T wave could remain inverted for a year. If all
Bipolar—A type of lead having one positive and one negative electrode
Bradycardia—An abnormally slow heartbeat.
Bundle of His—A group of special heart muscle fibers that transmit electrical impluses to the ventricles, beginning the contractions that pump blood into the aorta and pulmonary artery.
Depolarization—The movement of an electrical charge through nerve or muscle tissue, changing its voltage.
Einthoven's triangle—The triangular arrangement of EKG electrodes on a patient, generally including the right arm, the left arm, and the left leg.
Electrode—The point of connection between the EKG unit and the patient.
Isoelectric—The baseline electrical level of the body.
Lead—A conductive connector between the electrode and the EKG unit or the signal derived from a group of electrodes.
Repolarization—The process in which a nerve or muscle cell returns to its normal electrical state after depolarization.
Tachycardia—An abnormally fast heartbeat.
Unipolar—A type of lead having one positive and multiple negative electrodes.
three of the layers of the heart have been affected by the attack, the Q wave will deflect more negatively. To be considered abnormal, the Q wave must be at least 0.04 seconds long (one small box) and be at least 25% of the height of the R wave. The Q wave will start large and shrink some over time, but will always be present after a so-called "Q-Wave" MI.
It is important to rule out artifacts as the cause of a patient's abnormal EKG. Common artifacts are patient movement, loose or defective electrodes, clammy skin, excessive chest hair, or improper grounding. The rule of thumb is to look to the patient and treat their distress, not what is showing on the monitor.
Electrocardiograph technicians are in charge of maintenance of EKG machines. Their tasks include changing graph paper and ink, maintaining the electrodes and leads, and monitoring the machine for malfunction.
Health care team roles
Specially trained assistants known as electrocardiograph technicians often operate and maintain EKG machines in larger hospital and cardiology group practice settings. In small settings, nurses and medical assistants perform the test. A doctor usually does the final interpretation of the tracing.
Many persons learn how to use an EKG machine through on the job training. However, training programs are available through outsourcing companies or in vocational and community colleges. The usual length of these college-based programs is 465 hours (four months). Program content includes classroom instruction in anatomy and physiology with an emphasis on the cardiac and vascular system, medical terminology, cardiovascular medications, patient care techniques, interpretation of cardiac rhythm, medical ethics, and a clinical practicum.
Beasley, Brenda M. Understanding EKGs: A Practical Approach. Upper Saddle River, NJ: Prentice Hall, 1999.
Dubin, Dale. Rapid Interpretation of EKGs: An Interactive Course. Tampa, FL: Cover Publishing Company, 2000.
Miracle, Vickie A. "Making Sense of the 12 Lead ECG." Nursing 99 (July 1999): 34.
Alliance for Cardiovascular Professionals. 910 Charles Street, Fredericksburg, Virginia 22401. 540-370-0102.
Yanowitz, Frank G. "The Standard 12 Lead ECG." The Alan E. Lindsay ECG Learning Center in Cyberspace. <http://medstat.med.utah.edu/kw/ecg/ecg_outline/Lesson1/index.html> (April 5, 2001).
Michelle L. Johnson, M.S., J.D.