Explain the following components of an electrocardiogram (ECG) machine; electrodes, leads, amplifiers and filters. Outline methods employed to reduce artefact.

Master answer

ECG

• 12 electrodes on the body surface to detect small potentials (0.5-2 mV) generated by the heart.
• Electrodes made of Ag/AgCl
• ECG signals detected by these electrodes
• Sent to electronic device that filters out noise and boosts the signal
• Displayed on oscilloscope

Signal Interpretation from oscilloscope readings via Enthoven’s triangle

• For orientation of the Bipolar and unipolar limb leads synthesised from the 12-lead ECG.
• Bipolar limb leads
  • Lead I: RA (-) to LA (+) at 0°
  • Lead II: RA (-) to LL (+) at 60°
  • Lead III: LA (-) to LL (+) at 120°
• Unipolar limb leads
  • aVR: RA (+) at -150°
  • aVL: LA (+) at -30°
  • aVF: LL (+) at 90°
• Positive deflection in a lead indicates depolarisation wave moving towards lead’s positive terminal

Calibration

• Vertical calibration
  • 1 mV signal → vertical deflection of 10 mm (2x large square or 10x small squares)
  • 0.1 mV per mm (or small square)
• Horizontal calibration
  • paper speed is 25 mm/s → 0.04 s/mm (or small square)

Errors:

Noise and interference caused by:

• Electrical interference by any device using AC current (esp high-frequency diathermy)
  • → Minimise by ECG filters, shielding of leads/cables, differential amplifers, Etc.
• Movement or shivering
  • → minimised by placement over bony prominences and use of ECG filters
• High skin impedance
  • → minimised by degreasing skin with EtOH and use of conducting gel with electrodes
• Incorrect electrode placement relative to heart

It looks like this Examiner reads Hemmings.

Gladwin 2016

Answer by Gladwin