Question 17

College Answer

Excellent answers described the function of the ECG device and its components. Components include
electrodes, which form leads (unipolar and bipolar), the amplifier and an output device. The process of
amplification and filtering (e.g., high and low pass filters), as well as monitoring and diagnostic ECG
modes were described. A comprehensive list of ways to reduce artefacts, including strategies to address
both patient and equipment factors was generally provided.

Discussion

• 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.