- Author: Ethan Levine, DO; Chief Editor: Richard A Lange, MD, MBA more...
The electrocardiogram (ECG) has grown to be one of the most commonly used medical tests in modern medicine. Its utility in the diagnosis of a myriad of cardiac pathologies ranging from myocardial ischemia and infarction to syncope and palpitations has been invaluable to clinicians for decades.
The image below depicts proper placement of the precordial leads for standard 12-lead electrocardiography.
See Are You Missing Subtle MI Clues on ECGs? Test Your Skills, a Critical Images slideshow, to help identify a variety of electrocardiographic abnormalities.
In this article, the authors touch briefly on the history of the ECG and then discuss in greater detail the equipment needed for ECG recording and how to record an ECG in a manner in keeping with the current standards and practices.
The ECG has its roots in the seminal work of Galvani who famously noted the contraction of frog legs when stimulated with electricity. Although he was incorrect in attributing this phenomenon to "animal electricity," it did usher in a period of research in the interaction of electricity and biological systems.
Matteucci, and later Kolliker and Muller, demonstrated that the electrical activity of a beating heart caused synchronous contractions in muscle preparations that were connected to the heart by nerve tissue.[2, 3] Gabriel Lippman developed the capillary electrometer, making possible the measurement of the minute currents in biological systems. With the Lippman electrometer, Agustus Waller performed the first ECG of a human heart at St. Mary’s Hospital in London in May of 1887.
Willem Einthoven made improvements on the ECG using the Lippman electrometer and then introduced the string galvanometer, which represented a great leap forward in electrocardiography. With Einthoven’s improvements, the now familiar P, Q, R, S, and T waves were apparent, whereas Waller’s work only demonstrated ventricular depolarization and repolarization. Interestingly, due to the size of the equipment and number of personnel needed to record the ECGs, the initial recordings were transmitted telemetrically from the physiology lab to the hospital.
Not only did Einthoven reveal the ECG waveforms as we know them today, but his convention for placement of leads is still in use. Einthoven placed leads on the right and left arms as well as the left leg, using the right leg as an electrical ground. With these leads, he demonstrated the now familiar "Einthoven’s triangle" (see image below).
One of Einthoven's students, Sir Thomas Lewis, used electrocardiography to reveal the mechanisms behind atrial fibrillation and other conduction abnormalities. In the United States, Wilson and Goldberger pioneered the use of additional leads, namely aVR, aVL, aVF, and the 6 precordial leads (see the images below).[8, 9]
After these advancements, the ECG as we know it today was created and, except for advances in the technology, it has changed little over the last 50 years.
The ECG has proven to be among the most useful diagnostic tests in clinical medicine. The ECG is now routine in the evaluation of patients with implanted defibrillators and pacemakers, as well as to detect myocardial injury, ischemia, and the presence of prior infarction as well. In addition to its usefulness in ischemic coronary disease, the ECG, in conjunction with ambulatory ECG monitoring, is of particular use in the diagnosis of disorders of the cardiac rhythm and the evaluation of syncope. Other common uses of the ECG include evaluation of metabolic disorders, effects and side effects of pharmacotherapy, and the evaluation of primary and secondary cardiomyopathic processes among others.
No absolute contraindications to performing an electrocardiogram, other than patient refusal, exist. Some patients may have allergies, or more commonly, sensitivities to the adhesive used to affix the leads; in these cases, hypoallergenic alternatives are available from various manufacturers.
For more information, see the following resources:
For a concise review of the recognition of artifacts which may be introduced by lead misplacement, see Surawicz, Borys, Timothy, K and Te-Chuan Chou. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric. W.B. Saunders and Company, 2001. 576-82
For further information on the technical standards of ECG recording devices, the see the Association for the Advancement of Medical Instrumentation and the American National Standards Institute.
The various components of an ECG (ie, P wave, QRS complex, and T waves) have different and distinct amplitudes and frequencies. Devices that record these signals must make use of various signal filtering and amplification algorithms to produce a clinically useful signal while excluding unwanted signals such as those resulting from skeletal muscle, respiratory variation, and electromagnetic interference from nearby equipment.
Currently, most ECG recording machines are digital as opposed to the analog devices previously in use. The American College of Cardiology, in conjunction with the American Heart Association and the Heart Rhythm Society, have published guidelines regarding the technical standards for ECG recording equipment.
In order to compensate for the effect of low-frequency signals, such as those due to chest wall movement with respiration, while also minimizing the effect of extraneous high frequency on the fidelity of the recording, various filters need to be applied to the raw electrical signal. For the current digital systems, these low-frequency and high-frequency cutoff values are 0.67 Hz and 150 Hz, respectively, for adult electrocardiography. The high-frequency filter should be increased to 250 Hz in pediatric electrocardiography. Additional guidelines are given with respect to signal amplification, compression and storage; these are beyond the scope of this article.
The standard 12-lead electrocardiogram is generally performed with the patient lying quietly in the supine position. Care should be taken to ensure that the skin is clean and is trimmed of excess hair in the areas in which the leads are to be placed. In some instances, a mild abrasive pad can be used to prepare the skin in these areas to aid in apposition of the leads.
Having prepared the patient, the leads should be applied in the following manner. The standard limb leads are recommended to be placed one on each limb distal to the shoulders and hips, but they do not necessarily need to be as far distal as the wrists and ankles. The precordial leads should be applied next, taking care to be as precise as possible in their positioning.
Lead V1 is placed at the fourth intercostal space at the right sternal border. Lead V2 is placed at the left sternal border directly across from V1, also in the fourth intercostal space. V4 is placed in the fifth intercostal space at the mid clavicular line, and then V3 can be placed midway between V2 and V4. V6 is placed in the horizontal plane of V4 at the mid-axillary line, and then V5 is placed in the same horizontal plane as V4 in the anterior axillary line or midway between V4 and V6 when the anterior axillary line is not readily discernible. Precordial electrode placement in women with large breasts can be problematic due to obfuscation of bony landmarks. The current recommendations are that the electrodes be placed beneath, rather than overlying, the breast.
The importance of proper lead placement cannot be stressed enough. Furthermore, properly trained ECG technologists have been noted to be more likely to properly place the leads than are nurses or physicians, including cardiologists. Improper placement of the leads can yield a tracing that gives the appearance of disease where none is present or vice versa.
Once the ECG is completed, it should be reviewed by the operator. If significant issues with the quality of the tracing exist, the cause of the issue should be addressed and the tracing repeated. When this is impractical or impossible, the practitioner ordering or reading ECG should be notified for further guidance.
After having obtained an acceptable tracing, the ECG leads are removed and the patient is helped off the examination table. In the event that the adhesive from the electrodes is not easily removed from the skin, the patient may be given an alcohol pad or moist paper towel.
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