Question 9

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.

Discussion

• 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

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