Question 9

Describe the essential components of an ECG monitor (80% of marks). Outline the methods employed to reduce artefact (20% of marks).

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College Answer

The ECG device detects and amplifies the small electrical changes on the skin that are caused 
when the heart muscle depolarizes (0.5 – 2 mV). This is reflected as rises and falls in the 
voltage between two electrodes placed either side of the heart which is displayed either on a 
screen or on paper. Usually more than 2 electrodes are used and they can be combined into a 
number of pairs (For example: Left arm (LA), right arm (RA) and left leg (LL) electrodes form 
the three pairs LA+RA, LA+LL, and RA+LL). The output from each pair is known as a lead. 
Each lead is said to look at the heart from a different angle.
Electrodes are commonly made of silver or silver chloride components that are attached to the 
main unit of the machine. Most ECG machines use 12 electrodes. Better answers made 
mention of the two lead types: unipolar and bipolar. 
Methods to reduce artefact include improving signal detection (conductive paste, skin 
preparation (dry, no hair, etc.)) and minimizing external electrostatic forces (common earthed 
environment, diathermy, etc.,) or patient environment (avoid shivering). The amplifier has three essential functions: High input impedance so as to minimize signal loss 
and reject interference (50 – 60 Hz), differential amplification, (to amplify the potential 
difference detected by the skin electrodes), and high common mode rejection (e.g. > 50Hz) to 
aid eliminating muscle artefact or electrical interference from the power grid.). 
Vector analysis is not a component of the ECG machine and so was not required to answer the question.


  • Relation of cellular ionic events to surface ECG
    • Extracellular charge of resting myocyte membrane is positive
    • Depolarisation makes it negative
    • This difference in charge along the myocardium produces an electric field
    • The difference between two surface measurements of electric field strength is the potential difference (voltage) measured by the ECG leads
    • Each pair of electrodes is a "lead"
  • Relation of surface ECG to events of the cardiac cycle
    • P wave: depolarisation of atrial muscle
    • PR interval: AV node onduction
    • QRS: depolarisation of the ventricular muscle
    • Peak of the R wave: beginning of isovolumetric contraction
    • T wave: ventricular repolarisation
  • Essential components of an ECG monitor
    • Signal transmission: by silver/silver chloride electrodes​​​​​​​
      • ​​​​​​​Thin and broad electrodes (10mm diameter)
      • Conducting gel to improve skin contact
      • Digital signal
      • High sampling rate (10,000-15,000 Hz) to detect pacing spikes
    • Amplification
      • ​​​​​​​Low signal amplitude (0.5-2.0 mV) requires a ~ 1,000 gain factor
      • Differential amplification only amplifies the difference between electrode leads, rather than the absolute voltages
      • This eliminates sources of noise which affect each electrode equally (this is called common-mode rejection)
    • Isolation removes mains interference and protects components
    • Earthing reduces interference
    • Filtering
      • Most ECG information is contained in signals 1.0-30  Hz
      • Monitoring mode filter the signal frequency to 0.5-30 Hz range
      • Diagnostic mode filter the signal frequency to 0.05-100 Hz range
      • High input impedance of the amplifier decreases the conduction of high-frequency signals, eliminating mains interference and EMG signal
      • ​​​​​​​Low pass filtering eliminates movement artifact
  • Methods used to decrease artifact and interferences 
    • Poor contact: skin prep, conductive gel, shaving the skin
    • ​​​​​​​Movement artifact: shivering, etc - rewarming the patient, NMJ blockade, low-pass filtering
    • Medical device interference: removing/repairing the devices, high-pass filtering


de Luna, Antoni Bayés. Chapter 2; "The ECG Curve: What Is It and How Does It Originate?" in:  ECGs for beginners. John Wiley & Sons, 2014.

Dupre, Anthony, Sarah Vincent, and Paul A. Iaizzo. "Basic ECG theory, recordings, and interpretation." Handbook of cardiac anatomy, physiology, and devices. Humana Press, 2005. 191-201.

Reisner, Andrew T., Gari D. Clifford, and Roger G. Mark. "The physiological basis of the electrocardiogram." Advanced methods and tools for ECG data analysis 1 (2006): 25.

Hurst, J. Willis. "Naming of the waves in the ECG, with a brief account of their genesis." Circulation 98.18 (1998): 1937-1942.

Geselowitz, David B. "Dipole theory in electrocardiography." The American journal of cardiology 14.3 (1964): 301-306.

Crouch, Catherine Hirshfeld, and John W. Hirshfeld Jr. "Teaching the electrical origins of the electrocardiogram: An introductory physics laboratory for life science students." American Journal of Physics 88.7 (2020): 526.

Bailey, James J., et al. "Recommendations for standardization and specifications in automated electrocardiography: bandwidth and digital signal processing. A report for health professionals by an ad hoc writing group of the Committee on Electrocardiography and Cardiac Electrophysiology of the Council on Clinical Cardiology, American Heart Association." Circulation 81.2 (1990): 730-739.

Wilson, Frank N. "The distribution of the potential differences produced by the heart beat within the body and at its surface." American Heart Journal 5.5 (1930): 599-616.

HOLT JR, JOHN H., et al. "A study of the human heart as a multiple dipole electrical source: I. Normal adult male subjects." Circulation 40.5 (1969): 687-696.

Hobbie, Russell K., and Bradley J. Roth. "The Exterior Potential and the Electrocardiogram." Intermediate Physics for Medicine and Biology. Springer, Cham, 2015. 185-212.

Hobbie, Russell K. "The electrocardiogram as an example of electrostatics." American Journal of Physics 41.6 (1973): 824-831.

Malmivuo, Jaakko, and Robert Plonsey. Bioelectromagnetism: principles and applications of bioelectric and biomagnetic fields. Oxford University Press, USA, 1995.

Vieau, Sarah, and Paul A. Iaizzo. "Basic ECG theory, 12-lead recordings, and their interpretation." Handbook of Cardiac Anatomy, Physiology, and Devices. Springer, Cham, 2015. 321-334.

Abdul Jamil, M. M., et al. "Electrocardiograph (ECG) circuit design and software-based processing using LabVIEW." (2017). Journal of Telecommunication, Electronic and Computer Engineering.Vol. 9 No. 3-8

Wilson, Frank N., A. Garrard Macleod, and Paul S. Barker. "The accuracy of Einthoven's equation." American Heart Journal 7.2 (1931): 203-206.

Wilson, Frank N., et al. "Electrocardiograms that represent the potential variations of a single electrode.American Heart Journal 9.4 (1934): 447-458.

Kligfield, P., et al. "American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; American College of Cardiology Foundation." Heart Rhythm Society. Recommendations for the standardization and interpretation of the electrocardiogram: part I: the electrocardiogram and its technology a scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology (2007): 1109-1127.

Bosznai, István, Ferenc Ender, and Hunor Sántha. "Web server based remote health monitoring system." 2009 32nd International Spring Seminar on Electronics Technology. IEEE, 2009.

Grassini, S. "Electrochemical Impedance Spectroscopy (EIS) for the in-situ analysis of metallic heritage artefacts." Corrosion and Conservation of Cultural Heritage Metallic Artefacts. Woodhead Publishing, 2013. 347-367.

Spach, Madison S., et al. "Skin-electrode impedance and its effect on recording cardiac potentials." Circulation 34.4 (1966): 649-656.

Davies, Alan. "Recognizing and reducing interference on 12-lead electrocardiograms." British journal of nursing 16.13 (2007): 800-804.

Patel, Santosh I., et al. "Equipment-related electrocardiographic artifacts: causes, characteristics, consequences, and correction." The Journal of the American Society of Anesthesiologists 108.1 (2008): 138-148.

Selvan, R. Barani, et al. "Earthing defect: A cause for unstable hemodynamics." Annals of cardiac anaesthesia 15.1 (2012): 47.

Somerville, T. "Isolation amps hike accuracy and reliability." Electronic Design 38.4 (1990): 127-132.