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Luc Dekie, PhD is the Director of Scientific Affairs, Europe.
This article discusses the importance of good patient preparation, lead hook-up, and the quality checks that can be performed to ascertain that an ECG is of good quality.
Electrocardiograms (ECGs) are frequently recorded in the clinic as an easy-to-perform, non-invasive method to collect diagnostic information about the status of the heart (rhythm, conduction, chamber enlargement, ischemia, infarction, the effect of electrolyte abnormalities and drug effects, and even non-cardiac disease effects on the heart). Properly recorded ECGs are very reproducible, and can be used serially to determine change in cardiac status. Recording a good quality ECG is straightforward as long as the proper technique is used. This test is usually performed by technicians, physicians, and nurses who have received the proper training to collect high quality recordings.
In clinical trials, ECGs are usually part of the safety assessments regardless of the therapeutic area. As a result, ECGs are sometimes recorded by nurses and technicians who lack recent training and who are unaware of all the factors that are important to record ECGs of high standard. Quantitative data from the recordings, and even computer generated interpretive statements, may be automatically transferred to trial databases or be used in case report forms. Unless an ECG is unreadable or technical statements are routinely included during the ECG review, little is known about the quality of the ECG when the database is examined.
This article will discuss the importance of good patient preparation, lead hook-up, and the quality checks that can be performed to ascertain that the ECG is of good quality.
High quality patient preparation is the key to collecting high quality ECGs. A simple patient preparation procedure tends to work best and is easily taught. The following steps are required for a good patient preparation.
ECG electrodes are typically placed using the Mason-Likar lead placement or the standard lead placement scheme. Although the rhythm analysis is not adversely affected by the Mason-Likar configuration, many studies have shown significant differences in morphology between ECGs acquired with the limb electrodes in the standard position and in the Mason-Likar position, and clinicians should be warned that ECGs acquired with the electrodes in the Mason-Likar configuration are not equivalent to those taken with the electrodes on the legs and arms and should not be directly compared.1,2
Six electrodes are placed on the chest in the following locations:
V1 is the important starting point since the V2-V6 electrode placement will be placed relative to the V1 electrode. It can be difficult locating the fourth intercostal space. The best way is to run your fingers down the sternum, starting at the heads of the clavicles, until you meet a bony horizontal ridge (the sternal angle or angle of Louis). This is easier to find in male patients. With your finger on this ridge, slide it to the patient's right, your finger will drop into an intercostal space, this is the second intercostal space; now move down to the third and then the fourth, that's where you place V1.1
A common error is superior misplacement of V1 and V2 in the second or third intercostal space. This can result in a reduction of initial R-wave amplitude in these leads, approximating 0.1 mV per interspace, which can cause poor R-wave progression or erroneous signs of anterior infarction. Superior displacement of the V1 and V2 electrodes will often result in rSr' complexes with T-wave inversion, resembling the complex in lead aVR.3 Misplacement of V1 and V2 electrodes too high or too low also results in misplacement of the other chest leads, often causing diagnostic errors in the interpretation of left ventricular hypertrophy.4
Arm lead electrodes are placed on the wrists. Leg lead electrodes are placed approximately 7-12 cm (3-5 inches) above the ankles.
Identical to the standard placement.
Arm electrodes are placed 2 cm below the lateral end of each clavicle, and the left leg electrode is placed in the anterior axillary line halfway between the anterior iliac spine and the costal margin. The ground electrode can be placed anywhere on the body.
Filtering of the ECG signal is used to reduce noise in the raw signal. The ECG signal is usually low (around 1 mV) so it is important that the filtering does not distort the ECG waveform. The noise may come from muscle noise, electrical interference (50/60Hz), electrical noise from equipment in the environment, and from within the ECG equipment itself, such as from internal AC/DC converters.
The low frequency noise typically corresponds to the baseline wandering produced by respiration, but baseline drift suppression by filters may affect the low frequency components of the ECG signal, namely the ST-T segments, and may produce artefactual ST-segment deviation.
Reduction of high frequency noise (typically muscle noise) by digital filters may affect the high frequency component of the ECG signal (e.g. QRS complex). Inadequate high frequency response reduces the amplitude of QRS measurements and the ability to detect small deflections.
In general, low-pass filters are used to reduce high frequency artefact, while high-pass filters are used to correct for low-frequency artefact. The notch filter is used to reduce electrical artefact of a specific frequency (e.g. 50 or 60 Hz interference from power sources).7,8
The international guidelines suggest to record the ECGs with a low-frequency cut-off at 0.05 Hz for routine filters and an upper frequency cut-off of at least 150 Hz (250 Hz being more appropriate for infants).9 An obvious consequence of these high frequency recommendations is that reduction of noise by setting the high frequency cut-off of a standard or monitoring ECG to 35 or 40 Hz will invalidate any amplitude measurements used for diagnostic classification (Figure 2).
It is highly recommended to perform ECG recordings without additional filters, and not change filter settings between recordings on the same patient.
Most ECG machines have a screen that shows the ECG signal prior to acquiring and printing the actual ECG. This feature should be used by the site to check the tracing for poor quality before pressing the record button. At all times, it is crucial to carefully review the recorded ECG on paper or on screen for completeness and good quality, before unhooking the patient, so that another ECG can be taken if needed.
Millions of electrocardiograms are recorded on a daily basis. Although this is considered a simple test to perform, the quality can be very poor, if not executed properly, and impact patient treatment. People recording ECGs need to be aware of the correct patient preparation and lead placement. The use of filters should be restricted, and the evaluation of ECGs should take the filter settings into consideration. A check should be performed immediately upon recording to assure the recording is of good quality.
Luc Dekie, PhD, is the Director of Scientific Affairs, Europe at Biomedical Systems.
The author would like to thank Dr. Richard Kovacs and Dr. Tim Callahan for their review of the article.
1Rautaharju. The effect of modified limb electrode positions on electrocardiographic wave amplitudes. J Electro 1980, 13:109.
2Marquette™ 12SL™ ECG Analysis Program, Revision A, 30 August 2010.
3Stroobandt RX, Barold SS, Sinaeve AF, ECG from basics to Essentials. Wiley Blackwell (2016)
4Elsayed Z. Soliman, MD, MSc, MS, Journal of Electrocardiology 41 (2008) 378–379.
5Mason RE, Likar I. A new system of multiple-lead exercise electrocardiography. Am Heart J 1966;71:196–205
6Trägårdh-Johansson et al, Journal of Electrocardiology 2011(44) 109–114.
8Gregg RE, Zhou SH, Lindaver JM, Helfenbein ED, Givliano KK. What is Inside the Electrocardiograph? J Electro. 2008 41:8-14.
9Kligfield P, Gettes LS, Bailey JJ, Childers R, Deal BJ, et al. Recommendations for the Standardization and Interpretation of the Electrocardiogram: Part 1: The Electrocardiogram and Its Technology: A Scientific Statement From the American Heart Association Electrocardiography and Arrhythmia Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society Endorsed by the International Committee for Computerized Electrocardiology. Circulation 2007; 115;1306-1324.
10R. Stroobandt et al. ECG from basics to essentials. Wiley Blackwell 2016.