Question 5

College Answer

Monitoring and monitors are essential to Intensive Care practice, and is the reason why it is 
included in the syllabus. Unfortunately candidates have performed poorly in this question, as they 
have in previous measurement and monitoring questions. Future candidates need to be aware 
that such questions WILL get asked again. 

For a good answer it was expected that mention would be made of what an ECG monitors does 
(ie detects and amplifies the small electrical changes on the skin that are caused when the heart 
muscle depolarizes), how (ie use of 2 or more electrodes, typically being made of silver or silver 
chloride), the type of leads (ie unipolar and bipolar, and a description of the latter), the way the 
signal is handled (isolation, amplification, gain, filtering) and displayed. Methods to reduce 
artefact and improve signal:noise ratio, should have included skin conductive measures, 
minimising external interference (filters, earthing), common mode signal artefact rejection, high 
input impedance amplification and mention of diagnostic and monitoring modes.

Recommended sources: Davis and Kenny pgs 160-178, also Sykes & Vickers Principles in 
measurement and monitoring in Anaesthesia and Intensive Care, Chps 4, 5, 6, 23.

Discussion

True to their threat, the college asked this again in Question 9 from the first paper of 2016; and it appears the trainees were prepared (pass rate was 50%).

• 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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